PATENT DOCUMENT

Publication Number: US-10097017-B2
Application Number: US-201615051228-A
Country: US
Kind Code: B2

Title: Systems and methods for bidirectional two-port battery charging with boost functionality

Abstract:
The disclosed embodiments provide a charging system for a portable electronic device. The charging system includes a first bidirectional switching converter connected to a first power port of the portable electronic device, a low-voltage subsystem in the portable electronic device, and a high-voltage subsystem in the portable electronic device and a second bidirectional switching converter connected to a second power port of the portable electronic device, the low-voltage subsystem, and the high-voltage subsystem. The charging system also includes a control circuit that operates the first and second bidirectional switching converters to provide and receive power through the first and second power ports and convert an input voltage received through the first or second power port into a set of output voltages for charging an internal battery in the portable electronic device and powering the low-voltage subsystem and the high-voltage subsystem.

Claims:
What is claimed is: 
     
       1. A charging system for a portable electronic device, comprising:
 a first bidirectional switching converter connected to each of a first bidirectional power port of the portable electronic device, a low-voltage subsystem in the portable electronic device, and a high-voltage subsystem in the portable electronic device; 
 a second bidirectional switching converter connected to each of a second bidirectional power port of the portable electronic device, the low-voltage subsystem, and the high-voltage subsystem; 
 an internal battery connected to the low-voltage subsystem; and 
 a control circuit configured to operate one of the first and second bidirectional switching converters to distribute power between the high-voltage subsystem, the low-voltage subsystem, the first and second bidirectional power ports and the internal battery. 
 
     
     
       2. The charging system of  claim 1 , wherein at least one of the first and second bidirectional switching converters is a single-input dual-output (SIDO) boost converter. 
     
     
       3. The charging system of  claim 1 , wherein the control circuit is further configured to:
 operate the first and second bidirectional switching converters based at least in part on a coupling of a power source, an external accessory, or an external battery to at least one of the first or second bidirectional power ports; and 
 operate the first and second bidirectional switching converters based on a state of the internal battery. 
 
     
     
       4. The charging system of  claim 1 , wherein the control circuit is further configured to:
 configure a set of switches in the first bidirectional switching converter to switch between up-converting power from the low-voltage subsystem to the high-voltage subsystem and up-converting power from the low-voltage subsystem to one of the first and second bidirectional power ports using a common inductor or mutually coupled inductors. 
 
     
     
       5. The charging system of  claim 1 , wherein the control circuit is further configured to:
 configure a set of switches in one of the first and second bidirectional switching converters connected to one of the first and second bidirectional power ports in the portable electronic device to:
 up-convert power from one of the first and second bidirectional power ports to the low-voltage subsystem; and 
 using a common inductor or coupled inductors, up-convert power from the low-voltage subsystem to the high-voltage subsystem. 
 
 
     
     
       6. The charging system of  claim 1 , wherein the control circuit is further configured to:
 during a low-voltage state or an under-voltage state of the internal battery and a coupling of an external battery to the first power port:
 operate the first bidirectional switching converter to down-convert power from the external battery to the low-voltage subsystem; and 
 operate the second bidirectional switching converter to up-convert power from the low-voltage subsystem to the high-voltage subsystem. 
 
 
     
     
       7. The charging system of  claim 1 , wherein at least one of the first and second bidirectional switching converters is a single-input dual-output (SIDO) buck-boost converter. 
     
     
       8. The charging system of  claim 7 , wherein the SIDO buck-boost converter is configured to distribute current in either direction between the low-voltage subsystem and the first and second bidirectional power ports. 
     
     
       9. The charging system of  claim 8 , wherein the state of the internal battery is at least one of a high-voltage state, a low-voltage state, and an under-voltage state. 
     
     
       10. The charging system of  claim 7 , wherein the SIDO buck-boost converter is configured to prioritize the distribution of power between the high-voltage subsystem, the low-voltage subsystem, and the first and second bidirectional power ports when the voltage of the first and second bidirectional power ports is higher or lower than the voltage of the low-voltage subsystem. 
     
     
       11. The charging system of  claim 1 , wherein at least one of the first and second bidirectional switching converters is a dual output SEPIC converter. 
     
     
       12. The charging system of  claim 1 , wherein the control circuit is configured to operate the first bidirectional switching converter to up-convert power from the low-voltage subsystem to the high-voltage subsystem with no power coming in or out of the first bidirectional power port. 
     
     
       13. The charging system of  claim 1 , wherein the control circuit is configured to operate the first bidirectional switching converter to up-convert power from a power source or an external battery connected to the first bidirectional power port to the low-voltage subsystem using an inductor or coupled inductors. 
     
     
       14. The charging system of  claim 13 , wherein the control circuit is further configured to operate the first bidirectional switching converter to up-convert power from the power source or the external battery connected to the first bidirectional power port to the high voltage subsystem using the inductor or coupled inductors. 
     
     
       15. The charging system of  claim 1 , wherein the control circuit is configured to operate the first bidirectional switching converter to down-convert power from a power source or an external battery connected to the first bidirectional power port to the low-voltage subsystem and up-convert power from the power source or the external battery connected to the first bidirectional power port to the high-voltage subsystem using a common inductor or coupled inductors. 
     
     
       16. The charging system of  claim 1 , wherein the control circuit is configured to operate the first bidirectional switching converter to up-convert power from the low-voltage subsystem to the high voltage sub-system and down-convert power from the low-voltage subsystem to an external accessory coupled to the first bidirectional power port using a common inductor or coupled inductors. 
     
     
       17. The charging system of  claim 1 , wherein the control circuit is configured to operate the first bidirectional switching converter to up-convert power from the low-voltage subsystem to the high-voltage subsystem and up-convert power from the low-voltage subsystem to an external accessory coupled to the first bidirectional power port using a common inductor or coupled inductors. 
     
     
       18. A method for managing power use in a portable electronic device having first and second bidirectional power ports, first and second bidirectional power converters respectively coupled to the first and second bidirectional power ports, and a low voltage subsystem, a high voltage subsystem, and an internal battery coupled to each of the first and second bidirectional switching converters, the method comprising:
 controlling the first and second bidirectional switching converters to distribute power among the low-voltage subsystem, the high-voltage subsystem, and the first and second bidirectional power ports responsive to a determination whether each of the first and second bidirectional power ports is coupled to an external power source, an external battery, or an external accessory and one or more voltages selected from a group consisting of: a voltage of the external power source, a voltage of the external battery, a voltage of the low-voltage subsystem, a voltage of the high-voltage subsystem, and a voltage of the internal battery. 
 
     
     
       19. The method of  claim 18 , further comprising configuring at least one of the first and second bidirectional switching converters to switch between up-converting power from the low-voltage subsystem to the high-voltage subsystem and up-converting power from the low-voltage subsystem to one of the first and second bidirectional power ports using a common inductor or mutually coupled inductors. 
     
     
       20. The method of  claim 18 , further comprising operating at least one of the first and second bidirectional switching converters to:
 up-convert power from one of the first and second bidirectional power ports to the low-voltage subsystem; and 
 using a common inductor or coupled inductors, up-convert power from the low-voltage subsystem to the high-voltage subsystem. 
 
     
     
       21. The method of  claim 18 , wherein the distributing of power further comprises prioritizing up-converting power from the low-voltage subsystem to the high-voltage subsystem over down-converting of the input voltage to the low-voltage subsystem. 
     
     
       22. The method of  claim 18 , wherein controlling the first and second bidirectional switching converters to distribute power among the low-voltage subsystem, the high-voltage subsystem, and the first and second bidirectional power ports further comprises operating the first bidirectional switching converter to up-convert power from the low-voltage subsystem to the high-voltage subsystem with no power coming in or out of the first bidirectional power port. 
     
     
       23. The method of  claim 18 , wherein controlling the first and second bidirectional switching converters to distribute power among the low-voltage subsystem, the high-voltage subsystem, and the first and second bidirectional power ports further comprises operating the first bidirectional switching converter to up-convert power from a power source or an external battery connected to the first bidirectional power port to the low-voltage subsystem using an inductor or coupled inductors. 
     
     
       24. The method of  claim 23 , wherein controlling the first and second bidirectional switching converters to distribute power among the low-voltage subsystem, the high-voltage subsystem, and the first and second bidirectional power ports further comprises operating the first bidirectional switching converter to up-convert power from the power source or the external battery connected to the first bidirectional power port to the high-voltage subsystem using the inductor or coupled inductors. 
     
     
       25. The method of  claim 18 , wherein controlling the first and second bidirectional switching converters to distribute power among the low-voltage subsystem, the high-voltage subsystem, and the first and second bidirectional power ports further comprises operating the first bidirectional switching converter to down-convert power from a power source or an external battery connected to the first bidirectional power port to the low-voltage subsystem and up-convert power from the power source or the external battery connected to the first bidirectional power port to the high-voltage subsystem using a common inductor or coupled inductors. 
     
     
       26. The method of  claim 18 , wherein controlling the first and second bidirectional switching converters to distribute power among the low-voltage subsystem, the high-voltage subsystem, and the first and second bidirectional power ports further comprises operating the first bidirectional switching converter to up-convert power from the low-voltage subsystem to the high-voltage subsystem and down-convert power from the low-voltage subsystem to an external accessory coupled to the first bidirectional power port using a common inductor or coupled inductors. 
     
     
       27. The method of  claim 18 , wherein controlling the first and second bidirectional switching converters to distribute power among the low-voltage subsystem, the high-voltage subsystem, and the first and second bidirectional power ports further comprises operating the first bidirectional switching converter to up-convert power from the low-voltage subsystem to the high-voltage subsystem and up-convert power from the low-voltage subsystem to an external accessory coupled to the first bidirectional power port using a common inductor or coupled inductors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Non-provisional application claims the benefit of Provisional Application No. 62/184,101 by Thomas Greening and Edrick C. G. Wong, filed on Jun. 24, 2015, and claims the benefit of Provisional Application No. 62/387,266 by Thomas Greening, Kamran Hasan and Edrick C. G. Wong, filed on Dec. 23, 2015, the entire contents of the provisional applications are herein incorporated by reference. 
     The subject matter of this application is related to the subject matter in a co-pending non-provisional application by inventors Thomas C. Greening, Qing Liu and William C. Athas, entitled “Battery Charging with Reused Inductor for Boost,” having Ser. No. 14/749,466, and filing date filed Jun. 24, 2015, the entire contents of which is herein incorporated by reference. 
     The subject matter of this application is also related to the subject matter in a co-pending non-provisional application by inventors Jamie Langlinais, Mark Yoshimoto and Lin Chen, entitled “Multi-Phase Battery Charging with Boost Bypass,” having Ser. No. 14/749,470, and filing date Jun. 24, 2015, the entire contents of which is herein incorporated by reference. 
    
    
     BACKGROUND 
     Field 
     The disclosed embodiments relate to batteries for portable electronic devices. More specifically, the disclosed embodiments relate to techniques for performing bidirectional two-port battery charging with boost functionality. 
     Related Art 
     Typical portable electronic devices have a power port for powering a connected device and charging an internal battery with an external direct current (DC) power supply. External accessories, such as game controllers, external memory or speakers, may also be plugged into the power port to support additional functionality. These external accessories are typically powered by separate pins on the connector. It may be desirable for portable electronic devices to have added flexibility in allowing the portable device to receive power from or provide power to external devices. 
     SUMMARY 
     The disclosed embodiments provide a charging system for a portable electronic device. The charging system includes a first bidirectional switching converter connected to a first power port of the portable electronic device, a low-voltage subsystem in the portable electronic device, and a high-voltage subsystem in the portable electronic device. The charging system also includes a second bidirectional switching converter connected to a second power port of the portable electronic device, the low-voltage subsystem, and the high-voltage subsystem. The charging system includes a control circuit that operates the first and second bidirectional switching converters to provide and receive power through the first and second power ports and convert an input voltage received through the first or second power port into a set of output voltages for charging an internal battery in the portable electronic device and powering the low-voltage subsystem and the high-voltage subsystem. 
     In some embodiments, the first and second bidirectional switching converters include a SIDO buck-boost converter and a buck converter. 
     In some embodiments, operating the first and second bidirectional switching converters includes operating the first and second bidirectional switching converters based at least in part on a coupling of a power source, an accessory, or an external battery to the first or second power ports, and operating the first and second bidirectional switching converters based on a state of the internal battery. 
     In some embodiments, the state of the internal battery is at least one of a high-voltage state, a low-voltage state, and an under-voltage state. 
     In some embodiments, operating the first and second bidirectional switching converters includes configuring an inductor and a set of switches in a bidirectional switching converter connected to a power port in the portable electronic device to perform at least one of:
         (i) up-converting power from the low-voltage subsystem to a high voltage external accessory or an external battery coupled to the power port;   (ii) down-converting power from the low-voltage subsystem to a low-voltage external accessory or an external battery coupled to the power port;   (iii) up-converting power from the low-voltage subsystem to the high-voltage subsystem with no power coming in or out of the power port;   (iv) down-converting power from a power source or the external battery coupled to the power port to the low-voltage subsystem;   (v) up-converting power from an external battery coupled to the power port to the low-voltage subsystem;   (vi) up-converting power from the low-voltage subsystem to the high-voltage subsystem and a high-voltage external accessory coupled to the power port; and   (vii) up-converting power from the low-voltage subsystem to the high-voltage subsystem while down-converting power from the low-voltage subsystem to a low-voltage external accessory coupled to the power port.       

     In some embodiments, operating the first and second bidirectional switching converters includes configuring an inductor and a set of switches in a bidirectional switching converter connected to a power port in the portable electronic device to switch between up-converting power from the low-voltage subsystem to the high-voltage subsystem and down-converting or up-converting the input voltage from a power source coupled to the power port to the low-voltage subsystem. 
     In some embodiments, during a low-voltage state or an under-voltage state of the internal battery and a coupling of an external battery to the first power port, operating the first and second bidirectional switching converters includes operating the first bidirectional switching converter to down-convert or up-convert power from the external battery to the low-voltage subsystems, and operating the second bidirectional switching converter to up-convert power from the low-voltage subsystem to the high-voltage subsystem. 
     In some embodiments, during a low-voltage state or an under-voltage state in the internal battery, a presence of the input voltage from a power source or an external battery through the first power port, and a coupling of an external accessory to the second power port, operating the first and second bidirectional switching converters includes operating the first bidirectional switching converter to switch between up-converting power from the low-voltage subsystem to the high-voltage subsystem and down-converting the input voltage from a power source to the low-voltage subsystem, and operating the second bidirectional switching converter to up-convert power from the low-voltage subsystem to the high-voltage subsystem, while either up-converting or down-converting power from the low-voltage subsystem to the external accessory. 
     In some embodiments, during a coupling of a power source to the first port and a coupling of an external battery to the second power port, operating the first and second bidirectional switching converters includes operating the first and second bidirectional switching converters to power the low-voltage subsystem and the high-voltage subsystem and charge the internal battery from the power source, and providing remaining power from the power source to charge the external battery. 
     In some embodiments, during a coupling of a power source to the first port and a coupling of an external battery to the second power port, operating the first and second bidirectional switching converters includes operating the first and second bidirectional switching converters to power the low-voltage subsystem and the high-voltage subsystem and charge the internal battery from the power source, and using the external battery to supplement the power to the low-voltage subsystem, the high-voltage subsystem, and the internal battery. 
     In some embodiments, the first bidirectional switching converter includes an inductor and a set of switches. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a charging circuit for a portable electronic device in accordance with the disclosed embodiments. 
         FIG. 2  shows a charging system for a portable electronic device in accordance with the disclosed embodiments. 
         FIG. 3  shows a charging circuit for a portable electronic device in accordance with the disclosed embodiments. 
         FIG. 4A  shows an exemplary bidirectional switching converter in accordance with the disclosed embodiments. 
         FIG. 4B  shows an exemplary bidirectional switching converter in accordance with the disclosed embodiments. 
         FIG. 4C  shows an exemplary bidirectional switching converter in accordance with the disclosed embodiments. 
         FIG. 5A  shows an exemplary charging system for a portable electronic device in accordance with the disclosed embodiments. 
         FIG. 5B  shows an exemplary charging system for a portable electronic device in accordance with the disclosed embodiments. 
         FIG. 5C  shows an exemplary charging system for a portable electronic device in accordance with the disclosed embodiments. 
         FIG. 6  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 7A  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 7B  shows a set of graphs of inductor current for a single switcher mode in accordance with the disclosed embodiments. 
         FIG. 7C  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 7D  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 7E  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 8  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 9A  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 9B  shows a set of graphs of inductor current for a single switcher mode in accordance with the disclosed embodiments. 
         FIG. 9C  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 9D  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 9E  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 10A  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 10B  shows a block diagram of a set of calculations associated with a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 10C  shows a set of graphs of inductor current for a single switcher mode in accordance with the disclosed embodiments. 
         FIG. 10D  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 10E  shows a block diagram of a set of calculations associated with a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 10F  shows a block diagram of a set of calculations associated with a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 11A  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 11B  shows a plot of inductor current for a single switcher mode in accordance with the disclosed embodiments. 
         FIG. 11C  shows a plot of inductor current for a single switcher mode in accordance with the disclosed embodiments. 
         FIG. 11D  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 12A  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 12B  shows a block diagram of a set of calculations associated with a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 12C  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 13A  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 13B  shows a block diagram of a set of calculations associated with a single switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 13C  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 14A  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 14B  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 14C  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 14D  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 15A  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 15B  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 16A  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 16B  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 16C  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 16D  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 16E  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 16F  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. 
         FIG. 17  shows a flowchart illustrating the process of managing use of a portable electronic device in accordance with the disclosed embodiments. 
         FIG. 18  shows a flowchart illustrating the process of operating a charging system for a portable electronic device in accordance with the disclosed embodiments. 
         FIG. 19  shows a portable electronic device in accordance with the disclosed embodiments. 
     
