Abstract:
Exemplary systems and methods for charging a battery with a digital charge reduction loop are described herein. In some embodiments, a system comprises an exemplary digital charge reduction loop which comprises a circuit for determining a charge-current adjustment signal, a counter for generating a digital count value, and a digital-to-analog converter. The circuit for determining a charge-current adjustment signal may base the determination on a source voltage of an input source. The counter may generate a digital count value based on the charge-current adjustment signal. The digital-to-analog converter (DAC) may generate a DAC control signal based on the digital count value of the counter, the DAC control signal being representative of an amount of charge current to be used to charge a battery.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation and seeks priority to U.S. Nonprovisional application Ser. No. 11/559,790, filed Nov. 14, 2006, entitled “Battery Charger Apparatus with a Digital Charge Reduction Loop” which is hereby incorporated by reference. 
    
    
     COPYRIGHT NOTICE 
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     TECHNICAL FIELD 
     This invention relates generally to battery chargers, and more particularly provides a battery charger apparatus with a digital charge reduction loop. 
     BACKGROUND 
     In recent years, the universal serial bus (USB) has become one of the most widely used techniques for interconnecting electronic devices. Originally used to interconnect computers and standard peripheral devices (e.g., printers, disk drives, etc.), USB has grown to support a vast array of portable USB devices (e.g., cellular telephones, personal digital assistants, cameras, personal music players, etc.). 
     USB devices are connected to a USB host (e.g., a PC, laptop, tablet PC, etc.) via USB ports. Generally, one USB host can support up to seven USB devices. In cases where more than seven USB devices are required, a USB hub may be used. Each USB hub may be connected to its own set of seven USB devices (or hubs as necessary). The USB connections between the USB host, USB hubs and USB devices allow data, e.g., names, phone numbers, calendars, photographs, music, etc., to flow between the USB host and USB devices. 
     Generally, a USB connection has a DC power line commonly referred to as the VBUS, a ground, and a twisted pair of data lines commonly referred to as D+ and D−. According to the USB 2.0 standard, a valid USB host should supply 5 volts and no more than 500 milliamps of current. Further, USB ports are supposed to provide current-limit protection, which can be set as high as 5.0 amps and as low as 100 milliamps. However, not all USB ports adhere to the USB standard. Further, USB hosts have different types of ports, including AC-powered ports (e.g., in host PCs and powered hubs), non-powered ports (e.g., in notebook computers operating on battery power), passive ports (e.g., in passive hubs), etc. Each port type has a different power delivery capability. For example, an AC-powered port may source 5 volts and 500 milliamps of current or more. A non-powered (battery-powered) port may source 5 volts, but may share 500 milliamps of current with other USB ports on the USB host. Thus, a non-powered port often sources less than 500 milliamps of current. A passive port may source 5 volts and only 100 milliamps of current. 
     Typical USB devices have a rechargeable lithium-ion/polymer battery for sourcing operating power and a control system to re-charge the battery. According to the USB standard, a USB device may take power from the power line of the USB port both to operate its system core and to re-charge its battery. A USB device with a battery charger typically includes an apparatus for regulating charge current to prevent the charging process from drawing excessive current and starving the remainder of the USB device. Charge-current regulation is typically accomplished using two current sense resistors to monitor the charge current to the battery and the load current to the system core. While effective, this mechanism has drawbacks. Among these is the fact that the battery charger can select only from one of two charging currents, namely, 500 milliamps or 100 milliamps. At 100 milliamps, battery charging could take as long as 8-10 hours for an 800-milliamp battery. Users may become irritated and return USB devices and/or USB chargers as defective. Also, when a USB host or hub is capable of supplying more than 100 milliamps but less than 500 milliamps, traditional charge-current regulation operates as though extra capability did not exist. Thus, traditional charge-current regulation unnecessarily slows the charging process. 
     A system and method are needed to facilitate more effective use of USB potential for charging a battery of a battery-powered USB device. 
     Similar problems exist for typical AC adaptor power supplies. An electronic device that uses a rechargeable battery is typically supplied with an AC power adaptor rated to support the charge-current demands of the electronic device. However, some electronic devices are not supplied with a power adaptor and, in many cases, third party power adapters are used. Using a non-approved power adapter on an electronic device risks starving the system core of the device due the drawing of excessive charge-current. Thus, when working with battery chargers, USB port and AC power adapter capabilities must assure that the battery charging process does not starve system operation. 
     SUMMARY 
     In accordance with one embodiment, the present invention provides a battery charger apparatus for charging a battery, comprising a charge-current control circuit for receiving a charge-current control signal to control an amount of charge current being drawn from an input source, e.g., a USB port or a line adapter; an analog control circuit for generating an analog control signal representative of a maximum amount of charge current to be used to charge a battery; a digital charge reduction loop including a circuit for determining a charge-current adjustment signal based on a source voltage of the input source, a counter for generating a digital count value based on the charge-current adjustment signal, and a digital-to-analog converter (DAC) for generating a DAC control signal based on the digital count value of the counter, the DAC control signal being representative of a reduced amount of charge current to be used to charge the battery; and a switching mechanism for selecting one of the analog control signal or the DAC control signal based on the source voltage, the selected control signal being used as the charge-current control signal to the charge-current control circuit. 
