Patent Publication Number: US-2019190373-A1

Title: Multi Output Three Level Buck Converter

Description:
BACKGROUND 
     Electronic devices continue their trend of becoming smaller, yet more computationally powerful. As a result, many existing power conversion solutions may be unable to provide desired levels of electrical power within acceptable size constraints. For example, in some devices, the power management integrated circuit (PMIC) and associated passive energy storage components (inductors and/or capacitors) may take up a substantial fraction of the entire device&#39;s area and/or volume. The traditional buck converter topologies that are included in a wide variety of electronic devices rely on inductors for energy storage. However, present passive device construction and materials technologies are such that capacitors can have much higher energy densities than inductors. This has led to interest in power conversion topologies that combine capacitor-based charge pumps with inductive topologies. An advantage of such combinations is that they can allow for “soft charging” of the capacitors in the charge pump, eliminating the ½CdV 2  losses and allowing for use of smaller capacitors than might be required in a traditional charge pump. Another advantage of such combinations is more precise output voltage regulation and improved efficiency beyond what is available with traditional charge pump arrangements. 
     Thus, there is a need for improved power conversion circuits that combine charge pumps with magnetic converters to provide desired power levels with reduced density and increased operating efficiency. 
     SUMMARY 
     One embodiment disclosed herein is a power converter including a charge pump configured to receive an input voltage and generate a flying rail voltage therefrom and a plurality of buck converters each configured to generate a regulated output voltage from the flying rail voltage. The power converter can include an asymmetric controller, the asymmetric controller having a first controller coupled to the charge pump and a first buck converter and configured to control the flying rail voltage and the regulated output voltage of the first buck converter. The asymmetric controller can also include a second controller coupled to a second of the plurality of buck converters, wherein the second control loop is configured to control the regulated output voltage of the second buck converter. The first controller can be a constant on time pulse frequency modulation controller. The power converter can alternatively include a symmetric controller, the symmetric controller having an outer control loop configured to regulate the flying rail voltage and a plurality of inner control loops in communication with the outer control loop. Each inner control loop can be configured to control one of the plurality of buck converters to generate a respective regulated output voltage responsive to one or more signals received from the outer control loop. The outer loop can be configured to regulate the flying rail voltage using a hysteretic controller configured to provide a signal to the plurality of inner control loops indicating whether the inner control loops should control respective buck converters to charge or discharge a capacitor supporting the flying rail voltage. 
     In the power converter described above, the charge pump can include a first charge pump switching device having first and second terminals, the first terminal of the first charge pump switching device being coupled to a first input voltage rail of the power converter. The charge pump can also include a second charge pump switching device having first and second terminals, the second terminal of the second charge pump switching device being coupled to a second input voltage rail of the power converter. The charge pump can also include a flying capacitor having a first flying capacitor terminal coupled to the second terminal of the first charge pump switching device and a second flying capacitor terminal coupled to the first terminal of the second charge pump switching device, wherein a voltage across the flying capacitor is the flying rail voltage. In such a converter, each of the plurality of buck converters can include a first buck converter switching device having first and second terminals, the first terminal of the first buck converter switching device being coupled to the first flying capacitor terminal. The buck converters can also include a second buck converter switching device having first and second terminals, the second terminal of the second buck converter switching device being coupled to the second flying capacitor terminal. The buck converter can also include an inductor having a first inductor terminal coupled to the second terminal of the first buck converters switching device and the first terminal of the second buck converter switching device and a second inductor terminal coupled to an output terminal. 
     In another embodiment, a power converter can include two charge pumps, a first charge pump coupled between an input of the power converter and a first pair of flying rails and configured to generate a first flying rail voltage across the first pair of flying rails, and second charge pump coupled between the first pair of flying rails and a second pair of flying rails and configured to generate a second flying rail voltage across the second pair of flying rails. Such a converter can have at least one buck converter coupled between the first pair of flying rails and configured to generate a first regulated output voltage from the first flying rail voltage, and at least one buck converter coupled between the second pair of flying rails and configured to generate a second regulated output voltage from the second flying rail voltage. The two charge pump converter can include a first controller configured to control the first charge pump and the at least one buck converter coupled between the first pair of flying rails and a second controller configured to control the second charge pump and the at least one buck converter coupled between the second pair of flying rails. One or both of the first and second controllers can be configured to operate at least one corresponding buck converter in a continuous conduction mode. Additionally, the first and second controllers can be implemented as a single controller. 
     Another embodiment disclosed herein relates to a method of generating a plurality of output voltages from an input voltage. The method can include using a charge pump to generate a flying rail voltage from the input voltage and using a plurality of buck converters to convert the flying rail voltage to the plurality of output voltages. Using the charge pump to generate a flying rail voltage from the input voltage can include operating a hysteretic controller to generate control signals sent to the buck converters indicating whether they charge or discharge the charge pump&#39;s capacitor. Using a plurality of buck converters to convert the flying rail voltage to the plurality of output voltages can include operating the plurality of buck converters responsive to one or more signals received from an outer loop controller of the charge pump indicating whether the plurality of buck converters are to charge or discharge a capacitor of the charge pump. Using a plurality of buck converters to convert the flying rail voltage to the plurality of output voltages can also include the use of a predictive control algorithm. 
     In still another embodiment, using a charge pump to generate a flying rail voltage from the input voltage can include using a first charge pump to generate a first flying rail voltage from the input voltage and using a second charge pump to generate a second flying rail voltage from the first flying rail voltage. In the same or other embodiments, using a plurality of buck converters to convert the flying rail voltage to the plurality of output voltages can include using at least one buck converter to convert the first flying rail voltage to a first regulated output voltage and using at least one buck converter to convert the second flying rail voltage to a second regulated output voltage. 
     Yet another embodiment relates to a controller for a power converter having a charge pump and a plurality of buck converters. The charge pump can be configured to receive an input voltage and generate a flying rail voltage therefrom. The plurality of buck converters can each being configured to receive the flying rail voltage and generate a regulated output voltage therefrom. The controller can include an outer control loop configured to regulate the flying rail voltage by generating a signal sent to the buck converters. The controller can also include a plurality of inner control loops in communication with the outer control loop, each configured to control a plurality of switches coupled between the flying rails to generate a respective regulated output voltages responsive to the one or more signals received from the outer control loop. The outer loop can include a hysteretic controller configured to provide a signal to the plurality of inner control loops indicating whether the inner control loops should control respective buck converters to charge or discharge a capacitor supporting the flying rail voltage. 
     Still another embodiment relates to a controller for a power converter having a first charge pump, at least one buck converter coupled to the output of the first charge pump, a second charge pump coupled to the output of the first charge pump, and at least one buck converter coupled to the output of the second charge pump. The controller can include a first controller configured to operate the first charge pump and the at least one buck converter coupled to the output of the first charge pump to generate a regulated output voltage at an output of each of the at least one buck converters coupled to the output of the first charge pump. The controller can also include a second controller configured to operate the second charge pump and the at least one buck converter coupled to the output of the second charge pump to generate a regulated output voltage at an output of each of the at least one buck converters coupled to the output of the second charge pump. At least one of the first and second controllers may be configured to operate at least one corresponding buck converter in a continuous conduction mode. Additionally, the first and second controllers may be implemented as a single controller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic of a three level buck converter. 
         FIG. 2  depicts the switching stages of a three level buck converter. 
         FIG. 3  depicts a schematic of a multi output three level buck converter. 
         FIG. 4  depicts a schematic of a multi output three level buck converter with asymmetric control. 
         FIG. 5  depicts a schematic of a multi output three level buck converter with symmetric control. 
         FIG. 6  depicts a block diagram of a multi output three level buck converter with symmetric control. 
         FIG. 7  depicts waveforms associated with a charge pump of a multi output three level buck converter. 
         FIG. 8A  depicts a state diagram of a buck converter controller for use with a symmetric controller of a multi output three level buck converter. 
         FIG. 8B  depicts various switching states of a three level buck converter operated by symmetric controller. 
         FIG. 9  depicts buck converter waveforms of a multi output three level buck converter corresponding to Case A in  FIG. 8B . 
         FIG. 10  depicts buck converter waveforms of a multi output three level buck converter corresponding to Case B in  FIG. 8B . 
         FIG. 11  depicts buck converter waveforms of a multi output three level buck converter corresponding to Case C in  FIG. 8B . 
         FIG. 12  depicts buck converter waveforms of a multi output three level buck converter corresponding to Case D in  FIG. 8B . 
         FIG. 13  depicts a schematic diagram of a multi output three level buck converter having a second charge pump. 
         FIG. 14  depicts a clock generation technique for a multi output three level buck converter having a second charge pump and symmetric control in which all buck converters operate in discontinuous conduction mode. 
         FIG. 15  depicts a block diagram of a second charge pump controller of a symmetric controller for a multi output three level buck converter having a second charge pump and symmetric control in which all buck converters operate in discontinuous conduction mode. 
         FIG. 16A  depicts a state diagram of a controller for a first charge pump and a CCM buck converter coupled to the output thereof. 
         FIG. 16B  depicts a state diagram of a controller for a second charge pump coupled to the output of the first charge pump and a CCM buck converter coupled to the output of the second charge pump. 
         FIG. 17  depicts a series of logic circuits for generating the gate drive signals for the various switches of a multi output three level buck converter having a second charge pump. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the disclosed concepts. As part of this description, some of this disclosure&#39;s drawings represent structures and devices in block diagram form for sake of simplicity. In the interest of clarity, not all features of an actual implementation are described in this disclosure. Moreover, the language used in this disclosure has been selected for readability and instructional purposes, has not been selected to delineate or circumscribe the disclosed subject matter. Rather the appended claims are intended for such purpose. 
     Various embodiments of the disclosed concepts are illustrated by way of example and not by way of limitation in the accompanying drawings in which like references indicate similar elements. For simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the implementations described herein. In other instances, methods, procedures and components have not been described in detail so as not to obscure the related relevant function being described. References to “an,” “one,” or “another” embodiment in this disclosure are not necessarily to the same or different embodiment, and they mean at least one. A given figure may be used to illustrate the features of more than one embodiment, or more than one species of the disclosure, and not all elements in the figure may be required for a given embodiment or species. A reference number, when provided in a given drawing, refers to the same element throughout the several drawings, though it may not be repeated in every drawing. The drawings are not to scale unless otherwise indicated, and the proportions of certain parts may be exaggerated to better illustrate details and features of the present disclosure. 
     Three Level Buck Converter Topology and Operation 
     A three level buck converter  100  is illustrated in  FIG. 1 . Three level buck converter  100  receives a DC input voltage  101  and converts it to a regulated output voltage Vout at output terminals  103 . Three level buck converter can include a ladder of four switching devices  102 ,  104 ,  106 ,  108  connected across the input voltage rails. The switching devices may be transistors, such as field effect transistors (FETs), metal-oxide semiconductor field effect transistors (MOSFETs), junction field effect transistors (JFETs), insulated gate bipolar transistors (IGBTs), or other types of switching devices. A flying capacitor  105  may be connected between the junction of first and second switches  102 ,  104  and the junction of third and fourth switches  106 ,  108 . An inductor  107  may be connected between the junction of second and third switches  104 ,  106  and positive output terminal  103 . A capacitor  109  may be connected across output terminals  103 . 