    
    
     In the figures, like reference numerals refer to the same figure elements. 
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     The data structures and code described in this detailed description are typically stored on a computer-readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. The computer-readable storage medium includes, but is not limited to, volatile memory, non-volatile memory, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs), DVDs (digital versatile discs or digital video discs), or other media capable of storing code and/or data now known or later developed. 
     The methods and processes described in the detailed description section can be embodied as code and/or data, which can be stored in a computer-readable storage medium as described above. When a computer system reads and executes the code and/or data stored on the computer-readable storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the computer-readable storage medium. 
     Furthermore, methods and processes described herein can be included in hardware modules or apparatus. These modules or apparatus may include, but are not limited to, an application-specific integrated circuit (ASIC) chip, a field-programmable gate array (FPGA), a dedicated or shared processor that executes a particular software module or a piece of code at a particular time, and/or other programmable-logic devices now known or later developed. When the hardware modules or apparatus are activated, they perform the methods and processes included within them. 
     The disclosed embodiments provide methods and systems for managing use of a battery in a portable electronic device. More specifically, the disclosed embodiments provide charging systems that may provide an up-converted voltage to one or more subsystems of the portable electronic device. 
       FIG. 1  shows an example of the charging systems as described herein. Specifically, the charging system includes a charging circuit comprising one or more power converters  114  and a control circuit  116 , which may control the one or more power converters. The charging system of  FIG. 1  may be used to supply power to components of the portable electronic device. In some variations, such as that shown in  FIG. 1 , the portable electronic device may include one or more high-voltage subsystems  106  and one or more low-voltage subsystems  104 . The portable electronic device may include an internal battery  112 , and the one or more high-voltage subsystems  106  and the one or more low-voltage subsystems  104  which may be powered by an internal battery  112 . The portable electronic device may have a predetermined cutoff voltage set for the battery  112 , which represents the minimum operating voltage of the battery  112  allowed by the electronic device. 
     During operation, the low-voltage subsystems  104  may require a first voltage that is less than a second voltage required by high-voltage subsystems  106 . For example, in some variations low-voltage subsystems  104  may require a first voltage (e.g., 3.0 V) at or below the cutoff voltage of battery  112  to power the components of the low-voltage subsystems  104 , while high-voltage subsystems  106  may require a second voltage (e.g., 3.4 V) above the cutoff voltage of the battery to power the components of the high-voltage subsystems  106 . In other variations, the first voltage required by low-voltage subsystems  104  may be above the cutoff voltage of battery  112 . The charging circuit may provide boost functionality (e.g., between battery  112  and the high-voltage subsystems  106 ), which may supply power to one or more high-voltage subsystems  106 , for example, when the voltage of battery  112  is below the second voltage. On the other hand, low-voltage subsystems  104  may require less voltage than high-voltage subsystems  106  and/or the cutoff voltage of battery  112 , and in some instances may be powered directly by battery  112 . 
     For example, many components in a portable electronic device, including the central processing unit (CPU), graphics-processing unit (GPU), and/or integrated circuit rails, may be powered by voltages much less than an exemplary 3.0V cutoff voltage for battery  112 . On the other hand, the radio and speaker subsystems of the portable electronic device may require an exemplary minimum voltage of 3.4V to operate. As a result, subsystems in the portable electronic device may be divided into two or more groups, such as low-voltage subsystems  104  that can be powered from the first voltage (e.g., 3.0V), and high-voltage subsystems  106  having components that require a minimum of the second voltage (e.g., 3.4 V) to be powered. It should be appreciated that not all components in the high-voltage subsystems  106  must require the second voltage to be powered, and that some components that could otherwise be placed in a low-voltage subsystem  104  (e.g., a component that can be powered by the first voltage) may be placed in a high-voltage subsystem  106  based on other design considerations. 
     In the variations described here, the portable electronic device may comprise a plurality of power ports, each of which may be used to receive power from or provide power to an external device depending on the mode of operation of the charging system. For example, in the variation shown in  FIG. 2 , the charging system may comprise two power ports (a first power port  108  and a second power port  110 ). In some instances, the power ports may be configured such that the same pins (e.g., a power pin and a common ground) are used to transfer power to and from the portable electronic device through the power port. 
     Power converters  114  may be operated by control circuit  116  to provide and/or receive power through power ports  108 - 110  and convert an input voltage received through one or both power ports  108 - 110  into a set of output voltages for charging battery  112  and powering low-voltage subsystems  104  and high-voltage subsystems  106 . For example, power converters  114  may include any type of or combination of power converters, such as one or more of a buck converter, boost converter, an inverting converter, a buck-boost converter, a Ćuk converter a single-ended primary-inductor (SEPICs), and/or a Zeta converter. 
     Each of the power ports  108 - 110  may be connected to an external device, and the portable electronic device may select a categorization for the external device based on three categories: a power supply, an external accessory, or an external battery. An external device categorized as a power supply may provide power to the portable electronic device via a power port, but the portable electronic device will not provide power to the power supply. For example, an AC-DC power adapter connected to mains electricity, for example, household power or line power, may be categorized as a power supply. Conversely, an external device categorized as an external accessory may draw power from the portable electronic, but the portable electronic device will not receive power from the external accessory. Examples of external accessories may include game controllers, external memory, speakers, or the like. An external device categorized as an external battery may receive power from the portable electronic device through the power port or provide power to the portable electronic device through the power port, depending on the operation of the portable electronic device. Accordingly, each of power ports  108 - 110  may have one of four connection states depending on the presence/classification of the external device connected to that port: unplugged (e.g., nothing connected), a power supply, an external battery, and/or an external accessory. 
     It should be appreciated that an external device may be categorized differently in different situations. For example, an external device may include a battery and a power adapter for converting mains power. In these instances, the external device may be classified as a power supply when the mains power is available, but may be classified as an external battery if mains power is not available. It should also be appreciated that the charging systems described here may provide for different combinations of connections statuses between the power ports. For example, the power ports may both be unplugged, one power port may be unplugged while the other power port is connected to either a power supply, an external accessory, or an external battery, or each power port may be connected to one of a power supply, an external accessory, or an external battery (e.g., a power supply connected to one power port and an external accessory connected to another power port, power supplies connected to each port, accessories connected to each port, etc.). 
     Collectively, external devices that provide power to the portable electronic device through a power port are referred to herein as “power sources”, while external devices that draw power from the portable electronic device via a power port are referred to herein as “powered devices.” For example, an external device that is categorized as an external accessory would be a powered device. Conversely, an external device categorized as a power supply would be a power source. An external device categorized as an external battery may act as a power source in instances where the external battery is providing power to the portable electronic device and may act as a powered device when receiving power to the portable electronic device (e.g., to charge the external battery). 
     In addition, the internal battery  112  of the portable electronic device may be in one of three states: a high-voltage state, a low-voltage state, and an under-voltage state. Battery  112  may be considered in an under-voltage state if the battery voltage of battery  112  is less than or equal to a designated cutoff voltage (e.g. a minimum operating voltage) of battery  112  (e.g., 3.0V) at which the portable electronic device will shut down or otherwise stop operating. This cutoff voltage may represent a battery voltage at which battery  112  has no useful remaining charge or at which further discharging of battery  112  may negatively impact future operation of the battery. In instances where the portable electronic device is configured to provide at least a specified voltage to the high-power subsystems, this voltage may be the dividing line between the internal battery&#39;s high-voltage and low-voltage state. In these instances, a low-voltage battery  112  may have a voltage that can be used directly by low-voltage subsystems  104  but not high-voltage subsystems  106  (e.g., between 3.0V and 3.4V in the examples discussed above). A high-voltage battery  112  may have a voltage that can be used directly by all subsystems (e.g., greater than 3.4V). In instances where the portable electronic device has three or more subsystems with different voltage requirements, battery  112  may have multiple low-voltage states. For example, battery  112  may have a first low-voltage state where the battery voltage is high enough to power the lowest-voltage subsystems, a second low-voltage state where the battery voltage is high enough to power the lowest-voltage subsystems and one or more mid-voltage subsystems, and a third low-voltage state where the battery voltage is high enough to power the lowest-voltage subsystems, mid-voltage subsystems, and one or more high-voltage subsystems but not one or more of the highest-voltage subsystems in the portable electronic device. In these examples, a high-voltage state battery  112  would have a voltage high enough to power the highest-voltage subsystems in the portable electronic device. 
       FIG. 2  shows a block diagram of a charging circuit  202  that provides both bi-directionality of two power ports  208 - 210  and a boosted rail for one or more high-voltage subsystems  206 , which have a minimum operating voltage that is higher than the cutoff voltage of an internal battery  212 . On the other hand, one or more low-voltage subsystems  104  may have a minimum operating voltage that is at or below the cutoff voltage of battery  212 , such as discussed above. This approach may achieve bi-directionality by using a power switch (e.g., a 4 FET power switch) on each power port to either provide power from a boost circuit to an external accessory or receive power from an external power supply to be passed to the battery charger. High-voltage subsystems  206  are powered by a boost/bypass circuit that boosts the voltage of battery  212  if the battery voltage is less than the minimum voltage limit of high-voltage subsystems  206 . 
     This charging circuit  202  may require several circuit components (e.g., as many as 19 power FETs and four inductors depending on the components. The charging circuit  202  may not be able to charge battery  212  simultaneously from both power ports  208 - 210  as the power switches may need to select either power port  208  or  210  to connect to the battery charger. 
     Accordingly, some embodiments as described herein may include a battery charging circuit capable of bi-directionally receiving power and providing power for at least two external power ports  208 - 210 , while providing a boosted voltage to high-voltage subsystems  206  that require a minimum voltage higher than the minimum internal battery voltage. These embodiments may reduce the board space required by using available inductors and power FETs for differing purposes, which may be especially beneficial given the limited space available on portable electronic devices. 
       FIG. 3  shows an example of the charging system of  FIG. 1  utilizing bidirectional switchers to control power distribution between power ports and the rest of the portable electronic device. As shown there, the charging system may be implemented using a circuit that includes a first bidirectional switcher  314  coupled to a first power port  308  (via voltage node V BUS,1 ), one or more low-voltage subsystems  304  (via voltage node V MAIN , to which an internal battery  312  may be coupled via a FET  320 ), and one or more high-voltage subsystems  306  (via voltage node V BOOST ). The circuit also includes a second bidirectional switcher  316  coupled to a second power port  310  (via voltage node V BUS,2 ), low-voltage subsystems  304  (via voltage node V MAIN ), and high-voltage subsystems  306  (via voltage node V BOOST ). Depending on the operation of the bidirectional switcher, each bidirectional switcher may be selectively controlled to control a voltage at one or more voltage nodes or act as a switch, and may include a FET, a variable resistor, or the like. Each of bidirectional switchers  314 - 316  may include one or more inductors and a set of switching mechanisms such as FETs, diodes, and/or other electronic switching components to facilitate operation of the bidirectional switcher. 
     As mentioned above, bidirectional switchers  314 - 316  may provide and receive power through power ports  308 - 310  and convert an input voltage received through one or both power ports  308 - 310  into a set of output voltages that may charge a battery  312  and power low-voltage subsystems  304  and/or may power high-voltage subsystems  306 . A single bidirectional switcher  314  or  316  may receive power from the other bidirectional switcher  316  or  314  or the internal battery  312 , and may provide power to the high-voltage subsystems  306 . The bidirectional functionality of the circuit may be represented by the ability of bidirectional switchers  314 - 316  to provide or receive power through the corresponding power ports  308 - 310  and low-voltage subsystems  304 . 
     Bidirectional switchers  314 - 316  may be operated by a control circuit (e.g., control circuit  116  of  FIG. 1 ) based on the connection state (e.g., unplugged, power supply, external accessory, external battery) of each the first or second power ports, and in some instances may also be controlled based on a state of the internal battery  312  in the portable electronic device. More specifically, the control circuit may configure bidirectional switchers  314 - 316  to prioritize the distribution and use of power among low-voltage subsystems  304 , high-voltage subsystems  306 , power ports  308 - 310 , and/or internal battery  312  based on the states of power ports  308 - 310  and battery  312 . As described in further detail below, the operation of bidirectional switchers  314 - 316  may include down-converting (e.g., bucking) power from a power source coupled to one or both power ports  308 - 310  to low-voltage subsystems  304 , up-converting (e.g., boosting) power from low-voltage subsystems  304  to an external accessory or external battery coupled to the power port, up-converting power from low-voltage subsystems  304  to high-voltage subsystems  306  with no power coming in or out of the power port, and/or up-converting power from low-voltage subsystems  304  to high-voltage subsystems  306  and the external accessory coupled to the power port. 
     Exemplary implementations of bidirectional switchers that may be used with the charging circuits described here are shown in  FIGS. 4A-4C . In  FIGS. 4A-4C , each bidirectional switcher  422 - 426  is shown as coupled to a power port  408 , one or more low-voltage subsystems  404  and one or more high-voltage subsystems  406 . Bidirectional switcher  422  of  FIG. 4A  may include a single-inductor dual-output (SIDO) boost converter, bidirectional switcher  424  of  FIG. 4B  may include a single-inductor dual-output (SIDO) buck-boost converter, and bidirectional switcher  426  of  FIG. 4C  may include a dual-output SEPIC converter. 
     Each bidirectional switcher may include one or more inductors and a number of FETs. In bidirectional switchers  422 - 426  of  FIGS. 4A-4C , FET A may be turned on to enable transmission of power to power port  408  at V BUS  and disabled to prevent transmission of power to power port  408 . For example, FET A may be turned off when power port  408  is unplugged and/or to provide reverse voltage protection from a power source that is incorrectly designed or connected backwards to power port  408 . 
     In bidirectional switchers  422 - 426  of  FIGS. 4A-4B , FETs B and C couple the input terminal of inductor L to a voltage node V X  and a reference voltage such as ground, respectively. FETs B and C may be switched to selectively couple the input of inductor L to voltage node V X  or the reference voltage. For example, voltage node V X  can either be at the reference voltage (when FET C is on) or at the voltage of the power port  408  (when FETs A and B are ON and FET C is OFF). FETs D and E may couple the voltage node V X  to high-voltage subsystems  406  at V BOOST . In  FIG. 4B , FETs F and G may couple the load terminal of inductor L to low-voltage subsystems  404  at V MAIN  and a reference voltage such as ground, respectively. 
     In bidirectional switcher  426  of  FIG. 4C , inductors L 1  and L 2  may be coupled inductors that are wound onto the same core. FET B may couple the input terminals of L 1  and L 2  to the voltage node V X , and FET C may couple the input terminals of L 1  and L 2  to a reference voltage such as ground. A capacitor C S  disposed between the input terminals of L 1  and L 2  may transfer energy between V X  and V MAIN  during an on-time when FET B is on and FET C is off. During an off-time when FET C is on and FET B is off, current may circulate from L 1  through C S  and FET C back to L 1 . FETs D and E may couple the voltage node V X  to high-voltage subsystems  406  at V BOOST . 
     When a charging system includes multiple bidirectional switchers, the same- or different-type of bidirectional switcher may be included in the charging system, depending on system design or requirements. For example, a SIDO boost converter may be used with the requirement that the power port  408  has a voltage that is higher than the voltage of the low-voltage subsystems  404 , while a SIDO buck-boost converter or dual-output SEPIC converter may be used without such a requirement at the expense of additional circuitry. 
     It should be appreciated that in some instances the systems described here do not require separation between high-voltage and low-voltage subsystems, and that in other instances the bidirectional switchers are not connected to the high-voltage subsystems (e.g., the high-voltage subsystems may receive power from an additional circuit). For systems that do not have a separation between low-voltage and high-voltage subsystems (e.g., all of the subsystems may be incorporated into the low-voltage subsystems described above) or provides power to the high-voltage subsystems using a separate circuit, the charging system can be simplified by removing the portion of the circuit including FETs D and E from the bidirectional switchers shown in  FIGS. 4A-4C . With respect to  FIG. 3 , in some of these instances the connection between the bidirectional switchers  314  and  316  may also be removed, as well as the FET  321  between the low-voltage subsystems and the high-voltage subsystems (if there are high-voltage subsystems), an additional circuit may be used to connect high-voltage subsystems  306  to either V MAIN  or V BAT  or high-voltage subsystem  306  may be removed and the components incorporated into the low-voltage subsystems  304 ). In some of these instances, the control of the bidirectional switchers may be identical to the control operations discussed below with the internal battery  312  in the high-voltage battery state. 
       FIG. 5A  shows an exemplary charging system for a portable electronic device in accordance with the disclosed embodiments. In particular,  FIG. 5A  shows the charging circuit of  FIG. 3  with each of bidirectional switchers  314 - 316  implemented as bidirectional switchers  514 - 516  including SIDO boost converters (as described above with respect to  FIG. 4A ). The charging system of  FIG. 5A  may support two bidirectional power ports  508 - 510 , powering of one or more low-voltage subsystems  504  from power ports  508 - 510  and/or an internal battery  512 , and a boosted rail to power one or more high-voltage subsystems  506  with a voltage requirement that is higher than the minimum operating voltage of internal battery  512 . The charging system of  FIG. 5A  uses 12 power FETs and two inductors and is approximately half the size of the traditional circuit of  FIG. 2 . Moreover, the charging system of  FIG. 5A  is capable of charging battery  512  and powering low-voltage subsystems  504  and high-voltage subsystems  506  from both power ports  508 - 510  at the same time if two power sources (e.g., devices characterized as external batteries and/or power supplies, etc.) are connected to power ports  508 - 510 . 
     As shown in  FIG. 5A , the charging system includes two bidirectional switchers  514 - 516  connected at both low-voltage subsystems  504  and high-voltage subsystems  506 . A first bidirectional switcher  514  is connected to power port  508  (at voltage node V BUS,1 ) and has an inductor L 1  and FETs that are sub-indexed with the label 1: A 1 , B 1 , C 1 , D 1 , and E 1 . A second bidirectional switcher  516  is connected to power port  510  (at voltage node V BUS,2 ) and has an inductor L 2  and FETs that are sub-indexed with the label 2: A 2 , B 2 , C 2 , D 2 , and E 2 . FET K connects battery  512  to low-voltage subsystems  504  (at voltage node V MAIN ), and FET J connects low-voltage subsystems  504  to high-voltage subsystems  506  (at voltage node V BOOST ). 
     In some instances, the charging system measures some or all of the voltages at V BUS,1 , V BUS,2 , V BOOST , V MAIN , and V BAT  and the current i BUS,1  through FET A 1 , current i BUS,2  through FET A 2 , and current i CHG  through FET K. The currents of inductors L 1  and L 2  may be measured for control, which may be performed in series with each inductor or in the respective switching FETs B, C, and E when the FETs are enabled. 
     In the exemplary charging system of  FIG. 5A , the voltage at the power port (for example, power port  510 ) may always be higher than the voltage of the low-voltage subsystem  504 . The charging system of  FIG. 5A  includes four modes of each power port that may be represented by the following: a power supply connected, an external battery connected, an external accessory connected, or nothing connected. Moreover, battery  512  may be in a high-voltage state, a low-voltage state, or an under-voltage state. The high-voltage state indicates that the internal battery voltage V BAT  is higher than the voltage requirement of high-voltage subsystems  506 , or V BOOST,MIN . In the high-voltage state, battery  512  can directly power both low-voltage subsystems  504  and high-voltage subsystems  506 . The low-voltage state indicates that V BAT  is between the voltage requirement of low-voltage subsystems  504 , or V MAIN,MIN , and the voltage requirement of high-voltage subsystems  506 . In the low-voltage state, battery  512  can directly power low-voltage subsystems  504 , but the battery voltage must be boosted above V BOOST,MIN  to power high-voltage subsystems  506 . In the under-voltage state, the battery voltage may be below the voltage requirements of both low-voltage subsystems  504  and high-voltage subsystems  506 , or may be above the voltage requirement of the low-voltage system, but at or below the minimum operating voltage of the device. 
     The positions of FETs A and B for each bidirectional switcher may be exchanged, with FET B closer to the corresponding power port and FET A closer to the corresponding inductor L. Particularly, when FET&#39;s A 1  and B 1  are exchanged, the drain terminal of FET A 1  is connected to node X and source terminal of FET A 1  is connected to voltage node V x1  while the drain terminal of FET B 1  is connected to node X and the drain terminal of FET B 1  is connected to power port  508 . Similarly, when FET&#39;s A 2  and B 2  are exchanged, the drain terminal of FET A 2  is connected to node Y and source terminal of FET A 2  is connected to voltage node V x2  while the drain terminal of FET B 2  is connected to node Y and the drain terminal of FET B 2  is connected to power port  510 . However, FET A may provide reverse voltage and current protection by preventing negative voltages from an incorrectly designed device connected to the power port from reaching FET B. In addition, the bi-directional current i BUS  may be more accurately measured through the non-switching FET A filtered by the capacitance between FETs A and B. To support a high-voltage power port, FET A does not need to be a higher voltage FET, but FET B and C may be required to be higher-voltage FETs as the voltage across these FETs would see the entire voltage swing from the power port voltage to ground. 
     Similarly, the positions of FETs D and E for each bidirectional switcher could be exchanged. Particularly, when FET&#39;s D 1  and E 1  are exchanged, the drain terminal of FET D 1  is connected to high-voltage subsystem  506  and the drain terminal of FET E 1  is connected to voltage node V x1  while the source terminals of FET&#39;s D 1  and E 1  are connected to each other. Similarly, when FET&#39;s D 2  and E 2  are exchanged, the drain terminal of FET D 2  is connected to high-voltage subsystem  506  and the drain terminal of FET E 2  is connected to voltage node V x2  while the source terminals of FET&#39;s D 2  and E 2  are connected to each other. However, by placing FET D next to the corresponding inductor, only FET D is required to be a higher-voltage FET to protect against a high voltage from the corresponding power port. While the voltage of high-voltage subsystems  506  V BOOST  is boosted, such a voltage is typically no higher than the maximum battery voltage, which allows FET E to be a lower-voltage FET. 
     As mentioned above, FET K may be used to facilitate charging and discharging of the internal battery  512 . The behavior of FET K may depend on the state of battery  512 , in terms of both voltage and charging. When the battery is charging (e.g., if an external power source is available such as through one or both power ports  508 - 510 ), FET K may be controlled to provide a target voltage at V BAT , and to charge battery  512  with a voltage limit of V BAT,MAX  and a current limit of i CHG,MAX . In some instances, FET K may be disabled to place battery  512  in a non-charging state (e.g., when battery  512  is fully charged and has reached the V BAT,MAX  voltage limit). Finally, FET K may allow for discharging of battery  512  (e.g., when no power supply or external battery is connected to the power ports, or where the external devices cannot provide sufficient power to power the portable electronic device). 
     For example, FET K may act as an ideal diode if a power source is available and battery  512  is in the non-charging state. To prevent discharge of battery  512 , the control circuit may attempt to control the voltage measured at V MAIN  to be higher than the measured battery voltage V BAT . If the control circuit cannot control V MAIN  to be higher than V BAT , the control circuit may enable FET K as an ideal diode to allow current to flow unimpeded from battery  512  to low-voltage subsystems  504 . 
     While battery  512  is charging in a low-voltage state or a high-voltage state, FET K may be fully enabled to connect low-voltage subsystems  504  directly to the charging battery  512  with the V BAT,MAX  voltage limit and the current limit i CHG,MAX . If battery  512  is charging in an under-voltage state, FET K may be operated linearly to keep the voltage on low-voltage subsystems  504  higher than the voltage requirement of low-voltage subsystems  504 , or V MAIN,MIN . 
     If no external power source is available, battery  512  may be discharging to power the portable electronic device. If battery  512  discharges in the low-voltage or high-voltage state, FET K may be fully enabled to directly power low-voltage subsystems  504  from battery  512 . If battery  512  is in the under-voltage state, the portable electronic device will switch off, since the battery voltage is too low to power low-voltage subsystems  504 . While the device is switched off, all FETs may be disabled, awaiting detection of a power source. 
     In some instances, battery  512  may be used to directly power high-voltage subsystems  506 . For example, in some instances the behavior of FET J may depend only on the voltage state of battery  512 . If the battery  512  is in the high-voltage state, FET J may be enabled to directly connect battery  512  to both low-voltage subsystems  504  and high-voltage subsystems  506 , since V BAT  is higher than V BOOST,MIN . If the battery  512  is in low-voltage or under-voltage state, FET J may be disabled, and the voltage for high-voltage subsystems  506  is provided by one or both bidirectional switchers. It should be appreciated that the charging systems described here need not include FET J connecting the high-voltage subsystems  506  and low-voltage subsystems  504 , such that the high-voltage subsystems  506  are always powered by one or both bidirectional switchers. In these instances, some efficiency may be lost when the battery voltage is high enough to power the high-voltage subsystems directly, but the control circuit may provide control without distinguishing between high- and low-battery voltage states, with the behavior of the charging system identical to the behavior in the low-voltage state, when FET J would be disabled. 
     For systems that do not require a high-voltage subsystem or do not provide power to the high-voltage subsystem using a separate circuit, then the charging system of  FIG. 3  may be implemented as the charging system shown in  FIG. 5B . The bidirectional switchers  514 - 516  in  FIG. 5B  may connect the power ports  508 - 510  to only the low-voltage subsystems  504 . The bidirectional switchers shown in  FIG. 4A ,  FIG. 4B , and  FIG. 4C , can also be simplified to the bidirectional switcher of  FIG. 5B . Particularly, SIDO boost converter of  FIG. 4A  may be implemented as the bidirectional switcher  516  of  FIG. 5B  by removing FETs D, E and the capacitance on the high-voltage subsystems  406  in bidirectional switcher  422  of  FIG. 4A , while bidirectional switcher  424  of  FIG. 4B  may be implemented as the bidirectional switcher  514  of  FIG. 5B  by removing FETs D, E and the capacitance on the high-voltage subsystem  406  in bidirectional switcher  424  of  FIG. 4B . Similarly, the bidirectional switcher  426  of  FIG. 4C  may be implemented in the charging system of  FIG. 5B  by removing DETs D, E and the capacitance on the high-voltage subsystems  406  in bidirectional switcher  426  of  FIG. 4C . Control of the switchers in  FIG. 5B  may be the same as the control of the switchers in  FIG. 3  with the internal battery  512  in the high-voltage battery state. 
       FIG. 5C  shows an exemplary charging system for a portable electronic device in accordance with the disclosed embodiments.  FIG. 5C  is substantially similar to the charging circuit of  FIG. 5A  with bidirectional switchers  518 - 520  connected at both low-voltage subsystems  504  and high-voltage subsystems  508 . Bidirectional switcher  520  may be implemented as a SIDO boost converter of  FIG. 4A . However, bidirectional switcher  518  is implemented as a SIDO buck-boost converter that was shown and described in reference to  FIG. 4B  while the rest of the components of  FIG. 5C  remain substantially the same as  FIG. 5A . It should be appreciated that the SIDO buck-boost converter of bidirectional switcher  518  may be operated as a SIDO boost converter by turning FET F 1  is ON and FET G 1  is OFF, and operating FET&#39;s A 1 , B 1 , C 1 , D 1  and E 1  the same as a SIDO boost converter would be operated, such as described in more detail below. In the charging systems of  FIGS. 5B and 5C , the bidirectional switcher may boost power from power port  508  to low-voltage subsystems  504  or buck power from power port  508  to low-voltage subsystems  504 . Therefore, the charging system of  FIGS. 5B-5C  may include additional modes at power port  508  such as accessory low voltage or power supply low-voltage in addition to the modes of charging system described above with respect to  FIG. 5A . 
     Bidirectional Switcher Operating Modes 
     The bidirectional switchers described here may be operated in a plurality of different modes depending on the connection state of the power ports and, in some instances, the battery state of the internal battery of the portable electronic device. In some instances, during operation of the charging system, a bidirectional switcher can be operated in one of a plurality of different modes, which depends primarily upon the mode of power ports and the voltage state of battery. Each bidirectional switcher mode determines the behaviors of the corresponding FETs (e.g., FETs A, B, C, D, and E in  FIG. 4A , or FETs A-G in  FIG. 4B ), which in turn control the current flowing through the corresponding inductor L. 
     For the purposes of discussion, positive current flow through an inductor of the bidirectional switchers  422 - 426  discussed above with respect to  FIGS. 4A-4C  is defined as flowing from low-voltage subsystems (voltage node V MAIN ) to the switching node V X  between the corresponding FETs B, C, and D. Positive current flow through FETs A and B is defined as current flowing from the switching node V x  to the corresponding power port (voltage node V BUS ). Positive current flow through FETs D and E is defined as current flowing from the switching node to high-voltage subsystems  506  (voltage node V BOOST ). Therefore, charging current flowing from each power port to low-voltage subsystems  504  is considered negative. 
     Operating modes of single bidirectional switchers are described below with respect to  FIGS. 6, 7A-7E, 8, 9A-9E, 10A-10F and 11A-11D . Exemplary modes discussed here include a disabled mode, a boost-accessory mode, a buck-boost accessory mode, a buck-boost main mode, a boost-internal mode, a buck mode, a buck accessory mode, a boost-main mode, a single-inductor dual-output (SIDO) mode, a SIDO buck-boost mode, a single-inductor sequential-control (SISC) mode, and a SISC boost-boost mode. Each switcher mode is described with respect to the bidirectional switchers of  FIGS. 4A and 4B . Control descriptions associated with the modes are based on current-mode control, where the outputs of the servo controllers determine the peak or valley inductor currents. Those skilled in the art will appreciate that other control techniques, such as slope compensation, adaptive dead-time control, voltage-mode control, and/or pulse-frequency modulation, may also be used with the modes. 
     Disabled Switcher Mode 
     In a disabled switcher mode, the bidirectional switcher is set to prevent power from being transferred through the bidirectional switcher. This may occur in instances where the power port connected to the bidirectional switcher is in an unplugged connection state.  FIG. 6  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 6  shows a disabled switcher mode for the bidirectional switcher  422  of  FIG. 4A , which is connected to an unplugged power port  608 , one or more low-voltage subsystems  604 , and one or more high-voltage subsystems  606 . In the disabled mode, all FETs A-E in the switcher are disabled, and no current can flow through the inductor L. Similarly, the bidirectional switcher  426  of  FIG. 4B  in a disabled switcher mode may have all FETs A-G in the bidirectional switcher  426  may be disabled and no current can flow through the inductor L. 
     Boost-Accessory Switcher Mode 
     A bidirectional switcher may be operated in a boost-accessory switcher mode in instances where a bidirectional switcher is used to take power from the V MAIN  voltage node and provide it to the power port connected to the bidirectional switcher. For example, the boost-accessory switcher mode may be used to provide power to the power port when the power port is in an external battery (when the external battery draws power from the portable electronic device) or external accessory connection state. More specifically,  FIG. 7A  shows a bidirectional switcher  422  of  FIG. 4A  in a boost-accessory mode. In the boost-accessory mode, the bidirectional switcher boosts power from one or more low-voltage subsystems  704  to a powered device  708  (e.g., an external accessory or an external battery) coupled to the corresponding power port. 
     The boost function in the boost-accessory mode is initiated by enabling FET C, which allows current to flow from low-voltage subsystems  704  through the inductor L. Once the inductor current reaches a given current i ACC,PK , FET C is disabled, and FET B is enabled to act as an ideal diode and allow the inductor current to flow to the powered device  708 , which may be connected at a higher voltage. The cycle of switching FETs C and B may be repeated (and in some instances may be repeated at a fixed frequency with period T S ). FET A may remain enabled to allow current to flow from the inductor to powered device  708  via power port, and FETs D and E may remain disabled to prevent current from flowing from the inductor to one or more high-voltage subsystems  706 . 
     In some instances, the transition from disabling FET C to enabling FET B occurs when the inductor current, measured either in series with the inductor or through FET C, reaches an adjustable i ACC,PK  current. The i ACC,PK  current threshold may be the output of a servo controller that simultaneously maintains the following limits:
 