     The charge-current control circuit may limit the charge current to the maximum charge current that the battery can handle. The circuit for determining a charge-current adjustment signal may include a first comparator for comparing a first voltage based on the source voltage against a reference voltage to generate a first comparator output signal, the first comparator output signal being for controlling the counter; and a second comparator for comparing a second voltage based on the source voltage against the reference voltage to generate a second comparator output signal, the second comparator output signal being for controlling the switching mechanism. The counter may count up when the first comparator output signal is a logical high value. The counter may count down when the first comparator output signal is a logical low value. The switching mechanism may select the DAC control signal as the charge-current control signal and the counter may reset to a predetermined digital value, when the second comparator output signal is a logical low value. The switching mechanism may select the analog control signal when the digital count value of the counter is at a predetermined digital value. The digital charge reduction loop may further comprise a clock oscillator for generating a clock signal, and the counter may modify the digital count value in coordination with the clock signal. The circuit for determining a charge-current adjustment signal may include a comparator for comparing a first voltage based on the source voltage against a reference voltage to generate a comparator control signal, the comparator control signal being for controlling the counter and the switching mechanism. The counter may count up when the comparator control signal is a logical high value. The counter may count down when the comparator control signal is a logical low value. The switching mechanism may select the DAC control signal as the charge-current control signal and the counter may reset, when the comparator control signal is a logical low value and the digital counter value is at a predetermined digital value. The switching mechanism may select the analog control signal as the charge-current control signal when the digital counter value is at a predetermined digital value. The switching mechanism may include an RS flip-flop for receiving a reset signal when the source voltage is below a trip threshold voltage, and for receiving a set signal when the digital count value is at a predetermined digital value, the RS flip-flop generating a RS output signal; and a multiplexer for selecting one of the analog control signal or the DAC control signal based on the RS output signal. 
     In accordance with another embodiment, the present invention provides a method of charging a battery, comprising generating a first analog control signal representative of a maximum amount of charge current to be used to charge a battery; generating a charge-current control signal based on a source voltage of an input source, e.g., a USB port or a line adapter; generating a digital count value based on the charge-current control signal; generating a second analog control signal based on the digital count value of the counter, the second analog control signal being representative of a reduced amount of charge current to be used to charge the battery; and selecting based on the source voltage one of the first analog control signal or the second analog control signal to use as a charge-current control signal to a charge-current control circuit. 
     The method may further comprise limiting the charge current to the maximum charge current that the battery can handle. The determining a charge-current adjustment signal may include comparing a first voltage based on the source voltage against a reference voltage to generate a first comparator output signal, the first comparator output signal being for controlling the generating the digital count value; and comparing a second voltage based on the source voltage against the reference voltage to generate a second comparator output signal, the second comparator output signal being for controlling the selecting. The generating the digital count value may include increasing the digital count value when the first comparator output signal is a logical high value. The generating the digital count value may include decreasing the digital count value when the first comparator output signal is a logical low value. When the second comparator output signal is a logical low value, the selecting may include selecting the DAC control signal as the charge-current control signal and the method may further comprise resetting the digital count value to a predetermined digital value. The selecting may include selecting the analog control signal when the digital count value is at a predetermined digital value. The method may further comprise generating a clock signal, and the selecting may include modifying the digital count value in coordination with the clock signal. The determining the charge-current adjustment signal may include comparing a first voltage based on the source voltage against a reference voltage to generate a comparator control signal, the comparator control signal being for controlling the generating the digital count value and the selecting. The generating the digital count value may include counting up when the comparator control signal is a logical high value. The generating the digital count value may include counting down when the comparator control signal is a logical low value. When the comparator control signal is a logical low value and the digital counter value is at a predetermined digital value, the selecting may include selecting the DAC control signal as the charge-current control signal and the method may further comprise resetting the digital count value. The selecting may include selecting the analog control signal as the charge-current control signal when the digital counter value is at a predetermined digital value. The method may further comprise receiving a reset signal when the source voltage is below a trip threshold voltage; receiving a set signal when the digital count value is at a predetermined digital value; and generating a RS output signal based on the received signal; wherein the selecting includes selecting one of the analog control signal or the DAC control signal based on the RS output signal. 
     In accordance with another embodiment, the present invention provides a digital charge reduction loop in a battery charger comprising a circuit for determining a charge-current adjustment signal based on a source voltage of an input source; a counter for generating a digital count value based on the charge-current adjustment signal; and a digital-to-analog converter (DAC) for generating a DAC control signal based on the digital count value of the counter, the DAC control signal being representative of an amount of charge current to be used to charge a battery. 
     In accordance with another embodiment, the present invention provides a digital charge reduction loop in a battery charger comprising means for determining a charge-current adjustment signal based on a source voltage of an input source; means for generating a digital count value based on the charge-current adjustment signal; and means for generating an analog control signal based on the digital count value, the control signal being representative of an amount of charge current to be used to charge a battery. 