       FIG. 2  illustrates at a high level the four phases of operation of a three level buck converter. In Phase I, first and third switches  102  and  106  are turned on, while second and fourth switches  104  and  108  are turned off. This causes current to flow from input voltage source  101 , through first switch  102 , flying capacitor  105 , and third switch  106 , thereby charging flying capacitor  105 . Current also flows thence through inductor  107  to charge output capacitor  109  and deliver power to a load connected across output terminals  103 . 
     In Phase II, third and fourth switches  106  and  108  are turned on, while first and second switches  102  and  104  are turned off. Because the current flowing through inductor  107  cannot change instantaneously, current continues to flow through inductor  107 . A load connected across output terminals  103  continues to receive power from inductor  107  and output capacitor  109 . Current returns to inductor  107  through third and fourth switches  106  and  108 . In other words, during Phase II, the circuit behaves like a synchronous buck converter. Charge on the flying capacitor  105  does not change during Phase II. 
     In Phase III, energy stored in flying capacitor  105  is delivered to the load. Second and fourth switches  104  and  108  are closed, providing a path for current to flow from flying capacitor  105 , through second switch  104  and inductor  107 , to output capacitor  109  and/or the load connected between output terminals  103 . Thus current recharges output capacitor  109  and delivers power to the load. Current returns to flying capacitor  105  via fourth switch  108 . 
     Phase IV is similar to Phase II. Third and fourth switches  106  and  108  are turned on, while first and second switches  102  and  104  are turned off. Because the current flowing through inductor  107  cannot change instantaneously, current continues to flow through inductor  107 . A load connected across output terminals  103  continues to receive power from inductor  107  and output capacitor  109 . Current returns to inductor  107  through third and fourth switches  106  and  108 . Charge on the flying capacitor  105  does not change during Phase IV. 
     It will be appreciated that flying capacitor  105  is not fully discharged (i.e., to 0V) during operation of the circuit. Rather, the circuit may be controlled so that the voltage of the flying capacitor remains at approximately some fraction of the input voltage Vin. For example, the circuit described above may be controlled to maintain flying capacitor  105  at approximately ½Vin. The instantaneous voltage across flying capacitor  105  will actually increase slightly above ½Vin during charging and decrease slightly below ½Vin during discharging. The magnitude of these voltage excursions above and below Vin are parameters that may be selected by a system designer to meet various performance objectives of the system. These performance objectives can include voltage ripple as the input voltage traverses the region of 2*Vout, and the frequency at which the system changes from charging to discharging the flying capacitor. 
     Use of an up-front charge pump to create a rail having a voltage equal to ½Vin provides various advantages with respect to the circuit components used to construct the converter. First, it will be appreciated that inductor energy storage, and therefore inductor size, increases proportionally to the square of the inductor flux (i.e., B 2 ). Inductor flux (B) is proportional to the applied voltage (i.e., ½Vin-Vout for Phases I and III) and Vout for Phases II and IV) times the amount of time (T) that current is driven into the inductor. Thus, by reducing the voltage applied to the inductor from Vin to ½Vin, smaller inductors may be used for any given switching frequency. Additionally, the switching devices need only be rated at ½Vin rather than Vin. In some applications this can allow for implementation with smaller, faster, lower voltage CMOS devices. This can, in some embodiments, mitigate the disadvantages of requiring two such switches in series, as compared to only one in a traditional buck converter topology. 
     Multi Output Three Level Buck Converter Topology 
     In some applications, it may be desirable to use the three level buck converter topology to provide a multiplicity of regulated output voltages. As an example, suppose that one wished to provide four regulated output voltages using this topology. One approach to such applications would be to implement four separate three level buck converters. For some applications, this approach may be undesirable because it would require sixteen switching devices and four flying capacitors and the associated routing on and off the chip as well as an inductor and capacitor for each output. Even in fairly low power applications, where the switching devices can be integrated into the same silicon as the controller, the additional flying capacitors and interconnect may increase the required volume beyond that which is available for a given design. Thus, it would be desirable to provide a single input charge pump stage that drives a plurality output buck stages. 
       FIG. 3  illustrates a multi output three level buck converter having a single input charge pump stage driving a plurality of output buck stages. A DC input voltage may be applied to input rails  301   a  and  301   b.  Charge pump switches Qa ( 302 ) and Qb ( 308 ) may be operated to charge and discharge flying capacitor  305 , which supports charge pump flying rails  310   a  and  310   b.  A plurality of buck converters may be connected to these flying rails. In the illustrated embodiment, four buck converters are provided, although more or fewer such buck converters may be provided. 
     Each buck converter receives its input voltage from flying rails  310   a  and  310   b  and produces a regulated output voltage at its output ( 303   a - 303   d ) by operating their high switches Q 2   a -Q 2   d  ( 304   a - 304   d ) and low side switches Q 3   a -Q 3   d  ( 306   a - 306   d ) as described in greater detail below. High side switches Q 2   a -Q 2   d  ( 304   a - 304   d ) are configured with their drain terminals coupled to the high flying rail  310   a  and their source terminals coupled to the drain terminals of low side switches Q 3   a -Q 3   d  ( 306   a - 306   d ), respectively. Low side switches Q 3   a -Q 3   d  ( 306   a - 306   d ) are configured with their drain terminals coupled to the source terminals of high side switches Q 2   a -Q 2   d  ( 304   a - 304   d ), respectively, and their source terminals coupled to low flying rail  310   b.  For each buck converter, the junction point of the high side switch&#39;s source terminal and the low side switch&#39;s drain terminal is coupled to a first terminal of inductor  307   a - 307   d.  The second terminal of this inductor  307   a - 307   d  is coupled to output capacitor  309   a - 309   d  and output terminal  303   a - 303   d.  The operation of high side switches Q 2   a -Q 2   d  ( 304   a - 304   d ) and low side switches Q 3   a -Q 3   d  ( 306   a - 306   d ) controls the current flowing through inductors  307   a - 307   d  and thus the voltage across output capacitors  309   a - 309   d.    
     In some embodiments, the high side switches Q 2   a -Q 2   d  ( 304   a - 304   d ) and low side switches Q 3   a -Q 3   d  ( 306   a - 306   d ) may be implemented with four-terminal MOSFETs having their body terminals coupled to ground. Tying the 4 th  terminal (body) to ground eliminates the parasitic diode associated with a source-body connection being connected to the inductors, thereby maintaining the ability to control the voltage applied to the inductors. 
     Asymmetric Control of a Multi Output Three Level Buck Converter 
     Controlling the multi output three level buck converter illustrated in  FIG. 3  is a multi-dimensional challenge. In a single output three level buck converter as illustrated in  FIG. 1 , there are two parameters that must be controlled: (1) the voltage across the flying capacitor  105 , and (2) the output voltage at output terminal  103 . In some embodiments, the voltage across the charge pump flying capacitor may be regulated to Vin/ 2  plus or minus some tolerance. As described with reference to  FIG. 2 , Phases I and II constitute a complete charge/discharge cycle for output inductor  107  while charging flying capacitor  105 , and Phases III and IV constitute a complete charge/discharge cycle for output inductor  107  while discharging flying capacitor  105 . Appropriately balancing these two phase pairs allows the flying capacitor voltage to be kept in regulation, while simultaneously keeping the output voltage in regulation. This balancing requires coordinated control between switching devices Q 1  and Q 4  and switching devices Q 2  and Q 3 . In a simple three level buck converter, the charge pump charge cycle (i.e., Phase I or III) occurs at a duty cycle D that is appropriate for the buck converter. (Duty cycle D represents the ratio of Phases I/II or III/IV and sets the gain of the buck converter.) With a single buck converter coupled to the charge pump, this mode of control works fine. However, with two buck converters coupled to a single charge pump (i.e., the flying rails of the charge pump) they cannot both control the duty cycle D of the charge pump, requiring an adaptation of the control strategy employed in a simple three level buck converter. 
     One approach to implementing such a control system for a multi output three level buck converter is to adapt the constant on time (“COT”) pulse frequency modulation (“PFM”) controller described in Chapter  2  of Cassidy, Brian Michael, “A Constant ON-Time 3-Level Buck Converter for Low Power Applications” (2015) (“Cassidy”), which is hereby incorporated by reference in its entirety. Cassidy&#39;s COT PFM controller is intended for use with a three level buck converter having only a single output stage. To adapt that controller for use with a multi output three level buck converter as illustrated in  FIG. 3 , an assumption may be made, with corresponding operational constraints placed on the multi output three level buck converter. The assumption being that the first output buck converter stage is always present (i.e., always in operation and loaded) and its energy requirements dominate the multi output converter (i.e., it has a larger output power requirement than the remaining stages). With this assumption, Cassidy&#39;s COT PFM controller may be implemented asymmetrically (i.e., to control the input charge pump stage and the first buck converter stage, with the remaining buck converter stages “along for the ride”). As used herein, “asymmetric” means that one buck stage is controlled in coordination with the charge pump stage, and remaining buck stages are slaved to the switching of the charge pump stage, with the attendant consequence that the one coordinated buck stage must be the dominant stage. 
     More specifically, with reference to  FIG. 4 , the input charge pump stage (made up of high and low side switches  302  and  303  and flying capacitor  305 ) and the first output buck stage (made up of high and low side switches  304   a  and  306   a,  output inductor  307   a  and output capacitor  309   a ) may be controlled by a COT PFM controller  402 , such as that described in Cassidy, or indeed any other control system suitable for a conventional three level buck. The remaining output buck converter stages (made up of high and low side switches  304   b - 304   d  and  306   b - 306   d,  output inductors  307   b - 307   d,  and output capacitors  309   b - 309   d ) may have their own buck controllers  404   b - 404   d.  Buck controllers  404   b - 404   d  may be conventional buck controllers that operate to generate the desired output voltage at the respective outputs  303   b - 303   d  while slaved to the operation of the charge pump stage (because the charge pump stage is controlling the state of the flying rails). However, because the main controller is controlling the duty cycle of the charge pump, the slave controllers cannot rely on any specific pulse width of the charge pump controller and so cannot be used to help regulate the voltage of the flying capacitor/flying rails. Furthermore, whereas the main buck converter may be operated in either continuous or discontinuous mode, the slaved converters must be operated in discontinuous mode, and must always require duty cycle D&lt;Dmain (the duty cycle of the main controller). 
     Symmetric Control of a Multi Output Three Level Buck Converter 
     As noted above, an asymmetric control implementation may be based on the assumption that the first buck converter stage&#39;s load is always present and dominates the multi-output controller. As a result, operation of a multi output three level buck converter controlled in this fashion may have operating constraints relating to a minimum current from the first stage as well as maximum currents from the remaining stages. In some embodiments, these constraints may be undesirable. Thus, a symmetric controller arrangement, in which each output is treated the same and each output may have any output power draw with good cross regulation. As used herein, “symmetric” means that each buck converter may supply an arbitrary amount of current/power to its load. 
     Additionally, it may be desirable for such a controller to be able to accommodate a wide range of input voltages. For example, if the multi output three level buck converter is to be powered by a lithium ion cell, it could be expected that the input voltage could range from 4.2V (or more) when the cell is charging, down to 3.2V or less at the end of charge. In such an embodiment, the flying voltage rails  310   a  and  310   b  could have a voltage ranging from a high of 2.1V or more to 1.6V or less. For cases in which the converter is powered by multiple cells in series, the input voltages may be an integer multiple of these values. For cases in which another cell chemistry is used, the voltages may vary accordingly. In any event, because the output voltages at the various outputs  303   a - 303 D may be either greater than or less than ½Vin, the control system may preferably be intelligent about the timing of the buck converter switch transitions relative to the state of the charge pump flying rails. For example, at higher input voltages it may be desirable to switch the inductor between ½Vin and 0V, while at lower input voltages it may be desirable to switch the inductor between Vin and ½Vin. 
     Described below is an integrated symmetric controller that operates all switching devices for a multi output three level buck converter. With reference to  FIG. 5 , symmetric controller  502  provides gate drive signals for switches Qa and Qb ( 302 ,  308 ), which, with capacitor  305 , make up the charge pump stage. Switches Qa and Qb are operated in coordination with the current draw from the buck converters (as described in greater detail below) to regulate the voltage appearing across flying capacitor  305 . The voltage across flying capacitor  305  is the voltage between flying rails  310   a  and  310   b  and the input voltage to each of the buck converter stages. Symmetric controller  502  also provides the gate drive signals to each of the buck converter stage&#39;s high side switches Q 2   a -Q 2   d  (a/k/a  304   a - 304   d ) and low side switches Q 3   a -Q 3   d  (a/k/a  306   a - 306   d ). The described control algorithm has each buck converter operating in a discontinuous current mode (DCM), which is suitable for many low power applications. In alternative embodiments, one buck converter attached to a charge pump may be operated in continuous current mode (CCM), while all other buck converters attached to that charge pump operate in DCM. Additionally, in some embodiments disclosed below, multiple charge pumps may be provided, with one buck converter coupled to each charge pump operating in the CCM and additional buck converters coupled to those charge pumps operating in DCM. 