V BUS ≤V ACC,MAX  
 
i BUS ≤i ACC,MAX  
 
V MAIN ≥V MAIN,MIN  
 
     In the above expressions, V ACC,MAX  is the maximum voltage target of powered device  708  and i ACC,MAX  is the maximum current target of powered device  708 . The desired boost target is to control V BUS  to V ACC,MAX , which must be higher than the V MAIN  voltage. To prevent powered device  708  from drawing too much current, the i BUS  current is limited to the settable i ACC,MAX  current, which allows the V BUS  voltage to droop if the current limit is reached. To prevent powered device  708  from browning out low-voltage subsystems  704 , the bidirectional switcher may also be controlled to prevent V MAIN  from drooping below V MAIN,MIN  even if the V BUS  and i BUS  targets are not met. 
       FIG. 7B  shows a set of graphs of inductor current for the different modes of controlling boost-accessory mode. During Stage I, as shown in the upper graph labeled Continuous Current Mode (CCM)  710 , FET C is enabled by a clock edge with period T S , allowing the linearly increasing current to flow from low-voltage subsystems  704  through the inductor. The transition from Stage I to Stage II occurs when the inductor current reaches the servo-controlled i ACC,PK  current. In Stage II, FET C is disabled and FET B is enabled, allowing the linearly decreasing inductor current to flow from the inductor to powered device  708  via the power port. When the next clock edge arrives, the cycle may be repeated. 
     To prevent the inductor current from exceeding the maximum allowed inductor current, the i ACC,PK  current is limited to a value represented by i PEAK,MAX . To prevent shoot-through current from flowing from powered device  708  to ground during the transitions between Stages I and II, both FET B and FET C may be simultaneously disabled before the stage transition continues. 
     To improve light-load efficiency, FET B can be designed to work as an active diode by not allowing current to flow in reverse from powered device  708  through the inductor. Such a mode is called Discontinuous Current Mode (DCM)  712 , with an inductor waveform shown in the middle graph of  FIG. 7B . When the inductor current reaches zero in Stage II, FET B may be turned off, and Stage III of DCM is entered. The inductor current in Stage II can be measured in series with the inductor or through FET B. 
     To further improve light-load efficiency, a cycle can be skipped when the i ACC,PK  current drops below a minimum i PEAK,MIN  current at the clock edge, which is labeled as Pulse Skipping Mode (PSM)  714  in the lower graph of  FIG. 7B . Since no power is provided to powered device  708 , the servo-controlled i ACC,PK  current will continue to increase until it eventually rises above the i PEAK,MIN  current threshold and triggers another cycle. 
     FET A may be disabled in Stage I, Stage III, and the transition between Stage III and Stage I, since no current flows to the power port during these stages. FET D may be enabled in Stage I, since no current flows to high-voltage subsystems  706  with FET E disabled and FET C enabled. 
       FIG. 7C  shows a bidirectional switcher that is operated in a boost-accessory switcher mode in accordance with the disclosed embodiments. Particularly,  FIG. 7C  shows a bidirectional switcher with a SIDO buck-boost bidirectional converter of  FIG. 4B  that may be operated in a boost-accessory switcher mode in instances where a bidirectional switcher is used to take power from the V MAIN  voltage node and provide it to powered device  708  at the power port connected to the bidirectional switcher of  FIG. 7C . For example, the boost-accessory switcher mode may be used to provide power to the powered device  708  when the power port is in an external battery or external accessory connection state. In the boost-accessory mode, the bidirectional switcher of  FIG. 7C  boosts power from one or more low-voltage subsystems  704  to powered device  708  (e.g., an external accessory or an external battery) coupled to the corresponding power port. The charging system of  FIG. 7C  is substantially the same as the charging system of  FIG. 7A  but includes additional FETs F and G that are operated in the boost-accessory switcher mode, while all other features remain substantially the same as the boost-accessory switcher mode of  FIG. 7A . In the boost-accessory switcher mode of  FIG. 7C , FET F is enabled (i.e., FET F is ON) and FET G is disabled (i.e., FET G is OFF) while FET&#39;s A, B, C, D and E may be operated as was just described in the embodiment of  FIG. 7A . Additionally, during operation of the boost-accessory switcher mode, FETs D and E may be controlled to be both OFF at the same time, while FET B may operate as an ideal diode allowing current to pass to, but not from, the powered device  708  connected to power port. By operating FET B as an ideal diode, the removal of a powered device  708  at the power port  708  can be detected as a drop in the V BUS  voltage. While an internal battery is not shown in  FIG. 7A or 7C , it should be appreciated than internal battery may provide power to the voltage node V MAIN  during a boost-accessory switcher mode. 
     Buck-Boost Accessory Switcher Mode 
     A bidirectional switcher may be operated in a buck-boost accessory switcher mode in instances where a bidirectional switcher is used to take power from the V MAIN  voltage node and provide it to the power port connected to the bidirectional switcher, but when V BUS  and V MAIN  are close in voltage levels and could pass over each other (e.g., voltage at powered device  708  is near the voltage at V MAIN ). For example, the buck-boost switcher mode may be used to either buck power or boost power to the power port when the power port is an external battery (when the external battery draws power from the portable electronic device) or external accessory connection state.  FIG. 7D  shows a charging system with a SIDO buck-boost converter of  FIG. 4B  that may be operated in a buck-boost accessory mode. In the buck-boost accessory mode, the bidirectional switcher of  FIG. 7D  may buck power from one or more low-voltage subsystems  704  to a powered device  708  (e.g., a powered device or an external accessory) coupled to the corresponding power port or may boost power from one or more low-voltage subsystems  704  to a powered device  708  coupled to the corresponding power port, such as for charging an external battery connected to the power port. The buck-boost accessory mode facilitates bucking and boosting from low-voltage subsystem  704  depending on which of voltages V BUS  and V MAIN  are higher. 
     The buck-boost functionality may be initiated by enabling FET&#39;s F and C, while disabling all other FETs. FET&#39;s D and E are disabled to remove high-voltage subsystem  706  from the charging system. Enabling FET&#39;s F and C allows current to flow from low-voltage subsystems  704  through the inductor L. Once the inductor current reaches a given current i ACC,PK , FET&#39;s G, A and B are enabled while other FET&#39;s are maintained in a disabled state to allow inductor current to flow to powered device  708  at power port. FET B may act as an ideal diode to allow the inductor current to flow to the powered device  708  at power port. The buck-boost accessory mode may allow for a smoother transition of currents between the low-voltage subsystems  704  and an external battery connected to powered device  708 . 
     Buck-Boost Main Mode 
     A bidirectional switcher may be operated in a buck-boost main mode in instances where a bidirectional switcher is used to take power from power source connected to a power port at node V BUS  and provide power to the V MAIN  voltage node, but when V BUS  and V MAIN  are close in voltage levels and could pass over each other. For example, a power source  710  such as an external battery may be used to provide power to the V MAIN  voltage node.  FIG. 7E  shows a charging system with the bidirectional switcher  424  of  FIG. 4B  operating in a buck-boost main mode. In this buck-boost main mode, the bidirectional switcher of  FIG. 7E  boosts power from power source  710  (e.g., an external battery coupled to a power port) to one or more low-voltage subsystems  704 . The buck-boost main mode facilitates boosting power from a power source  710  at the power port when voltages of V BUS  and V MAIN  are close in voltage levels and could pass over each other. 
     The buck-boost main functionality may be initiated by enabling FET&#39;s G and A, while disabling all other FETs. For example, FET&#39;s D and E may be disabled to remove high-voltage subsystem  706  from the charging system. Enabling FET&#39;s G and A allows current to flow from an external battery connected to power source  710  to inductor L and charge the inductor L. Once the inductor current reaches a given current i ACC,PK , FET&#39;s F and C may be enabled while other FET&#39;s are disabled to allow inductor current from inductor L to flow into low-voltage subsystem  704 . FET C may act as an ideal diode to allow the inductor current from inductor L to flow to the low-voltage subsystem  704 . The buck-boost main mode implementing the boost function may allow for a smoother transition of currents between the low-voltage subsystems  704  and power source  710  (e.g., an external battery) connected to the power port. 
     Boost-Internal Switcher Mode 
     A bidirectional switcher may be operated in a boost-internal switcher mode in instances where a bidirectional switcher is used to take power from the V MAIN  voltage node and provide it to the power port connected to the bidirectional switcher.  FIG. 8  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 8  shows the bidirectional switcher  422  of  FIG. 4A  in a boost-internal mode. In the boost-internal mode, the bidirectional switcher boosts power from one or more low-voltage subsystems  804  to one or more high-voltage subsystems  806  with no power coming in or out of an unplugged power port  808 . 
     The boost function of the boost-internal mode may be initiated by enabling FET C, which allows current to flow from low-voltage subsystems  804  through the inductor L. Once the inductor current reaches a target current i BOOST,PK , FET C is disabled, and FET E is enabled to act as an ideal diode and allow current to flow to high-voltage subsystems  806  at a higher voltage. The cycle of switching FETs C and E may be repeated (and in some instances may be repeated at a fixed frequency with period T S ). FET D may remain enabled to allow current to flow from the inductor to high-voltage subsystems  806 , and FETs A and B may remain disabled to prevent current from entering or leaving unplugged power port  808 . 
     The transition from disabling FET C to enabling FET E occurs when the inductor current, measured either in series with the inductor or through FET C, reaches an adjustable i BOOST,PK  current. The i BOOST,PK  current threshold is the output of a servo controller that simultaneously maintains the following limits:
 