     Exemplary systems and methods for charging a battery with a digital charge reduction loop are described herein. In some embodiments, a system comprises an exemplary digital charge reduction loop which comprises a circuit for determining a charge-current adjustment signal, a counter for generating a digital count value, and a digital-to-analog converter. The circuit for determining a charge-current adjustment signal may base the determination on a source voltage of an input source. The counter may generate a digital count value based on the charge-current adjustment signal. The digital-to-analog converter (DAC) may generate a DAC control signal based on the digital count value of the counter, the DAC control signal being representative of an amount of charge current to be used to charge a battery. 
     In some embodiments, the circuit for determining the charge-current adjustment signal comprises a first comparator and a second comparator. The first comparator may be configured to compare a first voltage based on the source voltage against a reference voltage to generate a first comparator output signal. The first comparator output signal may be for controlling the counter. The second comparator may be configured to compare a second voltage based on the source voltage against the reference voltage to generate a second comparator output signal. The second comparator output signal may be configured to control the switching mechanism. In various embodiments, the counter is operative to count up when the first comparator output signal is a logical high value. In other embodiments, the counter is operative to count down when the first comparator output signal is a logical low value. 
     In some embodiments, the digital charge reduction loop may further comprise a clock oscillator. The clock oscillator may be configured to generate a clock signal. Further, the counter may be operative to modify the digital count value in coordination with the clock signal. 
     In various embodiments, the circuit for determining the charge-current adjustment signal may include a comparator for comparing a first voltage based on the source voltage against a reference voltage to generate a comparator control signal. The comparator control signal may be configured to control the counter. The counter may be operative to count down when the comparator control signal is a logical low value. 
     The system may also comprise a charge-current control circuit, an analog control circuit, and a switching mechanism. The charge-current control circuit may be configured to receive a charge-current control signal to control an amount of charge current being drawn from the input source. The analog control circuit may be configured to generate an analog control signal representative of a maximum amount of charge current to be used to charge the battery. The switching mechanism may be configured to select one of the analog control signal or the DAC control signal, the selected control signal being used as the charge-current control signal to the charge-current control circuit. 
     In some embodiments, the charge-current control circuit is operative to limit the charge current to the maximum charge current that the battery can handle. The switching mechanism may be operative to select the analog control signal when the digital count value of the counter is at a predetermined digital value. 
     In various embodiments, an exemplary system comprises a digital charge reduction loop. The digital reduction loop comprises a means for determining a charge-current adjustment signal based on a source voltage of an input source, a means for generating a digital count value based on the charge-current adjustment signal, and a means for generating an analog control signal based on the digital count value, the control signal being representative of an amount of charge current to be used to charge a battery. 
     An exemplary comprises determining a charge-current adjustment signal based on a source voltage of an input source, generating a digital count value based on the charge-current adjustment signal, and generating a DAC control signal based on the digital count value of the counter, the DAC control signal being representative of an amount of charge current to be used to charge a battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a battery charging system with a USB host and USB device, in accordance with an embodiment of the present invention. 
         FIG. 2  is a circuit diagram illustrating details of a battery charger with a digital charge reduction loop, in accordance with an embodiment of the present invention. 
         FIG. 3  illustrates a voltage/time diagram for the battery charger of  FIG. 2  in operation, in accordance with an embodiment of the present invention. 
         FIG. 4  illustrates voltage/time and current/time diagrams for the battery charger of  FIG. 2  in operation, in accordance with an embodiment of the present invention. 
         FIG. 5  is a circuit diagram illustrating details of a battery charger with a digital charge reduction loop, in accordance with another embodiment of the present invention. 
         FIG. 6  illustrates a voltage/time and current/time diagrams of a digital charge reduction loop managing charge current from a line adapter capable of sourcing greater amounts of current than a typical USB port. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is provided to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the embodiments are possible to those skilled in the art, and the generic principles defined herein may be applied to these and other embodiments and applications without departing from the spirit and scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles, features and teachings disclosed herein. 
     U.S. application publication no. 2006/0033474, entitled “USB Battery Charger,” Ser. No. 11/916,339, filed on Aug. 11, 2004 by inventor K. Shum is hereby incorporated by reference. 
       FIG. 1  is a circuit diagram of a battery charging system  100  with a USB host  105  and battery-powered USB device  120 , in accordance with an embodiment of the present invention. The USB host  105  may be coupled to the USB device  110  directly or via a USB cable  115 . When the USB host  105  and the USB device  110  are interconnected, the USB host  105  and USB device  110  may exchange data, and/or the USB host  105  may charge the battery of the USB device  110 . 
     The USB host  105  may include a desktop computer, a laptop computer, a tablet PC, etc. The USB host  105  includes a power source  165 . When the USB host  105  is a desktop computer, the power source  165  may include an AC power supply  120  (and corresponding circuit). When the USB host  105  is a laptop, tablet PC or other portable computer system, the power source  165  may include both an AC power supply  120  and a battery  125  (and corresponding circuit), such that the AC power supply  120  powers the USB host  105  when connected to an AC outlet (not shown) and the battery  125  powers the USB host  105  when not connected to an AC outlet. The USB host  105  also includes one or more USB ports  130  (only one shown), each capable of connection to a USB device  110  directly or via the USB cable  150 . The power source  165  powers the USB ports  130 . 