     Symmetric controller  502  may be implemented in various ways, including digital, analog, or hybrid digital/analog circuitry. The controller may be constructed from discrete components, integrated circuits, or combinations thereof, and by fixed logic devices, programmable logic devices, or field programmable gate arrays (FPGAs) and the like. In some embodiments, symmetric controller  502  and the switching devices for the charge pump stage and the buck converter stages may be constructed as a single integrated circuit, with only passive components such as the identified inductors and capacitors being external to the IC. In still other embodiments, some or all of the passive components may be integrated with the controller and switching components, although this may not be desirable in all embodiments for efficiency or other reasons. Thus, the foregoing description of symmetric controller  502  focuses on the control loops, algorithms, and logic implemented by the controller, without regard to any particular physical form that such controller may take. 
       FIG. 6  depicts, in block diagram form, a multi output three level buck converter implementing a symmetric controller  620 . Power flow through the converter is depicted using dashed lines, and control signals are depicted using solid lines. The upper portion of  FIG. 6  depicts the power components of the multi-output three level buck converter. In the illustrated example, an input voltage Vin is received by charge pump  611 . The flying rails Vfly of charge pump  611  are coupled to four buck converters  613   a - 613   d.  Buck converters  613   a - 613   d  produce output voltages Vout 1 -Vout 4 , respectively. It will be appreciated that more or fewer buck converters may be provided in a given implementation. The lower portion of  FIG. 6  depicts symmetric controller  620 . Symmetric controller  620  includes an outer loop charge pump controller  630  and inner loop buck converter controllers  640   a - 640   d,  each corresponding to one of the buck converters  613   a - 613   d.    
     Symmetric controller  620  implements a separate outer loop controller  630  to regulate the charge pump  611 . This allows the four buck converters  613   a - 613   d  to operate independently of one another (but not independently of charge pump  611 ). In other words, the charge pump regulator is not slaved to any one of the buck converters  613   a - 613   d  and/or respective controllers  640   a - 640   d.  First, outer loop controller  630  can generate a “steer” state signal  634  having two states, “steer” and “˜steer,” discussed in greater detail below. Second, the outer loop controller  630  can generate a system clock  636  that controls the timing (i.e., switching frequency and phase) of the charge pump and the buck converters. Third, the outer loop controller  630  can control the pulse width of the charge pump switching signals. 
     Steer state signals  634  can be used to instruct the buck converter controllers  640   a - 640   d  to either discharge (steer) or charge ˜(steer) the charge pump capacitor. These operations are described in greater detail below. In one embodiment, steer signal  634  may be generated by implementing a hysteretic regulator around the voltage across the flying capacitor ( 305 ). A simple hysteretic regulator may be constructed from a comparator and a voltage reference. The comparator generates an output signal based on comparing the feedback voltage (e.g., the voltage across flying capacitor  305 ) to the reference voltage (e.g., ½Vin). When the flying capacitor voltage is greater than reference voltage, the comparator generates the steer signal, instructing the buck converter controllers to cause the buck converters to discharge the flying capacitor. When the flying capacitor voltage is less than the reference voltage, the comparator generates the steer signal, instructing the buck converters to charge the flying capacitor. Adding hysteresis to this comparator forms a hysteretic controller. In some embodiments, it may be desirable that the transition between the steer and steer states occur at a point in time when none of the buck converter switches are turned on. Otherwise, miss-switching may occur at the steer to steer state transition boundary. Symmetric controller  630  may also implement other controllers or controller types for generating the steer signals  634   a - 634   b  for the respective buck controllers  640   a - 640   d.    
     Outer loop charge pump controller  630  can also generate a variable frequency clock  636  for use internally within outer loop charge pump controller  630  and throughout symmetric controller  620  in what is a form of pulse frequency modulation (PFM) that works well with the discontinuous current mode (DCM) operation of the buck converters. Variable frequency clock  636  may be responsive to the maximum error of the buck converters, such that a large maximum error results in a higher switching frequency, while a smaller maximum error results in a lower switching frequency. The variable clock frequency could also be proportional to the load of the most heavily loaded buck converter. Thus, outer loop charge pump controller  630  may be configured to receive an error signal  638  from each of the buck converter controllers  640   a - 640   d.  Further, outer loop charge pump controller  630  may include a comparator/selector circuit for identifying which of the buck converter error signals has the greatest maximum error. A variable frequency clock (VCO)  1412 ;  FIG. 14 ) may be used to control the switching frequency the charge pump and the buck converters, which may be configured to run at the same frequency. Thus, the variable frequency clock signal may be provided to each of the inner loop buck controllers  640   a - 640   d.    
     Charge pump pulse width, i.e., the on time of the charge pump switching devices  302  and  304 , may also be controlled by the outer loop charge pump controller  630 . In some embodiments it may be desirable to have this pulse width as a constant, with all control of the charge pump being controlled by the switching frequency, i.e., variable frequency clock  636 . For example, a pulse width set at a constant 250 ns would result in a 50% duty cycle at maximum switching frequency of 2 MHz, although other values may be selected. Alternatively, the charge pump pulse width may be a controlled variable, rather than a constant. Such dynamic control of the charge pump pulse width could be used to improve the efficiency of the converter and/or reduce the output ripple. Alternatively, the charge pump pulse width may be controlled to be equal to the required duty cycle of one of the converters, thus allowing one of the converters to operate in continuous current mode. It should be noted, however, that the predictive control for States A and D, described below, relies on the inner regulators knowing this pulse width. 
     Outer loop charge pump controller also generates an output signal  621  that is the gate drive signals for the charge pump FETs  302  and  304 . These gate control signals may be produced by a gate drive controller implemented within outer loop charge pump controller  630  and responsive to the variable frequency clock signal and pulse width signals discussed above. 
     Exemplary waveforms for the charge pump stage of such an embodiment are illustrated in  FIG. 7 . Waveform  710   a  depicts the voltage of high flying rail  310   a,  and waveform  710   b  depicts the voltage of low flying rail  310   b.  The difference between these two voltages is the voltage across the flying capacitor  305 . The steering signal is depicted in three zones:  734   a,  having a low voltage corresponding to the “˜steer” state in which the buck converters are charging the flying capacitor;  734   b,  having a high voltage corresponding to the “steer” state in which the buck converters are discharging the flying capacitor; and  734   c,  having a low voltage again corresponding to the “˜steer” state. As can be seen, the high flying rail  310   a  is switching between Vin and ½Vin, and the low flying rail  310   b  is switching between ½Vin and 0V. The steering signal  734  is toggling between its two states at a rate determined by the hysteresis of its regulator, and the output power of the buck converters. At low output powers the toggling frequency will be low, while at high output powers the toggling frequency will be higher. The toggling frequency will be inversely proportional to the hysteresis of the regulator, i.e., the larger the hysteresis, the lower the toggling frequency, for a given output power. 
     As illustrated in  FIG. 6 , each buck converter  613   a - 613   d  has a corresponding inner loop buck controller  640   a - 640   d.  A state diagram of an exemplary inner loop buck controller is diagrammed in  FIG. 8A .  FIG. 8B  illustrates the switching states of a buck converter implementing such a controller, along with partial schematics illustrating current flow for inductor charge (upper) and discharge (lower) for each case. Each inner loop buck controller may implement an identical controller. Alternatively, in some embodiments, if it is known that the target voltage/setpoint of the buck converter (i.e., Vset) is always greater than the flying rail/flying capacitor voltage (e.g., ½Vin), then the half of the inner loop buck controller corresponding to the case in which the target voltage/setpoint of the buck converter is less than the flying rail/flying capacitor voltage may be omitted. Conversely, if the target voltage/setpoint of the buck converter (i.e., Vset) will always be less than the flying rail/flying capacitor voltage (e.g., ½Vin), then the half of the inner loop buck controller corresponding to the case in which the target voltage /setpoint of the buck converter is greater than the flying rail/flying capacitor voltage may likewise be omitted. The following description focuses on the general case in which the target voltage/setpoint of the buck converter (i.e., Vset) may be greater than or less than the flying rail/flying capacitor voltage (e.g., ½ Vin). 
     Each inner loop buck controller  640   a - 640   d  receives from its respective buck converter an input  614   a - 614   d  that is a feedback signal indicating output voltage of the buck regulator. In other embodiments, this feedback signal could include a load on the buck converter or an output current of the buck converter. In any case, the controller can use this feedback signal to derive the required switching signals to keep the buck converter&#39;s output in regulation. For example, each inner loop buck controller  640   a - 640   d  can implement a voltage control loop. This voltage control loop can derive from its respective feedback signal  614   a - 614   d  and its setpoint an error signal “err”  638 . In some switching states, this error signal  638  may be used as a peak current target (like current mode control) for the current through inductor  307 . However, for other switching states, the time at which the rising current ramp is terminated is not under control of the inner loop buck regulators  640   a - 640   d,  but rather under the control of outer loop charge pump controller  630 ). In those switching states, to control the buck converters  613   a - 613   d,  the inner loop buck controllers  640   a - 640   d  must use the known rate at which the current ramps (which is determined by the input and output voltages and the inductance values of the buck converters as described below) to calculate the time at which the current ramp should begin. It should be noted that the inner loop controllers also are looking at inductor current. For the ‘current mode control’ control states (B and C, discussed below), the inductor current is compared to the error and used to terminate the inductor charging state (i.e., States B1 and C1). For all four control states the inductor current value is used to terminate the inductor discharging state (A2-D2). Negative values of inductor current should be prevented, as negative inductor current would discharge the output capacitor. 
     Each inner loop buck controller  640   a - 640   d  also receives three inputs (collectively labeled  634 ) from the outer loop charge pump controller  630 . These inputs are: the steer signal discussed above, the clock signal discussed above, and a variable associated with the pulse width (the time high side charge pump switch Qa  302  is on), which the inner loop buck controllers  640   a - 640   d  use to know when the charge pump will change stage for use with the predictive control algorithm. Each inner loop buck controller has another output in addition to error signal  638  discussed above, namely the switching gate drive signals  644   a - 644   d,  which are generated as described below and provided to the respective buck converters  613   a - 613   d.    
     As noted above,  FIG. 8A  illustrates a state diagram of an inner loop buck converter controller. The inner loop buck converter controller has five states. State 0,0 ( 850 ) corresponds to the state in which both buck converter switches (i.e., switches Q 1  ( 304 ) and Q 2  ( 306 ) in  FIG. 8B ) are off. In state 0,0 ( 850 ) charge pump switches Qa ( 302 ) and Qb ( 308 ) may be in any state as determined by outer loop charge pump controller  630 . State A1,B1 ( 860 ) corresponds to the first state of Cases A and B (described further below with reference to  FIG. 8B ) in which charge pump switch Qa ( 302 ) and buck converter switch Q 1  ( 304 ) are on. State A2,C1 ( 870 ) corresponds to the second state of Case A and the first state of Case C (described further below with reference to  FIG. 8B ) in which charge pump switch Qb ( 308 ) and buck converter switch Q 1  ( 304 ) are on. State B2,D1 ( 880 ) corresponds to the second state of Case B and the first state of Case D (described further below with reference to  FIG. 8B ) in which charge pump switch Qa ( 302 ) and buck converter switch Q 2  ( 306 ) are on. State C2,D2 ( 890 ) corresponds to the second state of Cases C and D (described further below with reference to  FIG. 8B ) in which charge pump switch Qb ( 308 ) and buck converter switch Q 2  ( 306 ) are on. Operation of the buck converter controller and transitions between the states are as described below with reference to  FIGS. 8A and 8B . 
     Case A 
     0,0:A1-- err&gt;vdeadband &amp; Vout&gt;Vin/2 &amp; Steer=True &amp; Pred_Timer=↑ 
     In Case A (described further below), the buck converter controller operates in discontinuous current mode cycling among three states. The first state is State 0,0 ( 860 ), in which the buck converter has zero current. The second state is State A1 ( 860 ) in which buck inductor  307  is charged from the high input voltage rail Vin using buck converter switch Q 1  ( 304 ) during the time period that charge pump high side switch Qa ( 302 ) is closed. The third state is A2 ( 870 ) in which buck inductor  307  and flying capacitor  305  are discharged using buck converter switch Q 1  ( 304 ) during the time period that charge pump low side switch Qb ( 308 ) is closed. Starting from State 0,0 ( 850 ), the buck controller transitions to state A1 (via transition 0,0:A1) when the following conditions are met:
         the buck controller&#39;s error signal “err” is greater than a deadband “Vdeadband” AND   the output setpoint voltage of the buck converter is greater than one-half the input voltage (i.e., Vout&gt;½Vin) AND   the “steer” signal generated by outer loop charge pump controller  630  is high (indicating that the buck converter should be operated to discharge flying capacitor  305 ) AND   buck controller&#39;s predictive timer has transitioned high, indicating that it is time to begin charging output inductor  307 .
 