V BOOST ≤V BOOST,TGT  
 
V MAIN ≥V MAIN,MIN  
 
In the above expressions, V BOOST,TGT  is the target voltage of high-voltage subsystems  806 , and the V MAIN  limit prevents a high load on high-voltage subsystems  806  from browning out low-voltage subsystems  804  by drooping the V BOOST  voltage if necessary.
 
     The inductor current behavior and FET control in the boost-internal mode may be the same as in the boost-accessory mode, except that FET E is enabled instead of FET B in Stage II, and the inductor current for detecting the DCM zero-current threshold in the transition to Stage III can be measured through FET E instead of FET B. FET D may be disabled in Stage III and the transition between Stage II and Stage I, since no current flows to high-voltage subsystems  806  during these stages. 
     It should be appreciated that the bidirectional switcher of  FIG. 8  may be implemented with the SIDO buck-boost converter of  FIG. 4B  by turning FET F ON and FET G OFF, and operating FET&#39;s A, B, C, D and E the same as the bidirectional switcher  422  of  FIG. 4A  would be operated in the boost-internal mode such as described in more detail above in  FIG. 8 . 
     Buck Switcher Mode 
     A bidirectional switcher may be operated in a buck switcher mode in instances where the bidirectional switcher is used to take power from a power source (e.g., a power supply or an external battery) connected to a power port and provide a voltage to V MAIN  voltage node via the bidirectional switcher.  FIG. 9A  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 9A  shows the bidirectional switcher  422  of  FIG. 4A  operating in a buck switcher mode. In this mode, the bidirectional switcher bucks power from a power source  908  to one or more low-voltage subsystems  904 . Power source  908  may be a power supply (e.g., a power adapter) or external battery coupled to a power port to which the bidirectional switcher is connected. 
     In the buck-switcher mode, FET A operates as an ideal diode allowing current to pass from, but not to, the power port. By operating FET A as an ideal diode, the removal of power source  908  at the power port can be detected as a drop in the V BUS  voltage. FET A is controlled by the measured bidirectional i BUS  current. If the i BUS  current is negative and below a fixed threshold, indicating that current is flowing in from the power port, then FET A is enabled. FET A is disabled otherwise, and FETs D and E are always disabled to prevent current from flowing from the inductor to one or more high-voltage subsystems  906 . 
     The buck function of the buck-switcher mode is initiated by enabling FET B, which allows current to flow from the power port through the inductor L to low-voltage subsystems  904  at a lower voltage. Once the inductor current reaches a target (e.g., a servo-controlled) i VALLEY  current, FET B is disabled and FET C, acting as an ideal diode, is then enabled to allow current to flow from the ground side of FET C through the inductor to low-voltage subsystems  904 . The cycle of switching FETs B and C may be repeated (and in some instances may be repeated at a fixed frequency with period T S ). 
     The transition from disabling FET B to enabling FET C occurs when the inductor current, measured in series with the inductor or through FET B, reaches an adjustable i VALLEY  current, where current flowing from the power port to the inductor is defined as negative. The i VALLEY  current threshold is the output of a servo controller that depends on one of four buck mode sub-states: charge, no-charge, primary and secondary. In the charge sub-state, the buck mode controls the voltage and current to the internal battery. The bidirectional switcher charges the battery with a servo controller that adjusts i VALLEY  to simultaneously maintain the following limits:
 
V BAT ≤V BAT,MAX  
 
i CHG ≤i CHG,MAX  
 
     In the above expressions, V BAT,MAX  is the maximum voltage limit of the internal battery, and i CHG,MAX  is the maximum current limit of the internal battery. 
     In the no-charge sub-state, the buck mode may control the voltage to low-voltage subsystems  904 , or V MAIN . FET G acts as an ideal diode to prevent charging of the battery, and i VALLEY  is adjusted to maintain the following limit:
 
 V   MAIN   ≤V   BAT   +V   BAT,OFF  
 
In the above expression, V BAT,OFF  is a voltage offset to be maintained above the measured battery voltage V BAT .
 
     In the primary sub-state, the buck mode pulls as much power as possible from the power port that can be used only with the charge and no-charge sub-states. The primary sub-state occurs only when two power sources are available (e.g., when coupled to both power ports) and the other power source is in the charge or no-charge sub-state. The aim of the primary sub-state is to pull as much power as possible from the attached power source  908  without directly controlling V MAIN , V BAT , or i CHG , which are controlled by the other bidirectional switcher in the buck mode. The power draw of the other bidirectional switcher is minimized by adjusting the i VALLEY  current to maintain the following limit:
 
i L,PRIMARY ≥i L,BUCK  
 
In the above expression, i L,PRIMARY  is the average inductor current for the bidirectional switcher in the primary sub-state, and i L,BUCK  is the average inductor current of the other bidirectional switcher in the charge or no-charge sub-state.
 
     In the secondary sub-state, the buck mode balances the current with the charge or no-charge sub-states. Like the primary sub-state, the secondary sub-state occurs only when there are two power sources, and the other power source is in the charge or no-charge sub-state. The aim of the secondary sub-state is to not directly control V MAIN , V BAT , or i CHG , but to balance the current load of the other bidirectional switcher by adjusting the i VALLEY  current to maintain the following limit:
 
i L,SECONDARY ≤i L,BUCK  
 
     In the above expression, i L,SECONDARY  is the average inductor current for the bidirectional switcher in the secondary sub-state, and i L,BUCK  is the average inductor current of the other bidirectional switcher in the charge or no-charge sub-state. For both the primary and secondary sub-states, instead of measuring the average inductor current, the previously generated i VALLEY  current thresholds can be used to indicate the current levels to control. 
     Regardless of the sub-state in the buck mode, the servo controller must also adjust i VALLEY  to keep the following additional limits within control:
 
 V   BUS   ≥V   MAIN   +V   BUS,OFF  
 
 i   BUS   ≥−i   BUS,MAX  
 
In the above expressions, V MAIN +V BUS,OFF  is the minimum allowed voltage to which the input voltage V BUS  is allowed to droop, and i BUS,MAX  is the maximum allowed current to be pulled from the power port. Since the i BUS  current flowing from the power port is defined as negative, the i BUS  current is compared to the negative of the positive i BUS,MAX  limit.
 
       FIG. 9B  shows a set of graphs of inductor current in the buck mode. During Stage IV, as shown in the upper graph of CCM  910 , FET B is enabled by a clock edge with period T S , allowing the linearly decreasing current to flow from the power port to low-voltage subsystems  904  through the inductor. The transition from Stage IV to Stage V occurs when the inductor current reaches the servo-controlled i VALLEY  current. In Stage V, FET B is disabled, and FET C is enabled to allow the linearly increasing inductor current to flow from the ground side of FET C to low-voltage subsystems  904 . When the next clock edge arrives, the cycle is repeated. 
     To prevent the inductor current from exceeding the maximum allowed inductor current, the i VALLEY  current, which is negative, is limited to a value less negative than −i PEAK,MAX . To prevent shoot-through current from flowing from the power port to ground during the transitions between Stage IV and Stage V, both FET B and FET C may be simultaneously disabled before the stage transition continues. 
     To improve light-load efficiency, FET C can be designed to work as an active diode by not allowing current to flow in reverse from the inductor to ground, as shown in the DCM  912  inductor current waveform in the middle graph of  FIG. 9B . In Stage V, when the inductor current reaches zero, FET C is turned off, entering Stage VI. The inductor current in Stage V can be measured in series with the inductor or through FET C. 
     To further improve light-load efficiency, a cycle can be skipped when the i VALLEY  current rises above −i PEAK-MIN  at the clock edge, as shown in the lower graph of PSM  914  in  FIG. 9B . Since no power is provided to low-voltage subsystems  904 , the servo-controlled i VALLEY  current will continue to decrease until it is eventually more negative than −i PEAK-MIN , which triggers another cycle. 
     A bidirectional switcher may be operated in a buck switcher in instances where a bidirectional switcher is used to take power from power source connected to a power port at node V BUS  and provide power to the V MAIN  voltage node. For example, a power source  908  such as an external battery may be used to provide power to the V MAIN  voltage node.  FIG. 9C  shows a bidirectional switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 9C  shows a charging system with the bidirectional switcher  424  of  FIG. 4B  in a buck switcher mode. In this mode, the bidirectional switcher of  FIG. 9C  may buck power from a power source  908  connected to a power port to one or more low-voltage subsystems  904 . Power source  908  may be a power supply (e.g., a power adapter) or external battery coupled to a power port to which the bidirectional switcher  424  of  FIG. 4B  is connected. The charging system of  FIG. 9C  may be substantially the same as the charging system of  FIG. 9A  but includes additional FETs F and G in a buck switcher mode. In the buck switcher mode of  FIG. 9C , FET F is ON and FET G is OFF, while FET&#39;s A, B, C, D and E may be operated as was just described in the embodiment of single switcher mode of  FIG. 9A . Additionally, during single switcher mode operation, FETs D and E may be controlled to be both OFF at the same time, while FET A operates as an ideal diode allowing current to pass from, but not to, the low-voltage subsystems  904 . By operating FET A as an ideal diode, the removal of an accessory (e.g. a power source  908 ) at a power port to which the bidirectional switcher is connected can be detected as a drop in the V BUS  voltage. 
     Buck Accessory Mode 
     A bidirectional switcher may be operated in a buck accessory mode in instances where a bidirectional switcher is used to take power from a V MAIN  voltage node and provide power to a powered device connected to a power port at node V BUS . For example, a powered device  916  such as an accessory or an external battery may receive power from the V MAIN  voltage node.  FIG. 9D  shows a bidirectional switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 9D  shows a charging system with the bidirectional switcher  424  of  FIG. 4B  in the buck accessory mode. In this mode, the charging system may buck power from one or more low-voltage subsystems  904  to a low-voltage powered device  916  (e.g., accessory, charging external battery, etc.) connected to a power port. Also, in the buck accessory mode, voltage of V BUS  (at powered device  916 ) is at a lower voltage than voltage of V MAIN  (at low-voltage subsystems  904 ). 
     The buck accessory mode of  FIG. 9D  may be initiated by enabling FET F, A and B (i.e., turning ON) and disabling FETs C, D, E and G (i.e., turning OFF). Enabling FETs F, A and B, allows current to flow from low voltage subsystems  904  through the inductor L to powered device  916 . Disabling FET&#39;s D and E disconnects high-voltage subsystems  906  from node V x . 
     Once the current in inductor L reaches an i VALLEY  current limit, in a second stage, FET F is disabled and FET G is enabled to disconnect low-voltage subsystems  904  from the inductor L and FET C is maintained in a disabled state. FET&#39;s F and G may be switched repeatedly to buck power from low-voltage subsystem  904  to powered device  916 . The cycle of switching FETs F and G may be repeated (and in some instances may be repeated at a fixed frequency with period T S ). FET G may operate as an ideal diode when FET F is OFF by allowing current to flow unimpeded from FET G to the inductor L. 
     Boost Main Mode 
     A bidirectional switcher may be operated in a boost main mode in instances where a bidirectional switcher is used to take power from a power source connected to a power port at node V BUS  and provide power to the V MAIN  voltage node. For example, a power source  908  such as an external battery may be used to provide power to the V MAIN  voltage node.  FIG. 9E  shows a bidirectional switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 9E  shows a charging system with the bidirectional switcher  424  of  FIG. 4B  in a boost main mode. In this boost main mode, the bidirectional switcher of  FIG. 9E  may boost power from a low-voltage power source  908  (e.g., external battery, etc.) connected to a power port to one or more low-voltage subsystems  904 . In addition, in the boost main mode, a voltage of V BUS  (at power source  908 ) is at a lower voltage than voltage of V MAIN  at low-voltage subsystems  904 . 
     The boost main mode of  FIG. 9E  may be initiated by enabling FET&#39;s A, B and G (i.e., turning ON the FETs) and disabling FET&#39;s F, C, D and E (i.e., turning OFF the FETs). Enabling FET&#39;s A, B and G allows current to flow from power source  908  to inductor L, disabling FET&#39;s F and C disconnects low voltage subsystems  904  from node V x  and inductor L, and disabling FET&#39;s D and E disconnects high-voltage subsystems  906  from inductor L. FET A may operate as an ideal diode allowing current to pass from, but not to, the power source  908 . By operating FET A as an ideal diode, the addition of an accessory at the power source  908  can be detected as a rise in the V BUS  voltage. 
     Once the current in inductor L reaches a current limit, FET G is disabled and FET F is enabled while FET C is continuously maintained OFF. Enabling FET F and disabling FET G connects low-voltage subsystems  904  to the inductor L to allow current from the inductor L to flow to low-voltage subsystems  904 . The cycle of switching FETs F and G may be repeated (and in some instances may be repeated at a fixed frequency with period T S ). FET F operates as an ideal diode to allow current to flow to low voltage subsystems  904  from the inductor L. FET A may also be an ideal diode so that power may not flow to power source  908  during the boost main mode. 
     SIDO Switcher Mode 
     A bidirectional switcher may be operated in a SIDO switcher mode in instances where the bidirectional switcher may be used to take power from the V MAIN  voltage node and provide power to both an external device connected to a corresponding power port and the high-voltage subsystems.  FIG. 10A  shows a single bidirectional switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 10A  shows the bidirectional switcher  422  of  FIG. 4A  in the SIDO switcher mode. In the SIDO switcher mode, the bidirectional switcher boosts power from one or more low-voltage subsystems  1004  to one or more high-voltage subsystems  1006  and a powered device  1008  (e.g., accessory, charging external battery, etc.) coupled to Power Port to which the bidirectional switcher is connected. 
     The SIDO switcher mode may require that the target voltage of high-voltage subsystems  1006  be less than the target voltage of powered device  1008 :
 
V BOOST,TGT ≤V ACC,MAX  
 
To prevent cross-contamination between the two outputs (e.g., high-voltage subsystems  1006  and powered device  1008 ), power is delivered to the lower-voltage rail of high-voltage subsystems  1006  first, followed by the higher-voltage rail of powered device  1008 . If the voltage target levels are reversed, the same procedure may be followed with the order reversed.
 
     The SIDO switcher mode has two sub-states: boost, which prioritizes power to high-voltage subsystems  1006 , and accessory, which prioritizes power to accessory  1008 . The SIDO function is initiated by enabling FET C, which allows current to flow from low-voltage subsystems  1004  through the inductor L. 
     For the SIDO switcher mode to simultaneously control two outputs, two servo controllers may be required. The first servo controller controls the peak current for the boosted subsystems, or i INTRNL,PK  (or i BOOST,PK ), and the second servo controller controls the peak current for accessory at powered device  1008 , or i ACC,PK . 
     The i INTRNL,PK  current threshold is the output of a servo controller that simultaneously maintains the following limits, which are the same as those for the servo controller of the boost-internal mode:
 
V BOOST ≤V BOOST,TGT  
 
V MAIN ≥V MAIN,MIN  
 
To prevent the inductor current from exceeding the maximum allowed inductor current, the i BOOST,PK  current is limited to i PEAK,MAX .
 
     The i ACC,PK  current threshold is the output of a servo controller that simultaneously maintains the following limits, which are the same as those for the servo controller of the boost-accessory mode:
 
V BUS ≤V ACC,MAX  
 
i BUS ≤i ACC,MAX  
 
V MAIN ≥V MAIN,MIN  
 
To prevent the inductor current from exceeding the maximum allowed inductor current, the i ACC,PK  current is limited to i PEAK,MAX .
 