     The USB device  110  may include a cellular telephone, a personal digital assistant (PDA), a personal music player, a camera, etc. The USB device  110  includes a rechargeable battery  135  (e.g., a lithium ion battery) and a battery charger  140  (with a digital charge reduction loop) coupled to the battery  135 . The USB device  110  includes a USB port  145  coupled to the battery charger  140  and to the system core  170  of the USB device  110 . The system core  170  includes the microprocessor(s), circuitry, memory, storage and/or the like, for operating the core functions of the USB device  110 , e.g., the cellular telephone, the PDA, the personal music player, the camera, etc. 
     When the USB device  110  is not connected to the USB host  105 , the system core  170  of the USB device  110  is powered by its internal battery  135 . However, when the USB device  110  is connected to the USB host  105 , the system core  170  may be powered fully or partially by the power supplied over the power line of the USB port  145  from the USB host  105 . Further, the battery charger  140  may draw power from the power line of the USB port  145  to charge the battery  135 . In one embodiment, the battery charger  140  is operative to take only as much current from the USB port  145  as possible without starving the system core  170 . If the system core  170  is inactive, then the battery charger  140  takes the maximum possible charge current, for example, 500 milliamps, that the USB port  145  can source and/or that the battery charger  140  can accept without damage. If the system core  170  is active and drawing a load current, then the battery charger  140  uses the remainder of the current available and/or that the battery charger  140  can accept. For example, if the system core  170  needs 200 milliamps of load current, then the battery charger  140  takes the remaining 300 milliamps of a USB port  130  that is capable of providing a maximum of 500 milliamps of current. A first example battery charger  140 ′ with a digital charge reduction loop is shown in  FIG. 2 . A second example battery charger  140 ″ with a digital charge reduction loop is shown in  FIG. 5 . 
     Generally, the battery charger  140  takes advantage of the fact that, when a circuit pulls more current than is available from an input source, the source voltage drops. Per the USB 2.0 standard, a USB port may sag as low as 4.4 volts, and thus USB devices  110  must be capable of operating with as little as 4.4 volts of input voltage. Per one embodiment, the battery charger  140  initially draws a maximum charge current. If the voltage of the USB port  130  drops below a trip voltage threshold, e.g., 4.4 volts, then the battery charger  140  draws only a minimum charge current until the source voltage returns above a safe voltage threshold, e.g., 4.5 volts. Then, the battery charger  140  step-wise increases the charge current it draws from the USB port  145  (e.g., in steps of predetermined amperage), and monitors the source voltage of the USB port  145  until the source voltage drops below the safe voltage threshold. When the source voltage drops below the safe voltage threshold, the battery charger  140  step-wise decreases the charge current it draws until the source voltage rises above the safe voltage threshold (or the maximum charge current is reached). The battery charger  140  continues to monitor the source voltage, and repeats step-wise increasing and step-wise decreasing the charge current, thus causing the charger current to oscillate about the maximum charge current possible at a given time. If at any time the load current changes, the battery charger  140  follows the above protocol. 
     For example, assume that the USB port  130  is capable of sourcing a maximum 500 milliamps of current. If the system core  170  draws no load current, then the battery charger  140  draws all 500 milliamps as charge current to charge the battery  135 . If the system core  170  draws 200 milliamps of load current, then the battery charger  140  notices a voltage drop at the USB port  145  below the trip voltage threshold. The battery charger  140  responsively drops to a minimum charge current until the source voltage returns above a safe voltage threshold. The battery charger  140  step-wise increases the charge current until the source voltage drops below the safe voltage threshold. Then, the battery charger  140  step-wise decreases the charge current and the source voltage rises above the safe voltage threshold. Accordingly, the source voltage stabilizes at (oscillates about) the safe voltage threshold, and the charge current stabilizes at (oscillates about) the maximum charge current possible. If the USB port  130  is capable of sourcing 500 milliamps and the load current is about 200 milliamps, then the charge current will stabilize at (oscillate about) 300 milliamps. 
       FIG. 2  is a circuit diagram illustrating details of a battery charger  140 ′ with a digital charge reduction loop  200 , in accordance with an embodiment of the present invention. The battery charger  140 ′ is an example embodiment of the battery charger  140  of  FIG. 1 . The battery charger  140 ′ includes a digital charge reduction loop  200  coupled to an input source  245  (e.g., the VBUS of the USB port  145 ), an analog charge-current control circuit  240 , a multiplexer  225  coupled to the digital charge reduction loop  200  and to the analog charge-current control circuit  240 , and a charge-current limit/control circuit  230  coupled to the input source  245  and to the multiplexer  225 . 