Each of these four conditions is explained further below.
       

     The first condition that must be satisfied for the 0,0:A1 transition to occur is that the buck controller&#39;s error signal be greater than a deadband. The buck controller may implement any of a variety of output controllers that compare the buck converters desired or setpoint output voltage (Vset) to its actual voltage (Vout) and implements an selected control law to control operation of the switches to keep the output voltage at the setpoint. In some embodiments, the control law may be a proportional-integral (i.e., PI control loop), although other control laws may also be used. Because the overall switching frequency of the system is proportional to the most heavily loaded converter (via the outer loop controller), A mechanism is required to reduce the switching frequency of the less heavily loaded converters. This is the function of the deadband. By requiring that a converter have some error greater than this deadband value, the less heavily loaded converters will switch at some sub-multiple of the main clock, with each switching event transferring a larger “packet” of energy than if it were switching at the main clock frequency. This reduces switching loss, which improves overall efficiency. 
     The second condition that must be satisfied for the 0,0:A1 transition to occur is that the output setpoint voltage of the buck converter be greater than one half the input voltage. If the output setpoint voltage is less than one half the input voltage, the buck converter controller will operate in one of Cases C or D, as described below. 
     The third condition that must be satisfied for the 0,0:A1 transition to occur is that the “steer” signal generated by outer loop charge pump controller be high, indicating that the buck converters should be operated to discharge flying capacitor  305 . If the “steer” signal is low, indicating that the buck converters should operate to charge flying capacitor  305 , the buck converter controller will operate in one of cases B or C, as described below. 
     The fourth condition that must be satisfied for the 0,0:A1 transition to occur is that the buck controller&#39;s predictive timer have transitioned high, indicating that it is time to begin charging output inductor  307 . In Case A, the buck converter controller does not have full control of the switches that control the current through output inductor  307  because the buck converter controller does not have control of the transitions of charge pump switches Qa ( 302 ) and Qb ( 308 ), which are controlled by outer loop charge pump controller  630  as described above. However, the buck converter controller can know the time at which the charge pump will transition from the high switch Qa ( 302 ) to low switch Qb ( 308 ), which information is received from outer loop charge pump controller  630  as described above. As a result, the buck converter controller can calculate (i.e., predict) the time at which buck converter high switch Q 1  ( 304 ) should be turned on to deliver sufficient energy to output inductor  307  by the time that the charge pump switch transition takes place. This prediction signal is then the final condition that must be satisfied for the 0,0:A1 transition. 
     As described above, in State A1 ( 860 ), the buck converter controller turns on high buck converter switch Q 1  ( 304 ). This begins charging output inductor  307  from the high input voltage rail through charge pump switch Qa ( 302 ), which was already turned on by outer loop charge pump controller  630 . 
     A1:A2-- Qb=↑ 
     Once in State A1 ( 860 ), the buck converter controller will transition to State A2 ( 870 ) when the charge pump transitions state. (As described above, the charge pump transitions are controlled by outer loop charge pump controller  630 .) More specifically, when the charge pump controller detects that charge pump switch Qb ( 308 ) has turned on, it makes the A1:A2 transition. As described above, in State A2 ( 870 ), the buck converter turns on low buck converter switch Q 2  ( 306 ). This begins discharging output inductor through flying capacitor  305 . This also has the effect of discharging flying capacitor  305  (as directed by the steer signal). 
     A2:0,0-- I(L)&lt;0 
     Once in State A2 ( 870 ), the buck converter controller will transition to State 0,0 ( 850 ) when output inductor  307  has completely discharged, i.e., when the inductor current becomes zero. To transition from State A2 to State 0,0 Q 2  ( 306 ) must be turned off, otherwise the inductor current will continue decreasing (and go negative). To do so, one can either sense the inductor current or use a predictive algorithm similar to that already described. This is the “discontinuous” current of the DCM mode of operation. Assuming that the first three conditions discussed above remain true, the buck converter controller will transition again to the A1 state when next indicated by the predictive controller. Otherwise:
         if the buck controller&#39;s error signal becomes less than the deadband, the converter will remain idle; OR   if the buck controller output setpoint changes to less than ½Vin, then the buck controller will transition to Case C or D operation as described below; OR   if the steer signal transitions low, then the buck controller will transition to Case B or D operation as described below.       