     Once the current reaches the sum of the servoed i ACC,PK  current limit and the servoed i INTRNL,PK  current limit, or i TOTAL,PK , FET C is disabled, and FET E is enabled to allow current to flow from low-voltage subsystems  1004  to high-voltage subsystems  1006  at a higher voltage. FET E operates as an ideal diode by allowing current to flow unimpeded to high-voltage subsystems  1006  from the inductor L. The i TOTAL,PK  current limit is the sum of the i ACC,PK  current threshold and the i INTRNL,PK  current threshold:
 
 i   TOTAL,PK =min( i   INTRNL,PK   +i   ACC,PK   ,i   PEAK,MAX )
 
where the i TOTAL,PK  current is limited to i PEAK,MAX  to prevent the inductor current from exceeding the maximum allowed inductor current.
 
     When the current level drops to a servoed i PARTIAL,PK  current, FET E is disabled, and FET B is enabled to allow current to flow from low-voltage subsystems  1004  to powered device  1008  at a higher voltage. The cycle of switching FETs C, E and B may be repeated (and in some instances may be repeated at a fixed frequency with period T S ). FETs A and D are enabled during certain stages to allow current to flow to either the powered device  1008  or high-voltage subsystems  1006 , as described below. 
     Depending upon the port configuration (e.g., information received from the external device), the system can decide to prioritize the SIDO power to either the power port  1008  or high-voltage subsystems  1006 . To prioritize the power to an accessory at powered device  1008  over high-voltage subsystems  1006  in the accessory sub-state, the i PARTIAL,PK  current is set to the i ACC,PK  current:
 
i PARTIAL,PK =i ACC,PK  
 
     To prioritize the power to high-voltage subsystems  1006  over accessory at powered device  1008  in the boost sub-state, the i PARTIAL,PK  current is set to the total peak current minus the i INTRNL,PK  current:
 
 i   PARTIAL,PK   =i   TOTAL,PK   −i   INTRNL,PK  
 
If the total peak current limit i TOTAL,PK  is not limited by i PEAK,MAX , the i PARTIAL,PK  current is equal to the i ACC,PK  current in both sub-states, where prioritization does not matter since both powered device  1008  and high-voltage subsystems  1006  are provided with their needed power. The mathematics used to calculate the partial and total peak current thresholds can be implemented in analog circuitry and are shown using the block diagram of  FIG. 10B .
 
       FIG. 10C  shows a set of graphs of inductor current in the SIDO switcher mode. During Stage I, as shown in the upper CCM  1010  graph, FET C is enabled by a clock edge with period T S , allowing the linearly increasing current to flow from low-voltage subsystems  1004  through the inductor L. FET D may also be enabled in Stage I, although no current would flow because FET E is disabled. The transition from Stage I to Stage IIa occurs when the inductor current reaches the servo-controlled i TOTAL,PK  current threshold. In Stage IIa, FET C is disabled, and FET E is enabled to allow the linearly decreasing inductor current to flow from the inductor to high-voltage subsystems  1006 . To prevent shoot-through current from flowing from high-voltage subsystems  1006  to ground during the transition from Stage I to Stage IIa, FET C is disabled before FET E is enabled. FET D continues to be enabled in Stage IIa. FET A is also enabled in Stage IIa, although no current would flow because FET B is disabled, and the power port voltage at powered device  1008  is higher than the voltage of high-voltage subsystems  1006 . The transition from Stage IIa to Stage IIb occurs when the inductor current reaches the servo-controlled i PARTIAL,PK  current. 
     In Stage IIb, FET D and FET E are disabled, and FET B is enabled to allow the decreasing inductor current to flow from the inductor to accessory at powered device  1008 . To prevent shoot-through current from flowing from high-voltage subsystems  1006  to powered device  1008  during the transition from Stage IIa to Stage IIb, FET D and FET E are disabled before FET B is enabled. FET A continues to be enabled in Stage IIb. The transition from Stage IIb to Stage I occurs on the next clock edge, which repeats the entire cycle. To prevent shoot-through current from flowing from powered device  1008  to ground during the transition from Stage IIb to Stage I, FET A and FET B are disabled before FET C and FET D are enabled. 
     To improve light load efficiency, FET B can be designed to work as an active diode by not allowing current to flow in reverse from powered device  1008  to the switching node, as shown in the middle graph of DCM  1012  inductor current in  FIG. 10C . When the inductor current reaches zero in Stage IIb, FET B and FET A are turned off, and Stage III is entered. The inductor current in Stage IIb can be measured either in series with the inductor or through FET B. 
     To further improve light-load efficiency, a cycle can be skipped when the i TOTAL,PK  current drops below a minimum i PEAK,MIN  current at the clock edge, as shown in the lower graph of PSM  1014 . Since no power is provided to either accessory at powered device  1008  or high-voltage subsystems  1006 , the servo-controlled i INTRNL,PK  and i ACC,PK  current levels will continue to increase until the i TOTAL,PK  level eventually rises above the i PEAK,MIN  current and triggers another cycle. 
     Note that the SIDO switcher mode behaves identically to the boost-accessory mode if the i INTRNL,PK  current is zero. Consequently, the SIDO switcher mode can realize the boost-accessory mode by forcing the i INTRNL,PK  current to zero. 
     To realize a boost-internal mode with a SIDO implementation, two changes are required to prevent unwanted current flow, since the power port voltage may no longer be higher than the voltage of high-voltage subsystems  1006 . First, the i ACC,PK  current is forced to zero to prevent controlled current from flowing to the power port. Second, FET A must be disabled in Stage IIa to prevent current from flowing from high-voltage subsystems  1006  to the powered device  1008  through the disabled FET B. In the standard SIDO switcher mode, FET A is enabled in Stage IIa, but no current flows because the voltage on the power port is higher than the voltage of high-voltage subsystems  1006  at the switching node, and no current can flow through the body diode of the disabled FET B. 
     At the expense of available current and bandwidth and the avoidance of CCM, the SIDO switcher mode could be implemented by alternating between the boost-internal and boost-accessory modes in DCM-only mode. 
     SIDO Buck-Boost Mode 
     A bidirectional switcher may be operated in a SIDO buck-boost switcher mode in instances where a bidirectional switcher may be used to take power from a V MAIN  voltage node connected to one or more low voltage subsystems and provide power to a voltage node connected to V BOOST  and to a powered device at a voltage node connected to V BUS , where a target voltage of V BOOST  may be greater than the target voltage of V MAIN  and voltage V BUS  may be higher or lower than voltage V MAIN . For example, a powered device  1008  such as an accessory or an external battery may receive power at the V BUS  node.  FIG. 10D  shows a single bidirectional switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 10D  shows a charging system with the bidirectional switcher  424  of  FIG. 4B  in the SIDO buck-boost mode. In the SIDO buck-boost mode, the charging system may boost power from V MAIN  connected to one or more low-voltage subsystems  1004  to V BOOST  connected to one or more high-voltage subsystems  1006  and buck or boost power from one or more low-voltage subsystems  1004  to V BUS  connected to a low-voltage external powered device  1008  (e.g., accessory, charging external battery, etc.) connected to V BUS  at a power port. More specifically, in the SIDO buck-boost mode, the charging system of  FIG. 10D  may be controlled to perform one of the following: boost power from one or more low-voltage subsystems  1004  to one or more high-voltage subsystems  1006 , boost power from one or more low-voltage subsystems  1004  to a high-voltage external powered device (e.g., accessory, charging external battery, etc.) coupled to the power port  1008  to which the bidirectional switcher is connected, buck power from one or more low-voltage subsystems  1004  to the low-voltage accessory coupled to the power port  1008  to which the bidirectional switcher is connected, boost power from an external battery coupled to the power port  1008  to one or more low-voltage subsystems  1004  coupled to the power port  1008  to which the bidirectional switcher is connected and buck power from an external battery coupled to the power port  1008  to one or more low-voltage subsystems  1004  coupled to the power port  1008  if the external battery is capable of providing a higher voltage than the low-voltage subsystems  1004 . 
     The control scheme for the SIDO buck-boost mode may be operated in a boost sub-state, which prioritizes power, first, to high-voltage subsystems  1006 ; and, second, to accessory, which prioritizes power to power port  1008 . The SIDO buck-boost mode of  FIG. 10D  is initiated in a first stage (stage I) by enabling FET F and C and disabling FET&#39;s A, B, D, E and G (i.e., turning OFF). Enabling FET&#39;s F and C allows current to flow from low voltage subsystems  1004  through the inductor L. Disabling FET&#39;s A, B, D, E and G disconnects high-voltage subsystems  1006  and power port  1008  from receiving current from the low-voltage subsystems  1004 . 
     Once the current in inductor L reaches a current limit, in a second stage (stage II), FET&#39;s F and C are disabled to disconnect low-voltage subsystems  1004  from charging the inductor L, FET&#39;s A and B are maintained in a disabled state to disconnect power port  1008  from the inductor L, and FET&#39;s G, D and E are enabled to allow current from the inductor L to flow to high-voltage subsystems  1006  at a higher voltage. FET E operates as an ideal diode by allowing current to flow unimpeded to high-voltage subsystems  1006  from the inductor L and does not allow current to flow in a reverse direction from the high-voltage subsystems  1006 . 
     When the current in inductor L drops to a current limit, in a third stage (stage III), FET&#39;s D and E are disabled to disconnect high-voltage subsystems  1006  from the inductor L, FET&#39;s C and F are maintained in a disabled state to disconnect low-voltage subsystems  1004  from charging inductor L, and FET&#39;s A, B and G are enabled to allow current from inductor L to flow to power port  1008 . FET B may operate as an ideal diode by allowing current to flow unimpeded to power port  1008  from the inductor L and does not allow current to flow back from power port  1008  to the low-voltage subsystems  1004 . 
     The cycle of switching for stages I, II and III is repeated at a fixed frequency with a predefined period until target voltages are achieved. 
       FIG. 10E  shows a block diagram of a set of calculations associated with a single bidirectional switcher  424  of  FIG. 4B  for a charging system operated in the SIDO buck-boost mode in accordance with the disclosed embodiments. As shown in  FIG. 10E , V Out   _   1,SetPoint  is the reference boost voltage V BOOST   _   setpoint  that is being controlled and V OUT   _   2,SetPoint  is the reference power port accessory voltage V ACC   _   setpoint  of power port  408 . Initially, error signals  1020  and  1022  for boost voltage, V BOOST , and accessory voltage, V ACC  respectively, may be input into respective controllers. In one non-limiting example, integrators  1024 ,  1026  may be implemented as the controllers that integrate the respective error signals  1020 ,  1022 . Current outputs I V   _   OUT   _   1  and I V   _   OUT   _   2  are received at a Summer block  1028  and I V   _   OUT   _   1  is received at a difference block  1030 . The current output  1032  of summer block  1028  is the total current I TOTAL,PK  current of the inductor that is being determined. Current Output  1032  and the maximum peak inductor current I MAX,PK  are inputs to a minimum block that outputs the minimum value between the i MAX,BK  and output  1032 . Output  1034  of minimum block is input to a difference block  1030  to subtract out current I V   _   OUT   _   1  to power port. If the total peak current limit I TOTAL,PK  is not limited by I MAX   _   PK , then I TOTAL   _   PK  is the sum and I PARTIAL   _   PK  will be I V   _   OUT   _   2 . I PARTIAL   _   PK  will not be equal to I V   _   OUT   _   2  if the inductor current limit has been reached. This determines the prioritization of power that can be delivered to the high-voltage subsystem and the power port. If there isn&#39;t enough power from the inductor, then the lower priority port, V BUS , is starved first in order to feed power to the higher priority port, the high-voltage subsystem at voltage rail, V BOOST . 
       FIG. 10F  shows a generalized block diagram of a set of calculations associated with multiple bidirectional switchers similar to bidirectional switcher  424  of  FIG. 4B  for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 10F  shows a set of calculations for n outputs, V OUT   _   1 , V OUT   _   2 , V OUT   _   n  that represent power that is delivered to one of power ports, low-voltage subsystem, or high-voltage subsystems by controlling the SIDO buck-boost converter of  FIG. 4B . The block diagram of  FIG. 10F  may be used to determine priority for delivering inductor currents to each port until there is no energy left in the inductor using the SIDO buck-boost mode to control FETs. The higher the number of the output voltage (e.g., V OUT1 , V OUT2 , V OUTN ), the lower the priority. In  FIG. 10F , block  1040  represents several prioritized current level calculations that may follow the computation depicted in block  1036  of  FIG. 10E . Block  1042  represents a buck-boost state machine for multiple outputs that includes logic for controlling FET&#39;s for multiple SIDO buck-boost switcher modes that may be implemented similar to  FIG. 4B  in the buck-boost mode. 
     In one embodiment, the bidirectional switcher of  FIG. 10D  may also be operated in a SIDO buck-boost switcher where the bidirectional switcher may be used to take power from an external battery or an external power supply connected to a power port at voltage node V BUS  and provide power to one or more low-voltage subsystems connected to a voltage node at V MAIN  and provide power to one or more high-voltage subsystem connected to V BOOST , where a voltage of V BUS  may be higher than voltage of V MAIN . For example, in the SIDO buck-boost mode, the charging system may buck power from an external battery connected to a power port at the V BUS  node to one or more low-voltage subsystems  1004  connected to V MAIN  and boost power to one or more high-voltage subsystems  1006 . 
     The SIDO buck-boost mode, in this embodiment, may be initiated in a first stage (stage I) by enabling FET A, B and G (i.e., turning ON) and disabling FET&#39;s F, C, D and E (i.e., turning OFF). Enabling FET&#39;s A, B and G allows current to flow from external battery at powered device  1008  through the inductor L so as to charge up the inductor L. Disabling FET&#39;s E, C, D and F disconnects high-voltage subsystems  1006  and low-voltage subsystem  1004  from receiving current from the external battery at the power port (labeled as powered device  1008 ). 
     Once the current in inductor L reaches a current limit, in a second stage (stage II), FET&#39;s A, B and G are disabled to disconnect external battery and FET&#39;s C, D and E are enabled to allow current from the inductor L to flow to high-voltage subsystems  1006  at a higher voltage (which is prioritized). FET E operates as an ideal diode by allowing current to flow unimpeded to high-voltage subsystems  1006  from the inductor L and does not allow current to flow in a reverse direction from the high-voltage subsystems  1006 . 
     When the current in inductor L drops to a current limit, in a third stage (stage III), keeping C enabled, FET&#39;s D and E are disabled and FET F is enabled to disconnect high-voltage subsystems  1006  from the inductor L and deliver power to the low-voltage subsystems  1004  from inductor L 
     SISC Switcher Mode 
     A bidirectional switcher may be operated in a SISC switcher mode in instances where it is desirable to provide power received from a power port to both the low- and high-voltage subsystems of a portable electronic device.  FIG. 11A  shows a single switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 11A  shows the bidirectional switcher  422  of  FIG. 4A  in a SISC switcher mode. In the SISC switcher mode, the bidirectional switcher switches between up-converting (e.g., boosting) power from one or more low-voltage subsystems  1104  to one or more high-voltage subsystems  1106  and down-converting (e.g., bucking) the input voltage from a power source  1108  (e.g., power supply, external battery, etc.) coupled to the power port to which the bidirectional switcher  422  is connected to low-voltage subsystems  1104 . Unlike the SIDO switcher mode, the SISC switcher mode cannot simultaneously buck and boost for the bidirectional switcher shown in  FIGS. 10A and 10D  and so must sequentially choose whether to boost power to high-voltage subsystems  1106  or buck power from the power port to low-voltage subsystems  1106 . 
     The SISC switcher mode may be implemented by switching between the boost-internal mode and the buck mode, depending on current levels and one of six SISC sub-states: boost with charge, boost with no-charge, boost-primary, boost-secondary, buck with charge, and buck with no charge. The SISC switcher mode may use the boost sub-states to prioritize the boost-internal mode, transitioning to the buck mode only when the boost-internal mode would otherwise skip a pulse with the i BOOST,PK  current less than the i PEAK,MIN  threshold. If the servo-controlled i BOOST,PK  current level is higher than i PEAK,MIN , the bidirectional switcher behaves identically to the boost-internal mode, and no power is bucked from the power port to low-voltage subsystems  1104 . 
     In the boost with charge sub-state, the SISC switcher mode prioritizes the boost but controls the voltage and current to an internal battery when bucking. In the boost with no-charge sub-state, the SISC switcher mode prioritize the boost but controls the voltage to low-voltage subsystems  1104 . In the boost-primary sub-state, the SISC switcher mode prioritizes the boost but pulls as much power as possible from power source  1108  during bucking. In the boost-secondary sub-state, the SISC switcher mode prioritizes the boost but balances current with the other bidirectional switcher in the SISC or buck mode during bucking. 
     The SISC switcher mode may use the buck sub-states to prioritize the buck mode, transitioning to the boost-internal mode only when the buck mode would otherwise skip a pulse with the i VALLEY  current less than −i PEAK,MIN . If the servo-controlled i VALLEY  current is more negative than −i PEAK,MIN , the bidirectional switcher behaves identically to the buck mode, and no power is boosted from low-voltage subsystems  1104  to high-voltage subsystems  1106 . 
     In the buck with charge sub-state, the SISC switcher mode prioritizes the buck and controls the voltage and current to the internal battery. In the buck with no-charge sub-state, the SISC switcher mode prioritizes the buck and controls the voltage to low-voltage subsystems  1104 . 
     The SISC switcher mode may further behave identically to the buck mode if the i BOOST,PK  current is zero. Similarly, the SISC switcher mode may behave identically to the boost-internal mode if the i VALLEY  current is zero. 
       FIG. 11B  shows a plot of inductor current in the four SISC boost sub-states. In the SISC boost sub-states, the i BOOST,PK  current is the servo-controlled limit that is identical to the i BOOST,PK  current in the boost-internal mode, and the i VALLEY  current is the servo-controlled limit that is identical to the i VALLEY  current in the buck mode with the buck sub-states of charge, no-charge, primary, or secondary described above with respect to  FIGS. 9A-9B . 
     The SISC boost sub-states preferentially provide power to high-voltage subsystems  1106  over providing power to low-voltage subsystems  1104  from power source  1108  by following the same procedure as the boost-internal mode, shown as Stages I, II, and III in  FIG. 11B , until the i BOOST,PK  current is less than i PEAK,MIN . In Stage I, FETs C and D are enabled as in the boost-internal mode. In the transition to Stage II, FET C is disabled, followed by the enabling of FET E, and FET D continues to be enabled. In Stage III, all FETs are disabled. 
     If the i BOOST,PK  current is less than i PEAK,MIN  at the clock edge, instead of skipping a boost pulse, a buck-like pulse is taken in Stage IV and V, which behave identically to Stages IV and V for the buck mode. In Stage IV, FET B is enabled to allow current to flow from power source  1108  to the inductor L. In the transition from Stage IV to Stage V, FET B is disabled, followed by the enabling of FET C. In both Stages IV and V, FET A operates as an ideal diode by allowing current to flow only from power source  1108 . By operating FET A as an ideal diode, the removal of power source  1108  from the power port can be detected as a drop in the V BUS  voltage. FET A is controlled by the measured bidirectional i BUS  current. If the i BUS  current is negative below a fixed threshold (e.g., indicating that current is flowing in from the power port), FET A is enabled; otherwise, FET A is disabled. 
     If the i BOOST,PK  current is less than i PEAK,MIN  and the negative i VALLEY  current is more negative than −i PEAK,MIN , the pulse is truly skipped because neither high-voltage subsystems  1106  nor low-voltage subsystems  1104  require a pulse of power. If the inductor current reaches zero in Stage V, which indicates DCM, then the stage transitions to Stage VI (not shown), and all of the FETs are disabled. The inductor current can be measured in Stage V in series with the inductor L or through FET C. 
     On the next clock edge in any buck stage (e.g., Stages IV, V or VI), if the i BOOST,PK  current exceeds the i PEAK,MIN  threshold, the SISC switcher mode returns to the boost-internal mode by transitioning through Stage VII before returning to Stage I. In Stage VII, FET A is disabled to prevent any current from returning to the power port, and FET C is enabled. Once the inductor current reaches zero, which is detected in series with inductor L or through FET C, the stage transitions from Stage VII to Stage I, where FET D is enabled along with the already enabled FET C. 
       FIG. 11C  shows a plot of inductor current in the two SISC buck sub-states. In the SISC buck sub-states, the i BOOST,PK  current is the servo-controlled limit that is identical to the i BOOST,PK  current in the boost-internal mode, and the i VALLEY  current is the servo-controlled limit that is identical to the i VALLEY  current in the buck mode with the buck sub-states of charge or no-charge described above. 
     The SISC buck sub-states preferentially provide bucked power from power source  1108  over providing boosted power to high-voltage subsystems  1106  by following the same procedure as the buck mode, shown as Stages IV, V and VI in  FIG. 11C , until the negative i VALLEY  current is less negative than −i PEAK,MIN . In Stage IV, FET B is enabled as in the buck mode. In the transition to Stage V, FET B is disabled, followed by the enabling of FET C. In Stage VI, all FETs are disabled. 
     In both Stages IV and V, FET A operates as an ideal diode by allowing current to flow from power source  1108  but not to power source  1108 . By operating FET A as an ideal diode, the removal of power source  1108  from the power port can be detected as a drop in the V BUS  voltage. FET A is controlled by the measured bidirectional i BUS  current. If the i Bus  current is negative below a fixed threshold (e.g., indicating that current is flowing in from the power port), FET A is enabled; otherwise, FET A is disabled. 
     If the negative i VALLEY  current is more negative than −i PEAK,MIN  at the clock edge, instead of skipping a buck pulse, a boost-internal-like pulse is taken, as shown in Stages I and II, which behave identically to Stages I and II for the boost-internal mode. In Stage I, FETs C and D are enabled to allow current to flow from the inductor L to ground. In the transition from Stage I to Stage II, FET C is disabled, followed by the enabling of FET E, and FET D continues to be enabled. 
     If the i BOOST,PK  current is less than i PEAK,MIN  and the negative i VALLEY  current is less negative than −i PEAK,MIN , the pulse is truly skipped because neither high-voltage subsystems  1106  nor low-voltage subsystems  1104  require a pulse of power. If the inductor current reaches zero in Stage II, which indicates DCM, then the stage transitions to Stage III (not shown) and all of the FETs are disabled. The inductor current can be measured in Stage II in series with the inductor L or through FET C. 
     On the next clock edge in any boost-internal stage (e.g., Stages I, II or III), if the negative i VALLEY  current is more negative than −i PEAK,MIN , the SISC switcher mode returns to the buck mode by transitioning to Stage IV. In the transition from Stage II to Stage IV, FETs E and D are disabled, followed by the enabling of FET B, with FET A operating as an ideal diode. Current will initially flow from inductor L through FET B into the input capacitance between FETs A and B. Once the current becomes negative, FET A will be enabled since it operates as an ideal diode, and current will flow in from the power port. 
     SISC Boost-Boost Mode 
     A bidirectional switcher may be operated in a SISC boost-boost switcher mode in instances where a bidirectional switcher may be used to provide power from a power source connected to a V BUS  node to a V MAIN  voltage node connected to one or more low voltage subsystems and provide power from the V MAIN  voltage node connected to one or more low-voltage subsystems to a voltage node at a high-voltage subsystem connected to V BOOST , where a voltage of V MAIN  may be at a greater voltage than the voltage node of V BUS  that is connected to the power source  1108 . For example, a power source  1108  such as a power source or an external battery may provide power to the V MAIN  node. More specifically,  FIG. 11D  shows a charging system with a bidirectional switcher  424  of  FIG. 4B  in a SISC boost-boost mode in accordance with the disclosed embodiments. The charging system of  FIG. 11D  is substantially similar to the charging system of  FIG. 11A , however, includes additional FET&#39;s F and G. In the SISC boost-boost mode, the bidirectional switcher switches between up-converting (e.g., boosting) power from one or more low-voltage subsystems  1104  to one or more high-voltage subsystems  1106  and up-converting (e.g., boosting) the input voltage from a power source  1108  (e.g., power supply, external battery, etc.) coupled to a power port at a V BUS  node to low-voltage subsystems  1104 . In SISC boost-boost mode, the bidirectional switcher must initially boost power from the power source  1108  to the low-voltage subsystems  1104  and, secondly, boost power from the low-voltage subsystems  1104  to the high-voltage subsystems  1106 . In the SISC boost-boost mode, Voltage V BUS  is at a lower voltage than voltage V MAIN . 
     The control scheme for the SISC boost-boost mode may prioritize boosting power from power source  1108  to low-voltage subsystems  1104  over boosting power from low-voltage subsystems  1104  to high-voltage subsystems  1106 . The SISC boost-boost mode of  FIG. 11D  is initiated in a first stage by enabling FET&#39;s A, B and G and disabling FET&#39;s C, D, E and F. Enabling FET&#39;s A, B and G allows current to flow from power port  1108  through the inductor L and store energy in inductor L, disabling FET&#39;s F and C disconnects low-voltage subsystems  1104  from power port  1108  and disabling FET&#39;s D and E disconnects high-voltage subsystems  1106  from the power source  1108  and low-voltage subsystems  1104 . Once the current in inductor L reaches a current limit, FET&#39;s A, B and F are enabled while all other FET&#39;s are disabled to allow current from inductor L to flow to V MAIN  and low-voltage subsystems  1104 . 
     In a second stage, FET&#39;s F and C are enabled while all other FETs are disabled. Enabling FET&#39;s F and C allows current to flow from low-voltage subsystems  1104  into inductor L. When the current in inductor L reaches a predetermined current limit, FET&#39;s F, D and E are enabled and other FET&#39;s are disabled to allow current from inductor L to flow to high-voltage subsystems  1106 . 
     Switcher Mode Transitions 
     When a bidirectional switcher changes from one mode to another (e.g., when a DC supply is connected to a power port), it may be desirable for the control circuit to transition smoothly between the former and current modes. The switcher modes can be classified into two styles: boost and buck. The boost-style modes include boost-accessory, boost-internal, SIDO, and the SISC boost sub-states. The buck-style modes include buck and the SISC buck sub-states. 
     Transitions between the disabled mode and either the boost-style or buck-style mode require no special transition considerations. Similarly, transitions between same style modes require no special transition considerations. 
     In some instances, it may be desirable to require that the inductor current reach zero before the transition occurs to provide smooth transition between a boost-style mode and a buck-style mode, the bidirectional switcher is immediately put into Stage II or Stage IIa until the inductor current reaches zero and Stage III is entered. After reaching Stage III, the mode may transition seamlessly to a buck-style mode. 
     Similarly, it may be desirable to require that the inductor current reach zero before the transition occurs to provide smooth transition between a buck-style mode and a boost-style mode. Thus, when transitioning from a buck-style mode, the bidirectional switcher is immediately put into Stage V until the inductor current reaches zero and Stage VI is entered. After reaching Stage VI, the bidirectional switcher may transition seamlessly to a boost-style mode. 
     Bidirectional Switcher Combinations 
     As discussed above, the mode of each bidirectional switcher depends mainly on what is plugged into the corresponding power port and the voltage state of the internal battery. However, some combinations of bidirectional switcher modes may have additional requirements to prioritize available power appropriately. In some instances, to improve bandwidth and reduce ripple current, clocks driving the two bidirectional switchers of the charging system may be driven 180 degrees out of phase. 
     Some combinations of bidirectional switcher modes do not require any coordination between the bidirectional switchers. Thus, the control of each bidirectional switcher is completely independent and defined by one of a plurality of modes described above. On the other hand, half of the switcher combinations may require codependent operation of the bidirectional switchers. Such codependent switcher combinations are described below with respect to  FIGS. 12A-12C, 13A-13C, 14A-14D, 15A-15B and 16A-16F . 
     Balancing in a Low-Voltage or Under-Voltage State with External Accessories 
       FIG. 12A  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. In particular,  FIG. 12A  shows a combination of bidirectional switcher modes when an internal battery  1212  connected to the charging system is in a low-voltage or under-voltage state and the power ports are either unplugged or connected to low- and high-powered devices  1208 - 1210  (e.g., external accessories or charging external batteries). In this mode, both bidirectional switchers may be used to boost power from one or more low-voltage subsystems  1204  to one or more high-voltage subsystems  1206  by balancing the total peak inductor current in the bidirectional switchers when possible. 
     The combinations defined by balancing the inductor currents with a low-voltage or under-voltage battery  1212  are summarized in the table below. 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 Power Port 1 
                 Power Port 2 
                 Switcher 1 
                 Switcher 2 
               