     Generally, the analog charge-current control circuit  240  operates during normal mode, when maximum charge-current is available from the input source  245 . The analog charge-current control circuit  240  may include a predetermined voltage signal, a modifiable voltage signal, or other analog control-signal generating circuit. The digital charge reduction loop  200  operates during reduced charge-current mode, when less than the maximum charge-current is available from the input source  245 . The multiplexer  225  operates to select one of the analog charge-current control circuit  240  or the digital charge reduction loop  200  as the charge-current controlling circuit to control the charge-current limit/control circuit  230 . The charge-current limit/control circuit  230  operates to draw the charge current from the input source  245  to the battery  135  based on the control signal it receives from the multiplexer  225 , however, limited to a maximum charge current that the battery  135  can accept without risking damage, e.g., 1 A. The charge-current limit/control circuit  230  may include a gated-MOS, e.g., PMOS or NMOS, circuit. 
     The digital charge reduction loop  200  includes a source voltage analysis circuit  205  coupled to the input source  245 , an up/down counter  210  (e.g., a shifter) coupled to the source voltage analysis circuit  205 , a digital to analog converter (DAC)  220  coupled to the up/down counter  210  and to the multiplexer  225 , an RS flip-flop  215  coupled to the source voltage analysis circuit  205 , to the up/down counter  210  and to the multiplexer  225 , and a clock oscillator  235  coupled to the up/down counter  210 . 
     The source voltage analysis circuit  205  monitors the source voltage of the input source  245 . If current demand on the input source  245  causes the source voltage to drop below a trip voltage threshold, then the multiplexer  225  selects the digital charge reduction loop  200  to control the charge current. Essentially, when the source voltage drops below the trip threshold voltage, the source voltage analysis circuit  205  resets the RS flip-flop  215 , which causes the multiplexer  225  to select the digital charge reduction loop  200  to control the charge-current limit/control circuit  230 . 
     Having been selected as the charge-current control circuit, the digital charge reduction loop  200  reduces the charge current to a predetermined minimum, thereby allowing the source voltage at the input source  245  to recover to a point above a safe voltage threshold (in this case measured at node V 1 , although measurable at any node). Once recovered, the digital charge reduction loop  200  steps up the charge current until the source voltage drops below the safe voltage threshold. At this time, the digital charge reduction loop  200  steps down the charge current, thereby allowing the source voltage to rise again. In a repeating fashion, the digital charge reduction loop  200  identifies a “steady-state” charge current where the input source  245  is not overloaded and the battery  135  is being charged using the maximum charge current available. 
     Should the capability of the input source  245  increase or should the load current demand reduce, the up/down counter  210  may eventually reach a maximum count (bn). When the maximum count is reached, the up/down counter  210  sends an END signal to set the RS flip-flop  215 , which causes the multiplexer  225  to return control of the charge current back to the analog control circuit  240 . 
     In this embodiment, the source voltage analysis circuit  205  includes three resistors, R 1 , R 2  and R 3 , coupled in series between the input source  245  and ground, respectively. A first comparator A is coupled to the node V 1  between the resistors R 1  and R 2  and to a reference voltage VREF, e.g., 2.0V (or other voltage). A second comparator B is coupled to the node V 0  between the resistors R 2  and R 3  and to the reference voltage VREF. The three resistors, R 1 , R 2  and R 3 , and the reference voltage VREF set the voltage thresholds, namely, the trip voltage threshold and the safe voltage threshold. The trip voltage threshold (digital charge reduction loop  200  enable), measured at node V 0 , is set for 1.98V, resets the up/down counter  210 , and causes selection of the digital charge reduction loop  200  by the multiplexer  225 . The safe voltage threshold (digital charge reduction loop  200  control), measured at node V 1 , is set at 2.02V and controls the up/down count of the digital counter  210 , which in turn controls the DAC  220 , which in turn controls the charge-current limit/control circuit  230 . 
     In a specific embodiment, when the input source  245  is greater than 4.5V, V 0  is greater than 1.98V. Thus, the analog control circuit  240  controls charge current to the battery  135 . Should the input source  245  be unable to supply the charge current or should the load current and charge current exceed the current capacity of the input source  245 , the source voltage at the input source  245  will drop below 4.5V. V 0  and V 1  will subsequently drop in a linear scale based upon the values of the resistors R 1 , R 2  and R 3 . When V 0  falls below the trip voltage threshold, e.g., 1.98V, comparator A will reset the RS flip-flop  215 , causing the multiplexer  225  to transition control of the charge current from the analog control circuit  240  to the digital charge reduction loop  200 . Further, comparator A will reset the up/down counter  210  to its lowest level, setting the DAC  220  to a predetermined minimum, which in turn reduces the charge current to a predetermined minimum. This restores the integrity of the input source  245  and prevents shutdown or potential damage to the input source  245  or the system core  170 . 
     The comparator B compares the voltage at node V 1  against the reference voltage VREF and responsively generates a control signal. If the voltage at node V 1  is greater than the safe voltage threshold (as measured at V 1 ), e.g., 2.02V, then the comparator B generates a logical-high control signal. Otherwise, comparator B generates a logical-low control signal. A clock period later, the up/down counter  210  samples the control signal from the comparator B. If the voltage seen by comparator B is logical-high (as expected with reduced load current), the logical-high control signal causes the up/down counter  210  to count up by one, thus setting the b0 bit and increasing the output voltage of the DAC  220  by one predetermined step. Thus, the DAC  220  increases the charge current by one predetermined step. As charge current is stepped up, charge current increases the current demand placed on the input source  245 , causing source voltage reduction. When the source voltage drops below the safe voltage threshold, the control signal transitions to a logical-low state, causing a down count in the up/down counter  210 . The down count causes a step down reduction in the output voltage of the DAC  220 , thus causing a step reduction in the charge current. After a number of clock cycles, the digital charge reduction loop  200  stabilizes (e.g., oscillates about) at the maximum charge current supported by the input source  245 . 