     A2:B2-- Qa=↑* 
     Ordinarily in State A2, the current in the inductor will decay to 0 and the controller will transition to State 0,0 before Qa is turned on again by the outer loop. However, in some cases (e.g., when Vout is very close to ½Vin) it may be possible for the inductor to discharge sufficiently slowly that it does not reach zero current before the charge pump switches transition from Qb ( 308 ) on to Qa ( 302 ) on. In those cases, to prevent uncontrolled current, the controller will transition from State A2 to State B2. 
     Case A, corresponding to block  822 , arises when the target voltage/setpoint (Vset) of the buck converter is greater than the flying rail/flying capacitor voltage (e.g., ½ Vin) and the outer loop charge pump controller is providing a steering signal indicating that flying capacitor  305  should be discharged by buck inductor  307 . The buck controller thus alternates between: (1) charging buck inductor  307  from the high input voltage rail Vin (using switch  304  during the time period that charge pump high side switch  302  is closed and (2) discharging buck inductor  307  and flying capacitor  305  (using switch  304  during the time period that charge pump low side switch  308  is closed). For case A, the inductor current cannot be controlled directly, so a predictive control algorithm may be implemented. The transition from inductor charge (ramp up current) to discharge (ramp down current) occurs when charge pump FETs  302  and  308  change state, which is controlled by the outer loop charge pump controller discussed above, and therefore is not under control of the buck converter control loops. Thus, for Case A, the proper switching time may be calculated using knowledge of the input and output voltages and inductor value. This calculated on time may then be used to determine the delay between the charge pump switch transition and the turn on time of the buck converter charging switch, i.e., a predicted turn on time. The reversal of state from charging inductor  307  to discharging inductor  307  will then be determined by the transition of the charge pump switches  302  and  308 , as described above and illustrated in the waveforms of  FIG. 9  discussed below. 
     Waveforms corresponding to Case A are illustrated in  FIG. 9 . The voltage of the high flying rail  710   a,  low flying rail  710   b,  and steering signal  734   a  are as described above with respect to  FIG. 7 . The steering signal waveform is constant at 1.0V, because for Case A the steering signal always indicates that the regulator is to discharge flying capacitor  305 . Waveform  902  illustrates the voltage appearing at the junction of buck converter high side switch  304  and buck converter low side switch  306 , which is determined by the switching states described above with respect to  FIG. 8 . Waveform  904  illustrates the current flowing through buck inductor  307 . It can be seen that the inductor is operated in discontinuous mode using predictive switching control as described above. 
     For instance, at time T=291.84 μs: switch Qa ( 302 ) is turned on, pulling the high flying rail  310   a  voltage V(fhi)  710   a  up to approximately Vin and the low flying rail  310   b  voltage V(flo)  710   b  to approximately ½Vin. Then, at time T=292 μs switch Q 1  ( 304 ) is turned on by the inner loop controller, using its predictive current algorithm, initiating the ramping of current in inductor  307 , at rate (Vin-Vout)/L. At time T=292.08 μs: switch Qa ( 302 ) is turned off, and switch Qb ( 308 ) is turned on, by outer loop charge pump controller  630 . Switch Q 1  ( 304 ) remains on. The voltage V(fhi) of high flying rail  310   a  transitions from approximately Vin to approximately ½Vin. Now, the voltage across inductor  307  is (½Vin-Vout)/L. Because Vout&gt;½Vin, the inductor voltage is negative, so the current in inductor  307  ramps down until it hits 0 at T=292.2 μs. 
     Case B 
     0,0:B1-- err&gt;vdeadband &amp; Vout&gt;Vin/2 &amp; Steer=False &amp; Qa=↑ 
     In Case B (described further below), the buck converter controller operates in discontinuous current mode cycling among three states. The first state is 0,0 State  850 , in which the buck converter has zero current. The second state is State B1 ( 860 ) (having the same switch positions as State A1, discussed above) in which buck inductor  307  is charged from the high input voltage rail Vin using buck converter switch Q 1   304  during the time period that charge pump high side switch Qa  302  is closed. The third state is B2 ( 880 ) in which buck inductor  307  is discharged and flying capacitor  305  is charged using charge pump switch Qa ( 302 ) and buck converter switch Q 2  ( 306 ). Starting from State 0,0 ( 850 ), the buck controller transitions to State B1 ( 870 ) (via transition 0,0:B1) when the following conditions are met:
         the buck controller&#39;s error signal “err” is greater than a deadband “Vdeadband” AND   the output setpoint voltage of the buck converter is greater than one-half the input voltage (i.e., Vout&gt;½Vin) AND   the “steer” signal generated by outer loop charge pump controller  630  is low (indicating that the buck converter should be operated to charge flying capacitor  305 ) AND   charge pump high side switch Qa  302  has turned on.
 
The first two conditions were explained above with reference to Case A. The remaining two conditions are explained further below.
       

     The third condition that must be satisfied for the 0,0:B1 transition to occur is that the “steer” signal generated by outer loop charge pump controller be low, indicating that the buck converters should be operated to charge flying capacitor  305 . If the “steer” signal is high, indicating that the buck converters should operate to charge flying capacitor  305 , the buck converter controller will operate in one of Cases A or D, as described elsewhere herein. 
     The fourth condition that must be satisfied for the 0,0:B1 transition to occur is that the charge pump&#39;s high side switch Qa ( 302 ) have turned on. This means that the controller can begin charging the output inductor. 
     As described above, in State B1 ( 860 ), the buck converter controller turns on high buck converter switch Q 1  ( 304 ). This begins charging output inductor  307  from the high input voltage rail through charge pump switch Qa ( 302 ), which was turned on by outer loop charge pump controller  630 , triggering the transition to state B1 ( 870 ). 
     B1:B2-- I(L)&gt;err 
     Unlike Case A (i.e., States 0,0; A1; A2), described above, in Case B (i.e., States 0,0; B1; B2) the buck converter controller has complete control of the inductor current. Having begun charging output inductor  307  on entry into State B1 ( 860 ), the controller can transition to State B2 ( 880 ) when the inductor current reaches its target value. For example, if the controller is implementing peak current mode control using a PI control loop like that described above, it would initiate the B1:B2 transition when the inductor current reached the peak current value corresponding to the error signal “err.” This transition results in the opening of buck converter switch Q 1  ( 304 ) and the closing of buck converter switch Q 2  ( 306 ), which begins the discharge of output inductor  307  (and the charging of flying capacitor  305 ). 
     B2:0,0-- I(L)&lt;0 
     Once in State B2 ( 880 ), the buck converter controller will transition to State 0,0 ( 850 ) when output inductor  307  has completely discharged, i.e., when the inductor current becomes zero. This is the “discontinuous” current of the DCM mode of operation. Assuming that the first three conditions discussed above remain true, the buck converter controller will transition again to the B1 state when charge pump switch Qa ( 302 ) next closes. Otherwise: 
     if the buck controller&#39;s error signal becomes less than the deadband, the converter will remain idle; OR
         if the buck controller output setpoint changes to less than ½Vin, then the buck controller will transition to Case C or D operation as described below; OR if the steer signal transitions high, then the buck controller will transition to Case A or C operation as described below.       

     Case B, corresponding to block  824 , arises when the target voltage/setpoint (Vset) of the buck converter is greater than the flying rail/flying capacitor voltage (e.g., ½ Vin) and the outer loop charge pump controller is providing a steering signal indicating that the flying capacitor  305  should be charged by buck inductor  307 . The buck controller thus alternates between: (1) charging the buck inductor from the high input voltage rail Vin (using switch  304 ) and (2) charging the flying capacitor  305  by discharging buck inductor  307  (using switches  306  and high side charge pump switch  302 ). In case B, the current through buck inductor  307  can be controlled directly, using peak current mode control, for example. In such an embodiment, the switching transition from buck converter high side switch  304  to buck converter low side switch  306  may be made when the inductor current reaches its target. 
     Waveforms corresponding to Case B are illustrated in  FIG. 10 . The voltage of the high flying rail  710   a,  low flying rail  710   b,  and steering signal  734   a  are as described above with respect to  FIGS. 7 and 8 . The steering signal waveform is constant at 0.0V, because for Case B the steering signal always indicates that the regulator is to charge flying capacitor  305 . Waveform  1002  illustrates the voltage appearing at the junction of buck converter high side switch  304  and buck converter low side switch  306 , which is determined by the switching states described above with respect to  FIG. 8 . Waveform  1004  illustrates the current flowing through buck inductor  307 . It can be seen that the inductor is operated in discontinuous mode, using peak mode current control as described above. 
     For instance, at time T=307.75 μs, both switches Qa ( 302 ) and Q 1  ( 304 ) are turned on (the former by outer charge pump control loop  630 , the latter by the inner loop  640   a ). This pulls the voltage V(fhi)  710   a  of flying rail  310   a  up to approximately Vin and the voltage V(flo)  710   b  of flying rail  310   b  to approximately ½Vin. This also initiates the ramping of current in inductor  307 , at a rate (Vin-Vout)/L. Then, at time T=307.8 μs, Q 1  ( 304 ) is turned off, and Q 2  ( 306 ) is turned on, both by inner loop buck controller  640   a,  in response to the current in inductor  307  reaching its target. At this point, the voltage across inductor  307  is (½Vin-Vout)/L. Because Vout is greater than ½Vin, the voltage across the inductor is negative. Thus, the current in the inductor ramps down until it reaches 0 at T=307.9 μs. 
     Case C 
     0,0:C1-- err&gt;vdeadband &amp; Vout&lt;Vin/2 &amp; Steer=True &amp; Qb=↑ 
     In Case C (described further below), the buck converter controller operates in discontinuous current mode cycling among three states. The first state is State 0,0  850 , in which the buck converter has zero current. The second state is State C1 ( 870 ) in which buck inductor  307  is charged from the flying rail/charge pump capacitor  305  using buck converter switch Q 1  ( 304 ) during the time period that charge pump low side switch Qb ( 308 ) is closed. The condition of the buck converter switches in State C1 ( 870 ) is the same as in State A2 ( 870 ) discussed above. The third state is C2 ( 890 ) in which buck inductor  307  is discharged using buck converter switch Q 2  ( 306 ) during the time period that charge pump low side switch Qb ( 308 ) is closed. Starting from State 0,0 ( 850 ), the buck controller transitions to state C1 (via transition 0,0:C1) when the following conditions are met:
         the buck controller&#39;s error signal “err” is greater than a deadband “Vdeadband” AND   the output setpoint voltage of the buck converter is less than one-half the input voltage (i.e., Vout&lt;½Vin) AND   the “steer” signal generated by outer loop charge pump controller  630  is high (indicating that the buck converter should be operated to discharge flying capacitor  305 ) AND   charge pump low side switch Qb  308  has turned on.
 
The first and third conditions were explained above with reference to Case A. The remaining two conditions are explained further below.
       