               
                   
               
             
            
               
                 Unplugged 
                 Unplugged 
                 Boost-internal 
                 Boost-internal 
               
               
                 Unplugged 
                 Powered Device 
                 SIDO Boost- 
                 SIDO Boost- 
               
               
                   
                 (HI) 
                 Internal 
                 Accessory 
               
               
                 Powered Device 
                 Unplugged 
                 SIDO Boost- 
                 SIDO Boost- 
               
               
                 (HI) 
                   
                 Accessory 
                 Internal 
               
               
                 Unplugged 
                 Powered Device 
                 Boost-Internal 
                 SIDO Buck- 
               
               
                   
                 (LO) 
                   
                 Accessory 
               
               
                 Powered Device 
                 Powered Device 
                 SIDO Boost- 
                 SIDO Boost- 
               
               
                 (HI) 
                 (HI) 
                 Accessory 
                 Internal 
               
               
                 Powered Device 
                 Powered Device 
                 SIDO Buck- 
                 SIDO Buck- 
               
               
                 (LO) 
                 (LO) 
                 Accessory 
                 Internal 
               
               
                 Powered Device 
                 Powered Device 
                 SIDO Buck- 
                 SIDO Boost- 
               
               
                 (LO) 
                 (HI) 
                 Accessory 
                 Internal 
               
               
                   
               
            
           
         
       
     
     In the dual switcher mode of  FIG. 12A , external accessories and/or other powered devices  1208 - 1210  are connected to both power ports, and a low-voltage state (e.g., “BATT_LO”) in battery  1212  requires a boost to high-voltage subsystems  1206 . In this case, both bidirectional switchers are in the SIDO switcher mode. As discussed above, the SIDO mode reduces to the boost-internal mode when the accessory peak current goes to zero. As a result, the SIDO-SIDO switcher mode represents the most general case, and all of the other cases in the table above can be realized by setting the accessory peak current to zero for power ports without an attached accessory. 
     The inputs to the balancing calculation are the accessory peak currents for the two power ports (i ACC,PK,1  and i ACC,PK,2  described above with respect to the boost-accessory switcher mode) and the peak current of high-voltage subsystems  1206  (i INTRNL,PK  described above with respect to the boost-internal switcher mode). If a power port is unplugged, the corresponding i ACC,PK,1  current is set to zero. The outputs of the balancing calculation are the two boost peak currents (i INTRNL,PK,1  and i INTRNL,PK,2 ), which are required to control each bidirectional switcher in the SIDO mode. 
     The most efficient distribution of power is to balance the total peak currents of the two bidirectional switchers, if possible. This is not possible when one accessory requires more power than the combined power of the other accessory and high-voltage subsystems  1206 . In this case, one bidirectional switcher should be dedicated to the more powerful accessory, and the second bidirectional switcher should provide power to the other accessory and high-voltage subsystems  1206 . The following calculations determine the i INTRNL,PK  and i ACC,PK  currents for each SIDO switcher. 
     First, the three peak currents are summed and divided by two to calculate the nominal total peak current available for each bidirectional switcher: 
               i     TOTAL   ,   PK       =         i     ACC   ,   PK   ,   1       +     i     ACC   ,   PK   ,   2       +     i     INTRNL   ,   PK         2           
The boosted peak current for the bidirectional switcher denoted by the sub-index  1  may be calculated as the nominal total peak current minus the accessory peak current for the bidirectional switcher, where the boosted peak current is limited to be positive and set to 0 of negative:
 
 i   INTRNL,PK,1 =max( i   TOTAL,PK   −i   TOTAL,PK,1,0 )
 
The boosted peak current for the bidirectional switcher denoted by the sub-index  2  may then be given by the original peak current of high-voltage subsystems  1206  minus the boosted peak current for the first bidirectional switcher:
 
 i   INTRNL,PK,2   =i   INTRNL,PK   −i   INTNL,PK,1 .
 
     With two connected accessories, power priority is given to high-voltage subsystems  1206  and the accessory connected to the first power port over the accessory connected to the second power port. Consequently, the bidirectional switcher for the first power port is controlled in the SIDO (accessory) mode, and the bidirectional switcher for the second power port is controlled in the SIDO (boost) mode. The mathematics used to calculate the i INTRNL,PK  and i ACC,PK  currents for each SIDO switcher can be implemented in analog circuitry, where a block representation is shown in  FIG. 12B . 
     For example, if i ACC,PK,1  is about 1.0 A, i ACC,PK,2  is about 1.5 A, and i INTRNL,PK  is about 2.5 A, the expressions above may be used to balance the peak inductor currents to about 2.5 A with i INTRNL,PK,1  equal to about 1.5 A and i INTRNL,PK,2  equal to about 1.0 A. If i ACC,PK,1  is increased to about 5.0 A, then the inductor peak currents can no longer be balanced. As a result, the total peak current of the first bidirectional switcher is about 5.0 A, the total peak current of the second bidirectional switcher is 4.0 A, i INTRNL,PK,1  is equal to about 0.0 A, and i INTRNL,PK,2  is equal to about 2.5 A. 
       FIG. 12C  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. In particular,  FIG. 12C  shows a combination of bidirectional switcher modes in a charging system that is substantially the same as the charging system of  FIG. 12A , however includes a SIDO buck-boost converter of  FIG. 4B  instead of a SIDO boost converter of  FIG. 4A . The SIDO buck-boost converter in  FIG. 12C  includes additional FETs F 1  and G 1  and with FET E 1  being operated as an ideal diode, while all other features remain substantially the same as the dual switcher mode of  FIG. 12A . It should be appreciated that in the control scheme for the charging system of  FIG. 12C , the SIDO buck-boost converter of  FIG. 12C  may be operated as a SIDO boost converter by turning FET F 1  ON and FET G 1  OFF and operating FET&#39;s A 1 , B 1 , C 1 , D 1  and E 1  the same as a SIDO boost converter as was described in the embodiment of  FIG. 12A . Additionally, during operation of charging system of  FIG. 12C , FETs D 2  and E 2  may be controlled to be, at the same time, both ON or both OFF. Adding the FET&#39;s F 1  and G 1  may support low-voltage accessories connected to the power ports and can provide an added functionality of bucking power from one or more low-voltage subsystems  1204  to a low-voltage external accessory connected to one or more power ports such as, for example, to a powered device (Low)  1208 . 
     Balancing in a Low-Voltage or Under-Voltage State with a Power Source and an Accessory 
       FIG. 13A  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 13A  shows a combination of bidirectional switcher modes when an internal battery  1312  connected to the charging system is in a low-voltage or under-voltage state, one power port is coupled to an accessory  1308  (e.g., a powered device or charging external battery), and another power port is coupled to a power source  1310  such as a power supply or a discharging external battery. In this mode, the bidirectional switchers are in a SISC-SIDO mode, with the bidirectional switcher connected to power source  1310  in the SISC switcher mode and the other bidirectional switcher in the SIDO switcher mode. 
     The combinations with power source  1310  (e.g., a power supply or discharging battery) and low-voltage and high-voltage powered devices  1308  (e.g., a low-voltage or a high-voltage accessory) coupled to the power ports and battery  1312  in a low-voltage or under-voltage state are summarized in the table below: 
                                         Power Port 1   Power Port 2   Switcher 1   Switcher 2                  Powered Device   Power Supply   SIDO Boost-   SISC Buck-       (HI)   (HI)   Accessory   Internal       Powered Device   Power Supply   SIDO Buck-   SISC Buck-       (LO)   (HI)   Accessory   Internal       Powered Device   Battery   SIDO Boost-   SISC Buck       (HI)       Internal   Boost                   with charge       Powered Device   Battery   SIDO Buck-   SISC Buck       (LO)       Internal   Boost                   with charge       Power Supply   Powered Device   SISC Buck-   SIDO Boost-       (HI)   (HI)   Internal   Accessory       Power Supply   Powered Device   SISC Buck-   SIDO Buck-       (HI)   (LO)   Internal   Accessory       Battery   Powered Device   SISC Buck   SIDO Boost-           (HI)   Boost   Internal               with charge       Battery   Powered Device   SISC Buck   SIDO Buck-           (LO)   Boost   Internal               with charge                    
With powered device  1308  and power source  1310  coupled to the power ports, the SIDO switcher may power one or more high-voltage subsystems  1306 , and the SISC switcher may pull as much power as possible from power source  1310  to power one or more low-voltage subsystems  1304 .
 