     When a source fault terminates or load current is reduced, the digital charge reduction loop  200  steps up the charge current. When the up/down counter  210  reaches the maximum level, namely, the bn level, the up/down counter  210  sends an END signal to set the SR flip-flop  215 , which controls the multiplexer  225  to shift control back to the analog control circuit  240 . 
     It will be appreciated that the up/down counter  210  may shift at any resolution, e.g., 32 steps or 64 steps, depending on how course or fine to make the voltage/current steps. A courser resolution may provide a faster response. Also, greater resolution may require a longer shifter. It will also be appreciated that an embodiment may be implemented with only a digital charge reduction loop  200  and without an analog control circuit  240 . 
       FIG. 3  illustrates a voltage/time diagram  300  for the battery charger  140 ′ of  FIG. 2 , in accordance with an embodiment of the present invention. As shown in the FIGS. the term “Digital Charge Reduction Loop” can be conveniently written as “DCHR.” The diagram  300  illustrates four (4) stages of the digital charge reduction loop  200 . During stage  305 , while the analog control circuit  240  controls the charge current, the digital charge reduction loop  200  measures the source voltage of the input source  245 . As shown, overload of the input source  245  occurs during stage  305 , thus causing the source voltage to drop. Stage  310  begins when the digital charge reduction loop  200  recognizes that the source voltage has dropped to a trip voltage threshold, thus causing transition of charge-current control from the analog control circuit  240  to the digital charge reduction loop  200 . The digital charge reduction loop  200  reduces charge current to a predetermined minimum, thus allowing the source voltage to recover back to a point above the safe voltage threshold (in this case, back to about 4.6V). Stage  315  begins when the source voltage has recovered to the point above the safe voltage threshold, at which time the digital charge reduction loop  200  begins to step up the charge current upon every clock cycle. In response to the stepping up of the charge current, stage  315  illustrates the source voltage stepping down until it reaches the safe threshold voltage. Stage  320  begins when the source voltage reaches the safe threshold voltage, at which time the digital charge reduction loop  200  begins to step down the charge current, thus causing the source voltage to rise above the safe voltage threshold. Having risen above the safe voltage threshold, the digital charge reduction loop  200  steps up the charge current, thus causing the source voltage to drop below the safe voltage threshold. Accordingly, during stage  320 , the digital charge reduction loop  200  causes a steady-state charge current as the source voltage stabilizes at (e.g., oscillates about) the safe voltage threshold. 
       FIG. 4  illustrates voltage/time and current/time diagrams  400  for the battery charger  140 ′ of  FIG. 2 , in accordance with an embodiment of the present invention. 
     Diagram  400 ( a ) illustrates load current over a time period of 12 milliseconds (ms). As shown, from t=0 ms to t=1 ms, the system core  170  of the USB device  110  draws no current. From about t=1 ms to about t=5 ms, the system core  170  draws about 300 mA of current. From about t=5 ms to about t=10 ms, the system core  170  draws about 230 mA of current. After about t−10 ms, the system core  170  draws no current. 
     Diagram  400 ( b ) illustrates source voltage of the input source  245  over the same time period. As shown, from t=0 ms until about t=1 ms (while the system core  170  draws no load current), the source voltage is stable at about 4.6V. At about t=1 ms, when the system core  170  begins drawing a load current of 300 mA, the source voltage drops below the trip voltage threshold of about 4.4V. Immediately thereafter (as the digital charge reduction loop  200  takes over charge-current control), the source voltage returns to a safe voltage threshold of about 4.5V and stabilizes at about 4.45V. At about t=5 ms, when the system core  170  reduces its load current from about 300 mA to about 230 mA, the source voltage rises slightly. From about t=5 ms to about t=5.5 ms (as the digital charge reduction loop  200  steps up the charge current), the source voltage lowers again to the safe voltage threshold. From about t=5.5 ms to about t=10 ms, the source voltage remains stable at the safe voltage threshold. At about t=10 ms (after the system core  170  stops drawing any load current), the source voltage rises upward. At about t=10.5 ms (as the analog control circuit  240  takes over charge-current control), the source voltage flattens at about 4.6V. 
     Diagram  400 ( c ) illustrates loop activation/deactivation over the same time period. As shown, the digital charge reduction loop  200  is deactivated at t=0 ms, activated at about t=1 ms (when the source voltage drops below the trip voltage threshold), and is deactivated at about t=10.5 ms (when the charge current rises above the maximum charge current controlled by the digital charge reduction loop  200 ). 