     The second condition that must be satisfied for the 0,0:C1 transition to occur is that the output setpoint voltage of the buck converter be less than one half the input voltage. If the output setpoint voltage is greater than one half the input voltage, the buck converter controller will operate in one of Cases A or B, as described above. 
     The fourth condition that must be satisfied for the 0,0:C1 transition to occur is that the charge pump&#39;s low side switch Qb ( 308 ) have turned on. This means that the controller can begin charging the output inductor. 
     As described above, in State C1 ( 870 ), the buck converter controller turns on high buck converter switch Q 1  ( 304 ). This begins charging output inductor  307  from flying capacitor  305  through charge pump switch Qb ( 308 ), which was already turned on by outer loop charge pump controller  630 . 
     C1:C2-- I(L)&gt;err 
     Unlike Case A (i.e., States 0,0; A1; A2) and like Case B (i.e., States 0,0; B1; B2) described above, in Case C (i.e., States 0,0; C1; C2) the buck converter controller has complete control of the inductor current. Having begun charging output inductor  307  on entry into State C1 ( 870 ), the controller can transition to State C2 ( 890 ) when the inductor current reaches its target value. For example, if the controller is implementing peak current mode control using a PI control loop like that described above, it would initiate the C1:C2 transition when the inductor current reached the peak current value corresponding to the error signal “err.” This transition results in the opening of buck converter switch Q 1  ( 304 ) and the closing of buck converter switch Q 2  ( 306 ), which begins the discharge of output inductor  307 . 
     C2:0,0-- I(L)&lt;0 
     Once in State C2 ( 890 ), the buck converter controller will transition to State 0,0 ( 850 ) when output inductor  307  has completely discharged, i.e., when the inductor current becomes zero. This is the “discontinuous” current of the DCM mode of operation. Assuming that the first three conditions discussed above remain true, the buck converter controller will transition again to the C1 state when charge pump switch Qb ( 308 ) next closes. Otherwise:
         if the buck controller&#39;s error signal becomes less than the deadband, the converter will remain idle; OR   if the buck controller output setpoint changes to greater than ½Vin, then the buck controller will transition to Case A or B operation as described above; OR   if the steer signal transitions low, then the buck controller will transition to Case B operation as described above or Case D operation as described below.       

     Case C, corresponding to block  832 , arises when the target voltage/setpoint (Vset) of the buck converter is less than the flying rail/flying capacitor voltage (e.g., ½ Vin) and the outer loop charge pump controller is providing a steering signal indicating that the flying capacitor  305  should be discharged by the buck inductor  307 . The buck controller thus alternates between: (1) charging buck inductor  307  by discharging flying capacitor  305  (using switches  304  and  308 ) and (2) discharging the buck inductor (using switches  306  and  308 ). In Case C, like Case B, the current through buck inductor  307  can be controlled directly, using peak current mode control, for example. In such an embodiment, the switching transition from buck converter high side switch  304  to buck converter low side switch  306  may be made when the inductor current reaches its target. 
     Waveforms corresponding to Case C are illustrated in  FIG. 11 . The voltage of the high flying rail  710   a,  low flying rail  710   b,  and steering signal  734   a  are as described above with respect to  FIGS. 7, 8, and 10 . The steering signal waveform is constant at 1.0V, because for Case B the steering signal always indicates that the regulator is to charge flying capacitor  305 . Waveform  1102  illustrates the voltage appearing at the junction of buck converter high side switch  304  and buck converter low side switch  306 , which is determined by the switching states described above with respect to  FIG. 8 . Waveform  1104  illustrates the current flowing through buck inductor  307 . It can be seen that the inductor is operated in discontinuous mode, using peak mode current control as described above. 
     For instance, at time T=315.15 μs, both switches Q 1  ( 304 ) and Qb ( 308 ) are turned on (the latter by outer charge pump control loop  630 , the former by the inner loop  640   a ). This pulls the voltage V(fhi)  710   a  of flying rail  310   a  down to approximately ½Vin and the voltage V(flo)  710   b  of flying rail  310   b  to approximately 0. This also initiates the ramping of current in inductor  307 , at a rate (½Vin-Vout)/L. Then, just before time T=315.3 μs, Q 1  ( 304 ) is turned off, and Q 2  ( 306 ) is turned on, both by inner loop buck controller  640   a,  in response to the current in inductor  307  reaching its target. Switch Qb ( 308 ) remains on. At this point, the voltage across inductor  307  is (0−Vout)/L. Because the voltage across the inductor is negative, the current in the inductor ramps down until it reaches 0 at T=315.3 μs. 
     Case D 
     0,0:D1-- err&gt;vdeadband &amp; Vout&lt;(Vin/2) &amp; Steer=False &amp; Pred_Timer=↑ 
     In Case D (described further below), the buck converter controller operates in discontinuous current mode cycling among three states. The first state is State 0,0 ( 850 ), in which the buck converter has zero current. The second state is State D1 ( 880 ) in which buck inductor  307  is charged from the flying rail/charge pump capacitor  305  using buck converter switch Q 2  ( 306 ) during the time period that charge pump high side switch Qa ( 302 ) is closed, which also charges flying capacitor  305 . The third state is D2 ( 890 ) in which buck inductor  307  is discharged using buck converter switch Q 2  ( 306 ) during the time period that charge pump low side switch Qb ( 308 ) is closed. Starting from State 0,0 ( 850 ), the buck controller transitions to state D1 (via transition 0,0:D1) when the following conditions are met:
         the buck controller&#39;s error signal “err” is greater than a deadband “Vdeadband” AND   the output setpoint voltage of the buck converter is less than one-half the input voltage (i.e., Vout&lt;½Vin) AND   the “steer” signal generated by outer loop charge pump controller  630  is low (indicating that the buck converter should be operated to charge flying capacitor  305 ) AND   buck controller&#39;s predictive timer has transitioned high, indicating that it is time to begin charging output inductor  307 .
 
The first three conditions have been explained above. The fourth condition is explained below.
       

     The fourth condition that must be satisfied for the 0,0:D1 transition to occur is that the buck controller&#39;s predictive timer has transitioned high, indicating that it is time to begin charging output inductor  307 . In Case D (like Case A, discussed above) the buck converter controller does not have full control of the switches that control the current through output inductor  307  because the buck converter controller does not have control of the transitions of charge pump switches Qa ( 302 ) and Qb ( 308 ), which are controlled by outer loop charge pump controller  630  as described above. However, the buck converter controller can know the time at which the charge pump will transition from the high switch Qa ( 302 ) to low switch Qb ( 308 ), which information is received from outer loop charge pump controller  630  as described above. As a result, the buck converter controller can calculate (i.e., predict) the time at which buck converter high switch Q 2  ( 306 ) should be turned on to deliver sufficient energy to output inductor  307  by the time that the charge pump switch transition takes place. This prediction signal is then the final condition that must be satisfied for the 0,0:D1 transition. 
     As described above, in State D1 ( 880 ), the buck converter controller turns on low buck converter switch Q 2  ( 306 ). This begins charging output inductor  307  from the flying rail/charge pump capacitor  305  through charge pump switch Qa ( 302 ), which was already turned on by outer loop charge pump controller  630 . 
     D1:D2-- Qb=↑&amp; Vout&lt;Vin/2 &amp; I(L)≥error 
     Once in State D1 ( 880 ), the buck converter controller will transition to State D2 ( 890 ) when the charge pump transitions state, provided that Vout remains less than ½ Vin and the inductor current is greater than the value indicated by the “err” signal. (As described above, the charge pump transitions are controlled by outer loop charge pump controller  630 .) More specifically, when the charge pump controller switches charge pump switch Qb ( 308 ) on, it makes the D1:D2 transition if Vout remains less than ½Vin and the inductor current is greater than or equal to the value specified by the current control loop. The condition that Vout remain less than ½Vin and that the inductor current have reached a minimum value indicated by the control loop error signal are to account for the case in which the output voltage is very close to the flying capacitor voltage. In such case, the inductor will charge very slowly. In some cases, the inductor might not have time to charge sufficiently to keep the buck converter output in regulation. It is desired that the control system prioritizes output regulation of the buck over voltage regulation of the flying capacitor. Therefore, if sufficient energy has not been transferred to the inductor by the time the charge pump switches transition from Qa ( 302 ) on to Qb ( 308 ), the controller will transition from D1 to state C1 as described in further detail below. Otherwise, as described above, in State D2 ( 890 ), the buck converter turns on low buck converter switch Q 2  ( 306 ). This begins discharging output inductor through flying capacitor  305 . 
     D2:0,0-- I(L)&lt;0 
     Once in State D2 ( 890 ), the buck converter controller will transition to State 0,0 ( 850 ) when output inductor  307  has completely discharged, i.e., when the inductor current becomes zero. This is the “discontinuous” current of the DCM mode of operation. Assuming that the first three conditions discussed above remain true, the buck converter controller will transition again to the D1 state when charge pump switch Qa ( 302 ) next closes. Otherwise:
         if the buck controller&#39;s error signal becomes less than the deadband, the converter will remain idle; OR   if the buck controller output setpoint changes to greater than ½Vin, then the buck controller will transition to Case A or B operation as described above; OR   if the steer signal transitions high, then the buck controller will transition to Case A or B operation as described above.       