     Inputs to the SISC-SIDO switcher mode of  FIG. 13A  are the i ACC,PK  current according to the servo described above with respect to the boost-accessory mode, the i VALLEY  current according to the servo described above with respect to the buck mode and the charge or no-charge sub-state, and the i INTRNL,PK  current described above with respect to the boost-internal switcher mode. The SIDO (accessory) mode is controlled by the i ACC,PK  current and the i INTRNL,PK  current, while the SISC switcher mode requires the i VALLEY  current and a residual peak current for high-voltage subsystems called i INTRNL,REMAINING . 
     As described above with respect to the SIDO switcher mode, the i TOTAL,PK  current is the sum of the accessory peak current i ACC,PK  and the peak current of high-voltage subsystems  1306  i INTRNL,PK , limited to the maximum inductor peak current i PEAK,MAX :
 
 i   TOTAL,PK =min( i   INTRNL,PK   +i   ACC,PK   ,i   PEAK,MAX )
 
The boosted peak current for the SISC (boost with charge) or SISC (boost with no-charge) modes i INTRNL,REMAINING  may then be calculated as the sum of the accessory peak current i ACC,PK  and the peak current of high-voltage subsystems  1306  i INTRNL,PK , minus the SIDO&#39;s total peak current i TOTAL,PK :
 
 i   INTRNL,REMAINING   =i   INTRNL,PK   +i   ACC,PK   −i   TOTAL,PK  
 
     If the total peak current i TOTAL,PK  is not limited by the maximum peak current i PEAK,MAX , then the i INTRNL,REMAINING  peak current will be zero, and the SISC (boost with charge) or SISC (boost with no-charge) mode will behave identically to the buck mode, which pulls the desired power from power source  1310  by controlling the i VALLEY  current. If the SIDO (accessory) switcher is peak-current-limited, then the SISC switcher will provide the residual current to high-voltage subsystems  1306  and no longer draw current from power source  1310  if the residual boost current i INTRNL,REMAINING  remains above the minimum peak current level i PEAK,MIN  and prevents the SISC mode from entering the buck mode. The mathematics used to calculate the separated boost peak currents i INTRNL,PK  and i INTRNL,REMAINING  for the SISC and SIDO switchers can be implemented in analog circuitry, where a block representation is shown in  FIG. 13B . 
       FIG. 13C  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 13C  shows a combination of bidirectional switcher modes in a charging system that is substantially the same as the charging system of  FIG. 13A  but includes a SIDO buck-boost converter of  FIG. 4B  instead of a SIDO boost converter of  FIG. 4A . The SIDO buck-boost converter in  FIG. 13C  includes additional FETs F 1  and G 1  and with FET E 2  being operated as an ideal diode, while all other features remain substantially the same as the charging system of  FIG. 13A . It should be appreciated that in the control scheme for the charging system of  FIG. 13C , the SIDO buck-boost converter may be operated as a SIDO boost converter by turning FET F 1  ON and FET G 1  OFF and operating FET&#39;s A 1 , B 1 , C 1 , D 1  and E 1  the same as a SIDO boost converter as was described in the embodiment of  FIG. 13A . Additionally, during operation, FETs D 1  and E 1  are controlled to be, at the same time, both ON or both OFF. Adding the additional FET&#39;s F 1  and G 1  may support low-voltage accessories connected to the power ports and can provide the additional functionality of bucking power from one or more low-voltage subsystems  1304  to a low-voltage external accessory connected to one or more power ports such as, for example, to a powered device (Low)  1308 . 
     Balancing with Two Power Sources 
       FIG. 14A  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 14A  shows a combination of bidirectional switcher modes when an internal battery  1412  connected to the charging system is in a high-voltage state (e.g., “BATT_HI”) and power sources  1408 - 1410  (e.g., DC power supplies, external batteries, etc.) are coupled to both power ports. In this combination, both bidirectional switchers may be used to buck power sources  1408 - 1410  to one or more low-voltage subsystems  1404  by balancing the current through the bidirectional switchers when possible. 
     As shown in  FIG. 14A , both bidirectional switchers may be in the buck mode, with the sub-state of each buck mode selected based on the charging state of battery  1412  and the capability of the corresponding power source. The capability of the power source may be determined by multiplying the power source&#39;s nominal open-circuit voltage by the power source&#39;s input current limit to obtain the power source&#39;s nominal maximum power. The capability of a given power source may also be advertised by the power source through the corresponding power port. 
     Of the two power sources  1408 - 1410 , the power source with highest nominal maximum power is considered the primary power source, and the weaker power source is considered the secondary power source. The buck sub-state for the primary power source is set to either charge or no-charge, depending on the charging state of battery  1412 , as described with respect to the behavior of FET G above. The buck sub-state for the secondary power source  1410  is set to secondary. As a result, the bidirectional switcher connected to the primary power source  1408  will control either the battery charging voltage and current or the voltage of low-voltage subsystems  1404 , and the power will be balanced between the two power sources  1408 - 1410  until the secondary power source  1410  becomes input-power-limited. 
     Operation of the charging system is similar when battery  1412  is in a low-voltage or under-voltage state, with the combination of bidirectional switcher modes and current flow shown in  FIG. 14B . Instead of operating both bidirectional switchers in the buck mode, the bidirectional switcher connected to the secondary power source  1410  is in the SISC (boost-secondary) mode. The bidirectional switcher connected to the primary power source  1408  is in the buck mode and provides power to low-voltage subsystems  1404 , while the other bidirectional switcher prioritizes providing power to high-voltage subsystems  1406  in the SISC (boost-secondary) mode. If the load on high-voltage subsystems  1406  is light, the SISC mode can pull current from the secondary power source  1410  and attempt to balance the current between the two power sources  1408 - 1410 , as with the mode of  FIG. 14A  with battery  1412  in a high-voltage state. 
       FIG. 14C  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 14C  shows a combination of bidirectional switcher modes for a charging system that is substantially the same as the charging system of  FIG. 14A  but includes a SIDO buck-boost converter of  FIG. 4B  instead of a SIDO boost converter of  FIG. 4A . The SIDO buck-boost converter in  FIG. 14C  includes additional FETs F 1  and G 1 , while all other features remain substantially the same as the charging system of  FIG. 14A . It should be appreciated that in the charging system of  FIG. 14C , the SIDO buck-boost converter may be operated as a SIDO boost converter by turning FET F 1  ON and FET G 1  OFF and operating FET&#39;s A 1 , B 1 , C 1 , D 1  and E 1  the same as a SIDO boost converter as was described in the embodiment of  FIG. 14A . Additionally, during operation, FETs D 1 , E 1  and D 2 , E 2  may be controlled to be both OFF at the same time. 
       FIG. 14D  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 14D  shows a combination of bidirectional switcher modes for a charging system that is substantially the same as the charging system of  FIG. 14B  but includes a SIDO buck-boost converter of  FIG. 4B  instead of a SIDO boost converter of  FIG. 4A . The SIDO buck-boost converter in  FIG. 14C  includes additional FETs F 1  and G 1 , while all other features remain substantially the same as the charging system of  FIG. 14B . It should be appreciated that in the charging system of  FIG. 14D , the SIDO buck-boost converter may be operated as a SIDO boost converter by turning FET F 1  ON and FET G 1  OFF and operating FET&#39;s A 1 , B 1 , C 1 , D 1  and E 1  the same as a SIDO boost converter ( FIG. 4A ) as was described in the embodiment of  FIG. 14B . Additionally, during operation, FETs D 1 , E 1  are OFF and DETs D 2  and E 2  may be controlled to be both ON or OFF at the same time. 
     Charging an External Battery with an Attached Power Source 
       FIG. 15A  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. In particular,  FIG. 15A  shows a combination of bidirectional switcher modes when an internal battery  1512  connected to the charging system is in a low-voltage state (e.g., “BATT_LO”) and a power source  1510  (e.g., a DC power supply or an external battery) coupled to one power port is used to charge an external battery  1508  coupled to the other power port. Such charging of external battery  1508  may be performed if available power remains after power source  1510  provides power to one or more low-voltage subsystems  1504 , one or more high-voltage subsystems  1506 , and battery  1512 . 
     No coordination between the two bidirectional switchers is required in this combination, but detection of available power from power source  1510  to charge external battery  1508  may be required. The six combinations with power source  1510  supplying power to low-voltage subsystems  1504 , high-voltage subsystems  1506 , internal battery  1512 , and external battery  1508  are summarized in the table below: 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                 Power Port 1 
                 Power Port 2 
                 Battery Voltage 
                 Switcher 1 
                 Switcher 2 
               
               
                   
               
             
            
               
                 Power Source 
                 Battery 
                 High-Voltage 
                 Buck (charge 
                 Buck Boost- 
               
               
                 (HI) 
                 (Charging) 
                   
                 or no-charge) 
                 Accessory 
               
               
                 Power Source 
                 Battery 
                 Low-Voltage or 
                 SISC Buck 
                 SIDO Buck- 
               
               
                 (HI) 
                 (Charging) 
                 Under-Voltage 
                 (charge or no- 
                 Boost-Internal 
               
               
                   
                   
                   
                 charge) 
               
               
                 Battery 
                 Power Source 
                 High-Voltage 
                 Buck-Boost 
                 Buck (charge 
               
               
                 (Charging) 
                 (HI) 
                   
                 Accessory 
                 or no-charge) 
               
               
                 Battery 
                 Power Source 
                 Low-Voltage or 
                 SIDO Buck 
                 SISC Buck 
               
               
                 (Charging) 
                 (HI) 
                 Under-Voltage 
                 Boost-Internal 
                 (charge or no- 
               
               
                   
                   
                   
                   
                 charge) 
               
               
                 Battery 
                 Battery 
                 High-Voltage 
                 Buck Boost- 
                 Buck Boost 
               
               
                 (Charging) 
                 (Discharging) 
                   
                 Accessory 
                 (charge or no- 
               
               
                   
                   
                   
                   
                 charge) 
               
               
                 Battery 
                 Battery 
                 Low-Voltage or 
                 SIDO Buck 
                 Buck Boost 
               
               
                 (Charging) 
                 (Discharging) 
                 Under-Voltage 
                 Boost-Internal 
                 (charge or no- 
               
               
                   
                   
                   
                   
                 charge) 
               
               
                   
               
            
           
         
       
     
     The power port connected to power source  1510  is operated in the buck mode in the charge or no-charge sub-state. If both power ports are connected to external batteries, then the external battery connected to the second power port is treated identically to a DC power supply. The switcher connected to external battery  1508  is in the boost-accessory or SIDO (boost) mode, depending on the voltage state of external battery  1508 . In this mode, power from power source  1510  is prioritized for low-voltage subsystems  1504 , high-voltage subsystems  1506 , and internal battery  1512 . External battery  1508  is charged only if additional power from power source  1510  is available after powering of low-voltage subsystems  1504 , high-voltage subsystems  1506 , and internal battery  1512 . 
     External battery  1508  remains in a charging state if the bidirectional switcher connected to power source  1510  is not input-power-limited, which is detected when the buck mode of the bidirectional switcher is not limited by either the V BUS  voltage or i BUS  current limits, as described above with respect to the buck mode. If the buck mode is limited by the V BUS  voltage or i BUS  current limits but the measured i BUS  current is positive into external battery  1508 , external battery  1508  may continue to be charged. If the measured i BUS  current into battery  1508  reaches zero and the buck mode is input-power-limited, battery  1508  switches from charging to discharging to supplement power from power source  1510 , as discussed below. 
       FIG. 15B  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 15B  shows a combination of bidirectional switcher modes for a charging system that is substantially the same as the charging system of  FIG. 15A  but includes a SIDO buck-boost converter of  FIG. 4B  instead of a SIDO boost converter of  FIG. 4A . The SIDO buck-boost converter of  FIG. 15B  includes additional FETs F 1  and G 1 , while all other features remain substantially the same as the charging system of  FIG. 15A . It should be appreciated that in the charging system of  FIG. 15B , the SIDO buck-boost converter may be operated in a buck-boost mode or a buck mode as described in the embodiments above by turning FET F 1  ON and FET G 1  OFF and operating FET&#39;s A 1 , B 1 , C 1 , D 1  and E 1  the same as a SIDO boost converter as was described in the embodiment of  FIG. 15A . Additionally, during operation, FETs D 2  and E 2  are OFF and FETs D 1  and E 1  may be controlled to be both ON or both OFF at the same time. 
     Supplementing a Power Source with an External Battery 
       FIGS. 16A-16F  show a dual switcher mode for a charging system in accordance with the disclosed embodiments. In particular,  FIGS. 16A-16F  show different combinations of bidirectional switcher mode when a power source  1610  connected to one power port does not have enough power to fully power one or more low-voltage subsystems  1604  and one or more high-voltage subsystems  1606  and charge an internal battery  1612 . In these modes, an external battery  1608  coupled to the other power port is used to supplement the power from power source  1610 . The combinations with external battery  1608  supplementing power source  1610  are summarized in the table below: 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                 Power Port 1 
                 Power Port 2 
                 Battery Voltage 
                 Switcher 1 
                 Switcher 2 
               
               
                   
               
             
            
               
                 Power Source 
                 Battery 
                 High-Voltage 
                 Buck (primary) 
                 Buck Boost (charge) 
               
               
                 (HI) 
                 (Discharging) 
               
               
                 Power Source 
                 Battery 
                 Low-Voltage or 
                 #1: SISC Buck- 
                 #1: SISC Buck Boost 
               
               
                 (HI) 
                 (Discharging) 
                 Under-voltage 
                 Internal (primary) 
                 (charge) 
               
               
                   
                   
                   
                 #2: SISC Buck 
                 #2: SISC (Buck Boost- 
               
               
                   
                   
                   
                 (charge) 
                 Internal(secondary) 
               
               
                 Battery 
                 Power Source 
                 High-Voltage 
                 Buck Boost (charge) 
                 Buck (primary) 
               
               
                 (Discharging) 
                 (HI) 
               
               
                 Battery 
                 Power Source 
                 Low-Voltage or 
                 #1: SISC Buck Boost 
                 #1: SISC Buck- 
               
               
                 (Discharging) 
                 (HI) 
                 Under-voltage 
                 (charge) 
                 Internal (primary) 
               
               
                   
                   
                   
                 #2: SISC (Buck Boost- 
                 #2: SISC Buck 
               
               
                   
                   
                   
                 Internal(secondary) 
                 (charge) 
               
               
                   
               
            
           
         
       
     
     If the buck switcher assigned to power source  1610  is limited by the V BUS  voltage or i BUS  current limits and the measured i BUS  current is zero into external battery  1608  in the other power port, the mode transitions from charging of external battery  1608  described above to using external battery  1608  to supplement power from power source  1610  in this section. If power source  1610  is not limited by the V BUS  voltage or i BUS  current limits, the mode transitions back to charging of external battery  1608 . 
     If internal battery  1612  is in a high-voltage state, most of the external power should be provided by power source  1610 , and external battery  1608  may supplement power as needed. As a result, the bidirectional switcher coupled to external battery  1608  may be operated in the buck mode in the charge or no-charge sub-state, and the bidirectional switcher coupled to power source  1610  may be operated in the buck mode in the primary sub-state, as shown in  FIG. 16A . 
     If internal battery  1612  is in a low-voltage state, the desired behavior is to pull as much power as possible from power source  1610 , while power from external battery  1608  is used to supplement power source  1610  and simultaneously boosted to high-voltage subsystems  1606 . However, the SISC switcher mode does not allow the bidirectional switcher connected to external battery  1608  to simultaneously buck and boost. 
     Consequently, the bidirectional switchers may be executed with two combinations of SISC switcher modes. The first combination may configure the bidirectional switcher coupled to power source  1610  in the SISC (boost-primary) mode and the bidirectional switcher coupled to external battery  1608  in the SISC (buck with charge or no-charge) mode, as shown in  FIG. 16B . If the boosted peak current i INTRNL,PK  is greater than the minimum peak current i PEAK,MIN , power source  1610  may be cut off as the SISC (boost-primary) mode reduces to the boost-internal mode. The SISC (buck with charge or no-charge) mode may reduce to the buck (charge or no-charge) mode when the negative i VALLEY  current is more negative than −i PEAK,MIN , and external battery  1608  may be the only external power source for the charging system. 
     The first combination may be associated with a number of disadvantages. First, if substantial current is needed for high-voltage subsystems  1606 , power source  1610  may not be able to supply power to the charging system. Second, because power source  1610  cannot become input-voltage- or input-current-limited, there is no way to detect when external battery  1608  should stop supplementing power source  1610  and return to charging from power source  1610 . 
     The second combination may configure the bidirectional switcher coupled to power source  1610  in the SISC (buck with charge or no-charge) mode and the bidirectional switcher coupled to external battery  1608  in the SISC (boost-secondary) mode, as shown in  FIG. 16C . If the boosted peak current i INTRNL,PK  is greater than the minimum peak current i PEAK,MIN , external battery  1608  may be cut off as the SISC (boost-primary) mode reduces to the boost-internal mode. The SISC (buck with charge or no-charge) mode may reduce to the buck (charge or no-charge) mode when the negative i VALLEY  current is more negative than −i PEAK,MIN , and power source  1610  may be the only external power source for the charging system. 
     The benefit of the second combination is that the transition from using external battery  1608  to supplement power source  1610  to charging from power source  1610  can be detected if power source  1610  is not limited by the V BUS  voltage or i BUS  current limits. On the other hand, external battery  1608  does not discharge, while internal battery  1612  may continue discharging if power source  1610  cannot provide sufficient power to all internal subsystems  1604 - 1606  and internal battery  1612 . Consequently, internal battery  1612  could discharge below the voltage requirement of low-voltage subsystems  1604 , and the portable electronic device may switch off. An undesirable user experience may then arise when power source  1610  is subsequently unplugged and external battery  1608  is able to provide power to the charging system. In other words, the charging system may shut down while plugged into an underpowered power source  1610  because not enough power is provided by power source  1610 , while at the same time external battery  1608  can sufficiently power the charging system but cannot be discharged until power source  1610  is unplugged. 
     The disadvantages of the first and second combinations may be balanced by alternately switching between the two combinations. For example, a majority of time may be spent in the first combination, and a switch to the second combination may occasionally be made to detect when a transition to the charging mode described above can be made. By spending most of the time in the first combination, external battery  1608  will drain while providing most of the power to low-voltage subsystems  1604 , high-voltage subsystems  1606 , and battery  1612 . Once the charge in external battery  1608  is fully depleted, only the second combination is used. 
     Because the operation of the charging system depends on power port states that includes low- and high-powered devices and low- and high-power sources (e.g., unplugged, low-power accessory, high-power accessory, low-power source, high-power source, external battery) for each of two power ports and two differentiated internal battery states (e.g., high-voltage, low-voltage or under-voltage), the charging system may be operated using a plurality of possible modes, which are shown in the following table: 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                 Power Port 1 
                 Power Port 2 
                 Battery Voltage 
                 Switcher 1 
                 Switcher 2 
               
               
                   
               
             
            
               
                 Unplugged 
                 Unplugged 
                 High-Voltage 
                 Disabled 
                 Disabled 
               