     Diagram  400 ( d ) illustrates charge current over the same time period. As shown, from t=0 ms to about t=1 ms (when the system core  170  draw no load current and the analog control circuit  240  is in control), the charge current is stable at about 420 mA. At about t=1 ms (upon detection of the source voltage dropping below the trip voltage threshold, after the load current increases to 300 mA, and after the digital charge reduction loop  200  takes control of the charge current from the analog control circuit  240 ), the charge current drops from 420 mA to a predetermined minimum of about 170 mA. Then, slightly after about t=1 ms (after the source voltage has recovered), the digital charge reduction loop  200  steps up the charge current until the source voltage reaches the safe voltage threshold of about 4.5V at about t=2 ms. At about t=2 ms, the charge current stabilizes at (e.g., oscillates about) about 270 mA. Then, at about t=5 ms (as the digital charge reduction loop  200  recognizes a slight increase in the source voltage due to the decrease in load current from 300 mA to about 230 mA), the charge current begins to step up. At about t=5.5 ms (as the source voltage stabilizes at the safe voltage threshold), the charge current stabilizes at about 350 mA. At about t=10 ms (as the digital charge reduction loop  200  recognizes an increase in the source voltage due to the system core  170  drawing no load current), the digital charge reduction loop  200  steps up the charge current. At about t=10.5 ms (as the analog control circuit  240  takes over control of the charge current from the digital charge reduction loop  200 ), the charge current flattens again at about 420 mA. 
       FIG. 5  is a circuit diagram illustrating details of a battery charger  140 ″ with a digital charge reduction loop  500 , in accordance with another embodiment of the present invention. The battery charger  140 ″ is a second example embodiment of the battery charger  140  of  FIG. 2 . Like battery charger  140 ′, battery charger  140 ″ includes a digital charge reduction loop  500  coupled to an input source  245  (e.g., the VBUS of the USB port  145 ), an analog charge-current control circuit  240 , a multiplexer  225  coupled to the digital charge reduction loop  500  and to the analog charge-current control circuit  240 , and a charge-current limit/control circuit  230  coupled to the input source  245  and to the multiplexer  225 . 
     As shown, the digital charge reduction loop  500  can be implemented with a low range hysteresis using only one comparator, namely, comparator C. In this case, trip and safe voltage thresholds are separated by the hysteresis level. When the source voltage of the input source  245  is maximized (such that the up/down counter  510  is at its maximum count), the multiplexer  245  causes the analog control circuit  240  to control the charge current. When the source voltage falls below the trip voltage threshold (comparator C high to low threshold), the digital charge reduction loop  500  is activated. The output signal of the comparator C goes low when the RS Flip-Flop  215  is at the ‘SET’ stage (Q output logic high), causing the up/down counter  510  to reset, reducing the digital count value of the up/down counter  510  to a predetermined minimum. Resetting the up/down counter  510  also causes the up/down counter  510  to reset the RS flip-flop  215 , which causes the multiplexer  225  to transition charge-current control from the analog control circuit  240  to the digital charge reduction loop  500 . 
     The digital charge reduction loop  500  reduces charge current to a predetermined minimum, thereby causing the source voltage to recover above the safe/trip voltage threshold. Every clock cycle that the output signal of the comparator C is high, the up/down counter  510  steps up the its output count (b0-bn) to the DAC  220 , which controls the charge-current limit/control circuit  230 , which in turn controls the charge current. For every clock cycle that the output signal of the comparator C is low (while the up/down counter  510  is not on its maximum count), the up/down counter  510  counts down. The source voltage stabilizes at (e.g., oscillates about) the trip/safe voltage threshold, causing the charge current to stabilize at the maximum current available. When the up/down counter  510  reaches its maximum count, the up/down counter  510  sends and END signal to the RS flip-flop  215 , which controls the multiplexer  225  to return control of the charge current back to the analog charge-current control circuit  240 . 
     Embodiments of the digital charge reduction loop may provide several advantages over an analog/linear charge reduction loop. For example, trip and safe voltage thresholds may be more easily adjusted, and hysteresis may be added to increase charge reduction loop stability to line and load transient effects with the source supply. The digital charge reduction loop  200 / 500  may provide greater ease of adjusting digital charge reduction loop start-up and response time. The response of the clock oscillator  235 , the up/down counter  210 / 510  speed, and the DAC  220  may be adjusted for the timing needs of a given application. The digital charge reduction loop may be less prone to adverse noise issues commonly associated with DC-DC switching charge-current regulation systems, making the digital charge reduction loop more desirable for implementation in integrated circuits for DC-DC switching battery chargers. Further, for battery charge integrated circuit (IC) applications, a digital charge reduction loop may save space and cost by virtue of the circuit being able to share elements (such as the system clock and counters) with other unrelated control circuits within the IC (such as temperature control). Sharing circuit elements allows for reduced IC die size, thus reducing space needs and manufacturing costs. 
     It will be appreciated that the digital charge reduction loop  200 / 500  is applicable to scenarios not involving battery charging using a USB port as an input source  245 . For example, battery charging can be performed with high-current external power supplies, commonly referred to as “line adaptors”. As stated above, designers may supply a line adaptor with specifications rated to meet or exceed the current requirements of the system and battery charging operations. However, when a non-approved input source  245  is used with a device  110  that has an insufficient current handling capacity, the digital charge reduction loop  200 / 500  may reduce charge current to a safe level, possibly better than the limits of a USB port. 