     D1:C1-- Qb=↑&amp; Vout&lt;Vin/2 &amp; I(L)&lt;err 
     As discussed briefly above, while operating in Case D, if the inductor does not sufficiently charge during state D1, the controller will transition to State C1 when the charge pump switches from Qa ( 302 ) on to Qb ( 308 ) on. This will allow output inductor  307  to continue charging, now from flying capacitor  305 , to ensure sufficient energy is transferred to the output of the buck converter. As noted above, this prioritizes buck converter output voltage regulation (which is typically more important) over flying capacitor voltage regulation (which is typically less important). This additional state transition may be omitted if, in some application, flying capacitor voltage regulation were more important than buck converter output voltage regulation. 
     Once the buck converter controller has transitioned from State D1 to State C1, it will either transition to State C2, and then to State 0,0 as described above (assuming the inductor is eventually fully charged and discharged) or it will transition back to state D1 as described below 
     Case D, corresponding to block  834 , arises when the target voltage/setpoint (Vset) of the buck converter is less than the flying rail/flying capacitor voltage (e.g., ½ Vin) and the outer loop charge pump controller is providing a steering signal indicating that the flying capacitor  305  should be charged by the buck inductor  307 . The buck controller thus alternates between: (1) charging the buck inductor  307  and flying capacitor  305  from the high input voltage rail Vin (using switch  306  while charge pump high side switch  302  is closed) and (2) discharging the buck inductor  307  (using switches  306  and  308 ). In case D, like Case A, the inductor current cannot be controlled directly, and thus predictive control may be used. For Case D, as with Case A, the turn on time of the switch may be predicted using knowledge of the input and output voltages and inductor value. This predicted on time may then be used to determine the delay between the charge pump switch transition (Qa,  302 ) and the turn-on time of the buck converter charging switch (Q 2 ,  304 ). The change of state from charging inductor  307  to discharging inductor  307  will be determined by the transition of the charge pump switches  302  and  308 , as illustrated in the waveforms of  FIG. 12 . 
     Waveforms corresponding to Case D are illustrated in  FIG. 12 . The voltage of the high flying rail  710   a,  low flying rail  710   b,  and steering signal  734   a  are as described above with respect to  FIGS. 7, 8, 10, and 11 . The steering signal waveform is constant at 0.0V, because for Case D the steering signal always indicates that the regulator is to charge flying capacitor  305 . Waveform  1202  illustrates the voltage appearing at the junction of buck converter high side switch  304  and buck converter low side switch  306 , which is determined by the switching states described above with respect to  FIG. 8 . Waveform  1204  illustrates the current flowing through buck inductor  307 . It can be seen that the inductor is operated in discontinuous mode using predicted switching control as described above. 
     For instance, at time T=13.4 μs: switch Qa ( 302 ) is turned on, pulling the high flying rail  310   a  voltage V(fhi)  710   a  up to approximately Vin and the low flying rail  310   b  voltage V(flo)  710   b  to approximately ½Vin. Then, at time T=13.6 μs, switch Q 2  ( 306 ) is turned on by the inner loop controller, using its predictive algorithm, initiating the ramping of current in inductor  307 , at rate (½Vin−Vout)/L. At time T=13.7 μs: switch Qa ( 302 ) is turned off, and switch Qb ( 308 ) is turned on, by outer loop charge pump controller  630 . Switch Q 2  ( 306 ) remains on. The voltage V(flo)  710   b  of low flying rail  310   b  transitions from approximately ½Vin to approximately 0. Now, the voltage across inductor  307  is (0−Vout)/L. Because the inductor voltage is negative, the current in inductor  307  ramps down until it hits 0 shortly after T=13.7 μs. 
     Multi Output Three Level Buck Converter With Multiple Charge Pumps 
     The embodiments above relate to a multi output three level buck converters having a single charge pump stage and a plurality of buck converter stages. However, in some applications, especially those with very low output voltages relative to the input voltage, it may be desirable to provide one or more additional charge pump stages. For example, a first charge pump having flying rails regulated to ½Vin may be used to power one or more buck converters having relatively higher output voltages with a second charge pump having flying rails regulated to ¼Vin may be used to power one or more buck converters having relatively lower output voltages. This may be advantageous because the losses associated with a buck converter increase with increased input voltage, so powering relatively lower voltage loads from a buck converter operating from a ¼Vin rail (rather than a ½Vin rail) may provide for improved efficiency. Such an embodiment is illustrated in  FIG. 13 . 
       FIG. 13  illustrates a multi charge pump multi output three level buck converter  1300 . Converter  1300  includes a first charge pump and first and second buck stages substantially similar to those described above. It will be appreciated that a single buck converter could be provided off of this first charge pump, or more than two buck converters could be provided. Converter  1300  also includes a second charge pump made up of switches Q 10  ( 1302 ), Q 12  ( 1308 ), Q 14  ( 1310 ) and flying capacitor  1355 . Flying capacitor  1355  is connected to second flying rails  1350   a  and  1350   b.  Two additional buck converters are connected to these second flying rails  13501  and  1350   b,  although only one or more than two buck converters could also be provided. 
     Control of the multi charge pump multi output three level buck converter may be implemented using a variation of the symmetric control arrangement described above.  FIG. 14  illustrates the variances between the symmetric controller described above and a symmetric controller for a multi charge pump three level buck converter as illustrated in  FIG. 13 . As an initial matter, each of the buck converter stages may be controlled with an individual corresponding buck controller substantially as described above. In the upper portion of  FIG. 14 , the error signals err1-err4, which are the error signals from each of the buck converter controllers, are input into control block  1410 , which provides a signal corresponding to the maximum of these four error signals to voltage controlled oscillator  1412 . Voltage controlled oscillator  1412  generates a clock signal  1414 , the frequency of which is proportional to the greatest error signal from the buck converters. This clock signal  1414  and an adjustable delay time  1416  (derived as described below) are provided to a delay circuit  1415 , which produces a delayed clock signal clk+pi  1418 . Clock signal  1414  and delayed clock signal  1418  are provided as inputs into SR flip flop  1420  to produce clock signal  1422 , which is used to control the switching of first charge pump switches Qa  302  and Qb  308  (illustrated in  FIG. 13 ). 
     The middle portion of  FIG. 14  illustrates steer signal  1424 , which is generated and functions substantially as described above. Also, the lower right portion of  FIG. 14  illustrates a flow chart showing generation of the steer signal. More specifically, at control block  1424   a,  it is determined whether the voltage of first charge pump flying capacitor  305  ( FIG. 13 ) is greater than ½Vin. If so, in control block  1424   b,  the steer signal is set high, indicating that the downstream converters should be operated so as to discharge first charge pump flying capacitor  305 . Otherwise, in control block  1424   c,  the steer signal is set low, indicating that the downstream converters should be operated so as to charge first charge pump flying capacitor  305 . 
     The lower central portion of  FIG. 14  illustrates generation of clock signal  1432  for control of the second charge pump switches Q 10   1302  and Q 12   1308  ( FIG. 13 ). Clock signal  1414  and the delayed clock signal  1418  are provided as inputs to switch  1425 . Switch  1425  is controlled by steer signal  1424  to select either the clock signal  1414  or the delayed clock signal  1418  responsive to the steer signal to generate second clock signal  1426 . Second clock signal  1426 , together with an adjustable delay time  1428 , are provided to time delay element  1427  to generate a second delayed clock signal  1430 . Adjustable delay time  1428  may be generated as described below. Second clock signal  1426  and second delayed clock signal  1430  are provided to SR flip flop  1431  to generate clock signal  1432 , which is used to control the switching of second charge pump switches Q 10   1302  and Q 12   1308  ( FIG. 13 ). 
     The lower left portion of  FIG. 14  illustrates the determination of the delay time  1428 , which is used to determine the duty cycles of first charge pump switches Q 10   1302  and Q 12   1308 . In block  1428   a  and  1428   b,  it is determined whether either the third or fourth buck converters, coupled upstream of the second charge pump, are operating in either Case B ( 1428   a ) or Case C ( 1428   c ). If not, the delay time is set to a specified pulse width in block  1428   e.  This pulse width may be a predetermined constant, or, in some embodiments, may be a control variable used to regulate the converter. Otherwise, if either the third or fourth buck converter is operating in Case C ( 1428   b ), then the delay time is shortened by a predetermined period. In some embodiments, the predetermined period may be 100 ns. Alternatively, if either of the third or fourth buck converters are operating in Case B ( 1428   a ), then the delay time is increased by a predetermined period, for example, 100 ns. Delay time  1416  may be generated in a corresponding manner. 
       FIG. 15  illustrates the operating conditions of the second charge pump (made up of switches Q 10   1302 , Q 12   1308 , and Q 14   1310  together with capacitor  1355 ) illustrated in  FIG. 13 . Cases E discharges first charge pump capacitor  305  and results in second charge pump first flying rail V(fhi2) being at roughly ½Vin and second charge pump second flying rail V(lo2) being at roughly ¼Vin. Cases F charges first charge pump capacitor  305  and results in the same second charge pump flying rail voltages as Case E. Case G decouples completely from first flying rail capacitor  305  and results in second charge pump first flying rail V(fhi2) being at roughly ¼Vin and second charge pump second flying rail V(lo2) being at roughly ground. Whether or not second flying rail capacitor  1355  is charged or discharged during these states is a function of variable steer 2 and the resulting current path of the downstream buck converters. 
     In another variation, the multi charge pump three level buck converter of  FIG. 13  may be operated so that one buck converter per charge pump is operated in continuous conduction mode (CCM) as opposed to all being limited to discontinuous conduction mode (DCM), as was discussed above.  FIG. 16A  illustrates a state controller  1600  that may be used to operate the first charge pump (e.g., switches Qa  302  and Qb  308  together with first flying capacitor  305 ) and a buck converter coupled to the output of the first charge pump and able to operate in CCM or DCM (e.g., switches Q 2   a    304   a  and Q 3   a    306   a  together with inductor  307   a ). Remaining buck converters coupled to the output of the first charge pump (e.g., switches Q 2   b    304   b  and Q 3   b    306   b  together with inductor  307   b ) may be operated in DCM using a controller substantially as described above.  FIG. 16B  illustrates a state controller  1601  that may be used to operate the second charge pump that is coupled to the output of the first charge pump (e.g., switches Q 10   1302 , Q 12 ,  1308 , and Q 14   1310  together with second flying capacitor  1355 ) and a buck converter coupled to the output of the second charge pump and able to operate in CCM or DCM (e.g., switches Q 2   c    304   c  and Q 3   c    306   c  together with inductor  307   c ). Although illustrated as two separate controllers, state controller  1600  and  1601  may be implemented as a single controller. Remaining buck converters coupled to the output of the second charge pump (e.g., switches Q 2   d    304   d  and Q 3   d    306   d  together with inductor  307   d ) may be operated in DCM using a controller substantially as described above. 
     As noted above,  FIG. 16A  illustrates a state controller  1600  that may be used to operate the first charge pump, i.e., switches Qa  302  and Qb  308  together with first flying capacitor  305  ( FIG. 13 ) and a buck converter coupled to the output of the first charge pump and able to operate in CCM or DCM (e.g., switches Q 2   a    304   a  and Q 3   a    306   a  together with inductor  307   a ). State controller  1600  is analogous to the state controller discussed above with respect to  FIGS. 8A and 8B , with switching states for switches Qa  302 , Q 2   a    304   a,  Q 3   a    306   a,  and Qb  308  generally corresponding to those illustrated for switches Qa  302 , Q 1   304 , Q 2   306 , and Qb  308  in  FIG. 8B . More specifically, in state controller  1600 , there are an all switches off 0,0 state  1610  and eight “active” switching states A1, A2, B1, B2, C1, C2, D1, D2 corresponding to the four Cases A, B, C, and D illustrated in  FIG. 8B . As above, certain of these states correspond to the same switch positions, i.e., states A1 and B1  1620 , states A2 and C1  1630 , states B2 and D1  1640 , and states C2 and D2  1650 , which are consolidated into single “states” in  FIG. 16A . 
     Case A 
     0,0:A1-- err1&gt;vdeadband, Vout1&gt;Vin/2, Steer=True, clk=0 
     Controller  1600  can cause the switches to transition from the 0,0 state  1610  to the A1  1620  state responsive to the occurrence of four conditions:
         err1&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband (as discussed above);   Vout1&gt;½Vin, meaning that the buck converter output voltage is greater than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 );   Steer=True, meaning that the buck converter is being instructed to discharge flying capacitor  305  of the first charge pump; and   Clock signal  1422 =0, indicating the minimum off time timer has timed out.
 
In the A1 state  1620 , buck converter inductor  307   a  is being charged from the high voltage input rail through switches Qa  302  and Q 2   a    304   a  while the charge on the flying capacitor  305  remains constant.
       