               
                 Unplugged 
                 Unplugged 
                 Low-Voltage or 
                 Boost-internal 
                 Disabled 
               
               
                   
                   
                 Under-Voltage 
               
               
                 Unplugged 
                 Powered 
                 High-Voltage 
                 Disabled 
                 Buck-Accessory 
               
               
                   
                 Device (LO or 
                   
                   
                 or Boost- 
               
               
                   
                 HI) 
                   
                   
                 Accessory 
               
               
                 Unplugged 
                 Powered 
                 Low-Voltage or 
                 Boost-internal 
                 Buck-Accessory 
               
               
                   
                 Device (LO or 
                 Under-Voltage 
                   
                 or Boost- 
               
               
                   
                 HI) 
                   
                   
                 Accessory 
               
               
                 Unplugged 
                 Power Source 
                 High-Voltage 
                 Disabled 
                 Buck 
               
               
                   
                 (HI) 
               
               
                 Unplugged 
                 Power Source 
                 Low-Voltage or 
                 Boost-internal 
                 Buck 
               
               
                   
                 (HI) 
                 Under-Voltage 
               
               
                 Unplugged 
                 Battery 
                 High-Voltage 
                 Disabled 
                 Buck Boost 
               
               
                 Unplugged 
                 Battery 
                 Low-Voltage or 
                 Boost-internal 
                 Buck Boost 
               
               
                   
                   
                 Under-Voltage 
               
               
                 Powered Device 
                 Unplugged 
                 High-Voltage 
                 Buck-Accessory 
                 Disabled 
               
               
                 (LO or HI) 
                   
                   
                 or Boost- 
               
               
                   
                   
                   
                 Accessory 
               
               
                 Powered Device 
                 Unplugged 
                 Low-Voltage or 
                 Buck-Accessory 
                 Boost-internal 
               
               
                 (LO or HI) 
                   
                 Under-Voltage 
                 or Boost- 
               
               
                   
                   
                   
                 Accessory 
               
               
                 Powered Device 
                 Powered 
                 High-Voltage 
                 Buck-Accessory 
                 Buck-Accessory 
               
               
                 (LO or HI) 
                 Device (LO or 
                   
                 or Boost- 
                 or Boost- 
               
               
                   
                 HI) 
                   
                 Accessory 
                 Accessory 
               
               
                 Powered Device 
                 Powered 
                 Low-Voltage or 
                 SIDO Buck- 
                 SIDO Buck- 
               
               
                 (LO or HI) 
                 Device (LO or 
                 Under-Voltage 
                 Accessory or 
                 Internal or SIDO 
               
               
                   
                 HI) 
                   
                 SIDO Boost- 
                 Boost-Internal 
               
               
                   
                   
                   
                 Accessory 
               
               
                 Powered Device 
                 Power Source 
                 High-Voltage 
                 Buck-Accessory 
                 Buck 
               
               
                 (LO or HI) 
                 (HI) 
                   
                 or Boost- 
               
               
                   
                   
                   
                 Accessory 
               
               
                 Powered Device 
                 Power Source 
                 Low-Voltage or 
                 SIDO Buck- 
                 SISC Buck- 
               
               
                 (LO or HI) 
                   
                 Under-Voltage 
                 Accessory or 
                 Internal 
               
               
                   
                   
                   
                 SIDO Boost- 
               
               
                   
                   
                   
                 Accessory 
               
               
                 Powered Device 
                 Battery 
                 High-Voltage 
                 Buck-Accessory 
                 Buck Boost 
               
               
                 (LO or HI) 
                   
                   
                 or Boost- 
               
               
                   
                   
                   
                 Accessory 
               
               
                 Powered Device 
                 Battery 
                 Low-Voltage or 
                 Buck-Accessory 
                 SISC Buck Boost- 
               
               
                 (LO or HI) 
                   
                 Under-Voltage 
                 or Boost- 
                 Internal 
               
               
                   
                   
                   
                 Accessory 
               
               
                 Power Source 
                 Unplugged 
                 High-Voltage 
                 Buck 
                 Disabled 
               
               
                 (HI) 
               
               
                 Power Source 
                 Unplugged 
                 Low-Voltage or 
                 Buck 
                 Boost-internal 
               
               
                 (HI) 
                   
                 Under-Voltage 
               
               
                 Power Source 
                 Powered 
                 High-Voltage 
                 Buck 
                 Buck-Accessory 
               
               
                 (HI) 
                 Device (LO or 
                   
                   
                 or Boost- 
               
               
                   
                 HI) 
                   
                   
                 Accessory 
               
               
                 Power Source 
                 Powered 
                 Low-Voltage or 
                 SISC Buck- 
                 SIDO Buck- 
               
               
                 (HI) 
                 Device (LO or 
                 Under-Voltage 
                 Internal 
                 Accessory or 
               
               
                   
                 HI) 
                   
                   
                 SIDO Boost- 
               
               
                   
                   
                   
                   
                 Accessory 
               
               
                 Power Source 
                 Power Source 
                 High-Voltage 
                 Buck (Charge) 
                 Buck 
               
               
                 (HI) 
                 (HI) 
                   
                   
                 (secondary) 
               
               
                 Power Source 
                 Power Source 
                 Low-Voltage or 
                 Buck (Charge) 
                 Buck 
               
               
                 (HI) 
                 (HI) 
                 Under-Voltage 
                   
                 (Secondary) 
               
               
                 Power source 
                 Battery 
                 High-Voltage 
                 Buck 
                 Buck Boost 
               
               
                 (HI) 
               
               
                 Power Source 
                 Battery 
                 Low-Voltage or 
                 Buck or SISC 
                 SIDO or SISC 
               
               
                 (HI) 
                   
                 Under-Voltage 
               
               
                 Battery 
                 Unplugged 
                 High-Voltage 
                 Buck Boost 
                 Disabled 
               
               
                 Battery 
                 Unplugged 
                 Low-Voltage or 
                 Buck Boost 
                 Boost-internal 
               
               
                   
                   
                 Under-Voltage 
               
               
                 Battery 
                 Powered 
                 High-Voltage 
                 Buck Boost 
                 Buck-Accessory 
               
               
                   
                 Device (LO or 
                   
                   
                 or Boost- 
               
               
                   
                 HI) 
                   
                   
                 Accessory 
               
               
                 Battery 
                 Powered 
                 Low-Voltage or 
                 SISC 
                 SIDO 
               
               
                   
                 Device (LO or 
                 Under-Voltage 
               
               
                   
                 HI) 
               
               
                 Battery 
                 Power Source 
                 High-Voltage 
                 Buck Boost 
                 Buck 
               
               
                   
                 (HI) 
               
               
                 Battery 
                 Power Source 
                 Low-Voltage or 
                 SIDO or SISC 
                 Buck or SISC 
               
               
                   
                 (HI) 
                 Under-Voltage 
               
               
                 Battery 
                 Battery 
                 High-Voltage 
                 Buck Boost 
                 Buck Boost 
               
               
                 Battery 
                 Battery 
                 Low-Voltage or 
                 SIDO or SISC 
                 Buck or SISC 
               
               
                   
                   
                 Under-Voltage 
               
               
                   
               
            
           
         
       
     
     Consequently, the above-described bi-directional two-port battery charging circuit may charge an internal battery if power is provided through other or both power ports. The power ports can also be used to provide power to attached external accessories. An external battery may be connected to a power port to provide additional power and/or obtain charge from a power supply connected to the other power port. The charging circuit may additionally extend runtime by providing a boosted voltage rail that allows the portable electronic device to continue to run after the internal battery voltage drops below the minimum voltage required by the high-voltage subsystems. Such functionality may be provided using a design that requires only two inductors to minimize the required board space. 
       FIG. 16D  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 16D  shows a combination of a charging system that is substantially the same as the charging system of  FIG. 16A  but includes a SIDO buck-boost converter of  FIG. 4B  instead of a SIDO boost converter of  FIG. 4A . The SIDO buck-boost converter of  FIG. 16D  includes additional FETs F 1  and G 1 , while all other features remain substantially the same as the charging system of  FIG. 16A . It should be appreciated that in the charging system of  FIG. 16D , the SIDO buck-boost converter may be operated as a SIDO boost converter by turning FET F 1  ON and FET G 1  OFF and operating FET&#39;s A 1 , B 1 , C 1 , D 1  and E 1  the same as a SIDO boost converter as was described in the embodiment of  FIG. 16A . Additionally, during operation, each FET pair D 1  and E 1  and FET pair D 2  and E 2  may be controlled to be both OFF at the same time. The circuit of charging system may also enable buck and buck-boost modes using FETs F 1  and G 1  and can support low- and high-powered devices and low- and high-power sources (e.g., unplugged, low-power accessory, high-power accessory, low-power source, high-power source, external battery) connected to each of two power ports and two differentiated internal battery states (e.g., high-voltage, low-voltage or under-voltage), 
       FIG. 16E  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 16E  shows a combination of a charging system that is substantially the same as the charging system of  FIG. 16B  but includes a SIDO buck-boost converter of  FIG. 4B  instead of a SIDO boost converter of  FIG. 4A . The SIDO buck-boost converter of  FIG. 16E  includes additional FETs F 1  and G 1 , while all other features remain substantially the same as the charging system of  FIG. 16B . It should be appreciated that in the charging system of  FIG. 16E , the SIDO buck-boost converter may be operated as a SIDO boost converter by turning FET F 1  ON and FET G 1  OFF and operating FET&#39;s A 1 , B 1 , C 1 , D 1  and E 1  the same as a SIDO boost converter as was described in the embodiment of  FIG. 16B . Additionally, during operation, FET pair D 1  and E 1  and FET pair D 2  and E 2  may be controlled to be, at the same time, both ON or both OFF. The circuit of charging system may also enable buck and buck-boost modes using FETs F 1  and G 1  and can support low- and high-powered devices and low- and high-power sources (e.g., unplugged, low-power accessory, high-power accessory, low-power source, high-power source, external battery) connected to each of two power ports and two differentiated internal battery states (e.g., high-voltage, low-voltage or under-voltage), 
       FIG. 16F  shows a dual switcher mode for a charging system in accordance with the disclosed embodiments. More specifically,  FIG. 16F  shows a combination of a charging system that is substantially the same as the charging system of  FIG. 16C  but includes a SIDO buck-boost converter of  FIG. 4B  instead of a SIDO boost converter of  FIG. 4A . The SIDO buck-boost converter of  FIG. 16F  includes additional FETs F 1  and G 1 , while all other features remain substantially the same as the charging system of  FIG. 16C . It should be appreciated that in the charging system of  FIG. 16F , the SIDO buck-boost converter may be operated as a SIDO boost converter by turning FET F 1  ON and FET G 1  OFF and operating FET&#39;s A 1 , B 1 , C 1 , D 1  and E 1  the same as a SIDO boost converter as was described in the embodiment of  FIG. 16C . Additionally, during operation, each FET pair D 1  and E 1  and FET pair D 2  and E 2  may be controlled to be both ON at the same time or both OFF at the same time. The circuit of charging system may also enable buck and buck-boost modes using FETs F 1  and G 1  and can support low- and high-powered devices and low- and high-power sources (e.g., unplugged, low-power accessory, high-power accessory, low-power source, high-power source, external battery) connected to each of two power ports and two differentiated internal battery states (e.g., high-voltage, low-voltage or under-voltage), 
       FIG. 17  shows a flowchart illustrating the process of managing use of a portable electronic device in accordance with the disclosed embodiments. In one or more embodiments, one or more of the steps may be omitted, repeated, and/or performed in a different order. Accordingly, the specific arrangement of steps shown in  FIG. 17  should not be construed as limiting the scope of the embodiments. 
     Initially, a charging circuit for providing and receiving power through two power ports and converting an input voltage received through one or both power ports into output voltages for charging an internal battery and powering low-voltage and high-voltage subsystems in the portable electronic device is operated (operation  1702 ). For example, the charging circuit may include one or more power converters and a control circuit that operates the power converter(s) in a mode, among others, that charges an external battery coupled to the first power port from a power source coupled to the second power port. 
     In particular, during operation of the charging circuit, the operation of bidirectional switching converters connected to the power ports is configured based on the coupling of a power source, external accessory, or an external battery to one or both power ports (operation  1704 ). Each bidirectional switching converter may include an inductor and a set of switching mechanisms. For example, the bidirectional switching converters may include a SIDO boost converter, a SIDO buck-boost converter, a dual-output SEPIC converter, and/or other types of bidirectional power converters. Operation of the bidirectional switching converters may include, but is not limited to: up-converting power from the low-voltage subsystem to an external accessory or an external battery coupled to the power port, up-converting power from the low-voltage subsystem to the high-voltage subsystem with no power coming in or out of the power port, down-converting power from a power source or the external battery coupled to the power port to the low-voltage subsystem, and/or up-converting power from the low-voltage subsystem to the high-voltage subsystem and the external accessory coupled to the power port. 
     The bidirectional switching converters are also operated based on the state of the internal battery (operation  1706 ). For example, the state of the internal battery may be a low-voltage state, a high-voltage state, and/or an under-voltage state. During the low-voltage or under-voltage state of the internal battery and a coupling of an external battery to the first power port, a first bidirectional switching converter may be operated to down-convert power from the external battery to the low-voltage subsystems. A second bidirectional switching converter may be operated to up-convert power from the low-voltage subsystem to the high-voltage subsystem. 
     During the low-voltage or under-voltage state of the internal battery, a presence of the input voltage from a power source or an external battery through the first power port, and a coupling of an external accessory to the second power port, the first bidirectional switching converter may be operated to switch between up-converting power from the low-voltage subsystem to the high-voltage subsystem and down-converting the input voltage from a power source to the low-voltage subsystem. The second bidirectional switching converter may be operated to boost power from the low-voltage subsystem to the high-voltage subsystem and the external accessory. 
     During a coupling of a power source to the first port and a coupling of an external battery to the second power port, the first and second bidirectional switching converters may be operated to power the low-voltage subsystem and the high-voltage subsystem and charge the internal battery from the power source. Remaining power from the power source may be used to charge the external battery, or the external battery may be used to supplement the power to the low-voltage subsystem, the high-voltage subsystem, and the internal battery from the power source. 
       FIG. 18  shows a flowchart illustrating the process of operating a charging system for a portable electronic device in accordance with the disclosed embodiments. In one or more embodiments, one or more of the steps may be omitted, repeated, and/or performed in a different order. Accordingly, the specific arrangement of steps shown in  FIG. 18  should not be construed as limiting the scope of the embodiments. 
     Initially, a charging circuit for providing and receiving power through two power ports and converting an input voltage received through one or both power ports into output voltages for charging an internal battery and powering low-voltage and high-voltage subsystems in the portable electronic device is operated (operation  1802 ). Next, during operation of the charging circuit, a first bidirectional switching converter connected to a first power port is configured to switch between up-converting power from the low-voltage subsystem to the high-voltage subsystem and down-converting the input voltage from a first power source coupled to the first power port to the low-voltage subsystem (operation  1804 ). 
     In other words, the first bidirectional switching converter may be operated in the SISC switcher mode described above. In the SISC switcher mode, up-converting power from the low-voltage subsystem to the high-voltage subsystem may be prioritized over down-converting of the input voltage to the low-voltage subsystem. Conversely, down-converting of the input voltage to the low-voltage subsystem may be prioritized over up-converting power from the low-voltage subsystem to the high-voltage subsystem. 
     During down-converting of the input voltage from the power source to the low-voltage subsystem, the SISC switcher mode may be operated in one of several sub-states. First, the input voltage may be down-converted to a target voltage of the internal battery. Second, the input voltage may be down-converted to a minimum voltage of the low-voltage subsystem. Third, as much power as possible may be pulled from the power source. Fourth, a first inductor current of the first bidirectional switching converter may be balanced with a second inductor current of a second bidirectional switching converter connected to the second power port. 
     The charging circuit may be operated based on the state of a second power port (operation  1806 ) connected to the charging circuit. If an accessory (e.g., a powered device or charging external battery) is coupled to the second power port, a second bidirectional converter connected to the second power port is configured to up-convert power from the low-voltage subsystem to the high-voltage subsystem and the accessory (operation  1808 ). In other words, the second bidirectional converter may be operated in the SIDO switcher mode described above. 
     If a second power source is coupled to the second power port, the second bidirectional converter is configured to down-convert power from the second power source to the low-voltage subsystem (operation  1810 ). For example, the second bidirectional converter may be operated in the buck (charge or no-charge) mode described above, depending on the charging state of the internal battery. 
     The above-described charging circuit can generally be used in any type of electronic device. For example,  FIG. 19  illustrates a portable electronic device  1900 , which includes a processor  1902 , a memory  1904  and a display  1908 , which are all powered by a power supply  1906 . Portable electronic device  1900  may correspond to a laptop computer, tablet computer, mobile phone, portable media player, digital camera, and/or other type of battery-powered electronic device. Power supply  1906  may include one or more power converters such as SIDO boost converters, SIDO buck-boost converters, dual-output SEPIC converters, and/or other bidirectional switching converters. In one or more embodiments, the power converter(s) include a first bidirectional switching converter connected to a first power port of the portable electronic device, a low-voltage subsystem in the portable electronic device, and a high-voltage subsystem in the portable electronic device, as well as a second bidirectional switching converter connected to a second power port of the portable electronic device, the low-voltage subsystem, and the high-voltage subsystem. 
     Power supply  1906  may also include a control circuit that operates the power converter(s) to provide and receive power through the first and second power ports and convert an input voltage received through the first or second power port into a set of output voltages for charging an internal battery in the portable electronic device and powering the low-voltage subsystem and the high-voltage subsystem. For example, the control circuit may use the power converter(s) to charge an external battery coupled to the first power port from a power source coupled to the second power port and/or use the external battery to supplement power from the power source. 
     The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention.

Metadata:
Filing Date: 20160223
Publication Date: 20181009
Grant Date: 20181009
Priority Date: 20150624
Inventors: GREENING, THOMAS C.
HASAN, KAMRAN M.
Wong, Edrick C. G.
Assignee: APPLE INC
CPC Classifications: [{"code": "H02J1/082", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J1/082", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02J7/0068", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02M3/1582", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M3/158", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M3/1582", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M3/158", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/266", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J1/102", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J7/34", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/26", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M3/158", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J1/102", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J1/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J7/0068", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/266", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J7/34", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M1/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J1/102", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J7/34", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M3/1582", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J7/0068", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J7/34", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J1/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M2001/009", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M3/1582", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J7/0044", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02J2001/008", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02J1/102", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J7/0068", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M3/158", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/26", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M2001/008", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M1/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M1/10", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02J1/082", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J7/0044", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M1/10", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02M1/009", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M1/008", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M1/009", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M1/009", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M1/008", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M1/008", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 56409686