       FIG. 6  illustrates a voltage/time and current/time diagrams  600  of a digital charge reduction loop managing charge current from a line adapter that is capable of sourcing amounts of current greater than a typical USB port. 
     Diagram  600 ( a ) illustrates load current over a time period of 12 ms. As shown, from t=0 ms to t=1 ms, the system core  170  of the USB device  110  draws no current. From about t=1 ms to about t=5 ms, the system core  170  draws about 1 A of current. From about t=5 ms to about t=10 ms, the system core  170  draws about 700 mA of current. After about t=10 ms, the system core  170  draws no current. 
     Diagram  600 ( b ) illustrates source voltage of the input source  245  over the same time period. As shown, from t=0 ms until about t=1 ms (while the system core  170  draws no load current), the source voltage is stable at about 4.6V. At about t=1 ms (when the system core  170  begins drawing a load current of 1 A), the source voltage drops below the trip voltage threshold of about 4.3V. Immediately thereafter (as the digital charge reduction loop  200 / 500  takes over charge-current control), the source voltage quickly returns to a safe voltage threshold of about 4.4V and becomes stable at about 4.4V. At about t=5 ms (when the system core  170  reduces its load current from about 1 A to about 700 mA), the source voltage rises slightly to about 4.5V. From about t=5 ms to about t=5.5 ms (as the digital charge reduction loop  200 / 500  steps up the charge current), the source voltage lowers again to the safe voltage threshold. From about t=5.5 ms to about t=10 ms, the source voltage remains stable at the safe voltage threshold. At about t=10 ms (after the system core  170  stops drawing any load current), the source voltage quickly rises upward. At about t=10.5 ms (as the analog control circuit  240  takes over charge-current control from the digital charge reduction loop  200 / 500 ), the source voltage flattens at about 4.5V. 
     Diagram  600 ( c ) illustrates loop activation/deactivation over the same time period. As shown, the digital charge reduction loop  200 / 500  is deactivated at t=0 ms, activated at about t=1 ms, and deactivated at about t=10.5 ms. 
     Diagram  600 ( d ) illustrates charge current over the same time period. As shown, from t=0 ms to about t=1 ms (when the system core  170  draws no load current and the analog control circuit  240  is in control), the charge current is stable at about 1.25 A. At about t=1 ms (upon detection of the source voltage dropping below the trip voltage threshold, after the load current increases to 1 A, and after the digital charge reduction loop  200 / 500  takes charge-current control from the analog control circuit  240 ), the charge current drops from 1.25 A to a predetermined minimum of about 500 mA. Then, the digital charge reduction loop  200 / 500  steps up the charge current until the source voltage reaches the safe voltage threshold of about 4.4V. At about t=1.2 ms, the charge current stabilizes at (e.g., oscillates about) about 650 mA. Then, at about t=5 ms (as the digital charge reduction loop  200  recognizes a slight increase in the source voltage due to the decrease in load current from 1 A to about 700 mA), the digital charge reduction loop  200 / 500  begins to step up the charge current. At about t=5.5 ms (as the source voltage stabilizes at the safe voltage threshold), the charge current stabilizes at about 950 mA. At about t=10 ms (as the digital charge reduction loop  200  recognizes an increase in the source voltage due to the system core  170  drawing no load current), the digital charge reduction loop  200 / 500  steps up the charge current. At about t=10.5 ms (as the analog control circuit  240  takes over control of the charge current from the digital charge reduction loop  200 / 500 ), the charge current flattens again at about 1.25 A. 
     By maximizing the charge current possible, the battery charger  140  fully utilizes power available from the particular input source  245 , dramatically reducing charging time. In some embodiments, battery chargers  140  need not be conFIG.d to limit maximum charge current to 500 milliamps, and may rely on digital charge reduction to drop charge current to the appropriate level. Also, if a USB device  110  with a USB controller (not shown) is connected to the USB host  105 , then the USB controller should recognize that it is plugged into a USB port  130  of a computer system and will distinguish between a type A (500 milliamp) port and a type B (100 milliamp) port. However, if a USB device  110  without a USB controller is connected to a USB host  105 , the USB device  110  will be incapable of recognizing that it is plugged into a USB port. (For example, a Bluetooth headset does not have a USB controller, since a Bluetooth headset is not used to exchange data.) Using digital charge reduction, the USB device  110  without a USB controller can be charged by a USB port. With a digital charge reduction loop, the USB device  110  will automatically step-wise reduce current drawn. No external control will be needed. 
     The foregoing description of the preferred embodiments of the present invention is by way of example only, and other variations and modifications of the above-described embodiments and methods are possible in light of the foregoing teaching. Components may be implemented using a programmed general-purpose digital computer, using application specific integrated circuits, or using a network of interconnected conventional components and circuits. The embodiments described herein are not intended to be exhaustive or limiting. The present invention is limited only by the following claims.