     A1:A2-- I(L1)&gt;err1, Steer=True 
     Controller  1600  can cause the switches to transition from the A1 state  1620  to the A2 state  1630  responsive to the occurrence of two conditions:
         I(L1)&gt;err1, meaning that the current through buck inductor  307   a  has reached the target value specified by the peak current mode controller for the buck converter; and   Steer=True, meaning that the buck converter is being instructed to discharge flying capacitor  305  of the first charge pump.
 
In the A2 state  1630 , buck converter inductor  307   a  and first flying capacitor  305  are discharged through switches Qb  308  and Q 2   a    304   a.  
       

     A2:0,0 (DCM)-- I(L1)&lt;0 
     Although the buck converter made up of switches Q 2   a    304   a  and Q 3   a    306   a  and inductor  307   a  is CCM capable, it may also operate in DCM when it is sufficiently lightly loaded. In such a case, controller  1600  can cause the switches to transition from the A2 state  1630  back to the 0,0 state  1610  responsive to the current through inductor  307   a  reaching zero (i.e., being completely discharged). 
     A2:A1 (CCM)-- err1&gt;vdeadband, Vout1&gt;Vin/2, Steer=True, clk=0, I(L1)&gt;0 
     Alternatively, if the buck converter is sufficiently heavily loaded, the buck converter may operate in CCM. In such a case, controller  1600  can cause the switches to transition from the A2 state  1630  back to the A1 state  1620  responsive to the occurrence of four conditions:
         err1&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout1&gt;½Vin, meaning that the buck converter output voltage is greater than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 );   Steer=True, meaning that the buck converter is being instructed to discharge flying capacitor  305  of the first charge pump;   Clock signal  1422 =0, indicating the minimum off time timer has timed out; and   I(L1)&gt;0, meaning that the current through inductor  307   a  has not reached zero.
 
As above, in the A1 state  1620 , the buck converter inductor is being charged from the high voltage input rail through switches Qa  302  and Q 2   a    304   a  while the charge on the flying capacitor  305  remains constant.
 
A2:B1 (CCM)-- err1&gt;vdeadband, Vout1&gt;(Vin-Vcap), Steer=False, clk=0, I(L1)&gt;0
       

     While the buck converter is operating in CCM, it may be that the steer signal changes from True to False, indicating that the buck converter should be operated so as to charge flying capacitor  305  of the first charge pump. In such a case, controller  1600  can cause a transition from the A2 state  1630  to the B1 state  1620 . (Recall that the switch positions for the A1 state and B1 state are the same, so the actual switching transitions are the same for either transition in the CCM mode of operation in Case A.) More specifically, the A2:B1 transition will occur responsive to the occurrence of five conditions:
         err1&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout1&gt;½Vin, meaning that the buck converter output voltage is greater than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 );   Steer=False, meaning that the buck converter is now being instructed to operate so as to charge flying capacitor  305  of the first charge pump;   Clock signal  1422 =0, indicating the beginning of the first charge pump&#39;s switching cycle; and   I(L1)&gt;0; meaning that the current through inductor  307   a  has not reached zero.
 
As above, in the B1 state  1620 , the buck converter inductor is being charged from the high voltage input rail through switches Qa  302  and Q 2   a    304   a  while the charge on the flying capacitor  305  remains constant.
       

     Case B 
     0,0:B1-- err1&gt;vdeadband, Vout1&gt;(Vin/2), Steer=False, clk=0 
     Controller  1600  can cause the switches to transition from the 0,0 state  1610  to the B1 state  1620  responsive to the occurrence of four conditions:
         err1&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband (as discussed above);   Vout1&gt;½Vin, meaning that the buck converter output voltage is greater than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 );   Steer=False, meaning that the buck converter is being instructed to charge flying capacitor  305  of the first charge pump; and   Clock signal  1422 =0, indicating the minimum off time timer has timed out.
 
In the B1 state  1620 , the buck converter inductor is being charged from the high voltage input rail through switches Qa  302  and Q 2   a    304   a  while the charge on the flying capacitor  305  remains constant.
       

     B1:B2-- I(L1)&gt;err1, Steer=False 
     Controller  1600  can cause the switches to transition from the B1 state  1620  to the B2 state  1640  responsive to the occurrence of two conditions:
         I(L1)&gt;err1, meaning that the current through buck inductor  307   a  has reached the target value specified by the peak current mode controller for the buck converter; and   Steer=False, meaning that the buck converter is being instructed to charge flying capacitor  305  of the first charge pump.
 
In the B2 state  1640 , buck converter inductor  307   a  is discharged and first flying capacitor  305  is charged through switches Qa  302  and Q 3   a    306   a.  
       

     B2:0,0 (SCM)-- I(L1)&lt;0 
     Although the buck converter made up of switches Q 2   a    304   a  and Q 3   a    306   a  and inductor  307   a  is CCM capable, it may also operate in DCM when it is sufficiently lightly loaded. In such a case, controller  1600  can cause the switches to transition from the B2 state  1640  back to the 0,0 state  1610  responsive to the current through inductor  307   a  reaching zero (i.e., being completely discharged). 
     B2:B1 (CCM)-- err1&gt;vdeadband, Vout1&gt;(Vin/2), Steer=False, clk=0, I(L1)&gt;0 
     Alternatively, if the buck converter is sufficiently heavily loaded, the buck converter may operate in CCM. In such a case, controller  1600  can cause the switches to transition from the B2 state  1640  back to the B1 state  1620  responsive to the occurrence of four conditions:
         err1&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout1&gt;½Vin, meaning that the buck converter output voltage is greater than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 );   Steer=False, meaning that the buck converter is being instructed to charge flying capacitor  305  of the first charge pump; and   Clock signal  1422 =0, indicating the minimum off time timer has timed out; and   I(L1)&gt;0, meaning that the current through inductor  307   a  has not reached zero.
 
As above, in the B1 state  1620 , the buck converter inductor is being charged from the high voltage input rail through switches Qa  302  and Q 2   a    304   a  while the charge on the flying capacitor  305  remains constant.
 
B2:A1 (CCM)-- err1&gt;vdeadband, Vout1&gt;Vin/2, Steer=True, clk=0, I(L1)&gt;0
       

     As discussed above, while the buck converter is operating in CCM, it may be that the steer signal changes from False to True, indicating that the buck converter should be operated so as to discharge flying capacitor  305  of the first charge pump. In such a case, controller  1600  can cause a transition from the B2 state  1640  to the A1 state  1620 . (Recall that the switch positions for the A1 state and B1 state are the same, so the actual switching transitions are the same for either transition in the CCM mode of operation in Case B.) More specifically, the B2:A1 transition will occur responsive to the occurrence of five conditions: 
     err1&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;
         Vout1&gt;½Vin, meaning that the buck converter output voltage is greater than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 );   Steer=True, meaning that the buck converter is being instructed to discharge flying capacitor  305  of the first charge pump;   Clock signal  1422 =0, indicating the minimum off time timer has timed out; and   I(L1)&gt;0; meaning that the current through inductor  307   a  has not reached zero.
 
As above, in the A1 state  1620 , the buck converter inductor is being charged from the high voltage input rail through switches Qa  302  and Q 2   a    304   a  while the charge on the flying capacitor  305  remains constant.
       

     Case C 
     0,0:C1-- err1&gt;vdeadband, Vout1&lt;(Vin/2), Steer=True, clk=0 
     Controller  1600  can cause the switches to transition from the 0,0 state  1610  to the C1  1630  state responsive to the occurrence of four conditions:
         err1&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout1&lt;½Vin, meaning that the buck converter output voltage is less than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 );   Steer=True, meaning that the buck converter is being instructed to discharge flying capacitor  305  of the first charge pump; and   Clock signal  1422 =0, indicating the minimum off time timer has timed out.
 
In the C1 state  1630 , buck converter inductor  307   a  is charged and first flying capacitor  305  is discharged through switches Qb  308  and Q 2   a    304   a.  
       

     C1:C2-- I(L1)&gt;err1, Vout1&lt;Vin/2, Steer=True 
     If the steer signal remains true, meaning that the buck converter continues to be instructed to discharge flying capacitor  305  of the first charge pump, controller  1600  can cause a transition from the C1 state  1630  to the C2 state  1650  responsive to the occurrence of three conditions:
         I(L1)&gt;err1, meaning that the current through buck inductor  307   a  has reached the target value specified by the peak current mode controller for the buck converter;   Vout1&lt;½Vin, meaning that the buck converter output voltage is less than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 ); and   Steer=True, meaning that the buck converter is being instructed to discharge flying capacitor  305  of the first charge pump.
 
In the C2 state  1630 , buck converter inductor  307   a  is discharged through switches Q 3   a    306   a  and Qb  308  while the charge on first charge pump flying capacitor  305  remains constant.
       

     C1:D2-- I(L1)&gt;err, Vout1&lt;Vin/2, Steer=False 
     Alternatively, if the steer signal transitions from True to False, indicating that the buck converter should be operated to charge flying capacitor  305  of the first charge pump, controller  1600  can cause a transition from the C1 state  1650  to the D2 state  1650 . (It will be appreciated that the switch positions for the C2 and D2 states are identical, so the actual switching transitions are the same for either transition.) More specifically, the transition from the C1 state to the D2 state can take place on the occurrence of three conditions:
         I(L1)&gt;err1, meaning that the current through buck inductor  307   a  has reached the target value specified by the peak current mode controller for the buck converter;   Vout1&lt;½Vin, meaning that the buck converter output voltage is less than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 ); and   Steer=False, meaning that the buck converter is being instructed to charge flying capacitor  305  of the first charge pump.
 
In the D2 state  1630 , as in the C2 state  1650 , buck converter inductor  307   a  is discharged through switches Q 3   a    306   a  and Qb  308  while the charge on first charge pump flying capacitor  305  remains constant.
       

     C2:0,0 (DCM)-- I(L)&lt;0 
     Although the buck converter made up of switches Q 2   a    304   a  and Q 3   a    306   a  and inductor  307   a  is CCM capable, it may also operate in DCM when it is sufficiently lightly loaded. In such a case, controller  1600  can cause the switches to transition from the C2 state  1650  back to the 0,0 state  1610  responsive to the current through inductor  307   a  reaching zero (i.e., being completely discharged). 
     C2:C1 (CCM)-- err1&gt;vdeadband, Vout1&lt;(Vin/2), Steer=True, clk=0 
     Alternatively, if the buck converter is sufficiently heavily loaded, the buck converter may operate in CCM. In such a case, controller  1600  can cause the switches to transition from the C2 state  1650  back to the C1 state  1630  responsive to the occurrence of four conditions:
         err1&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout1&lt;½Vin, meaning that the buck converter output voltage is less than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 );   Steer=True, meaning that the buck converter is being instructed to discharge flying capacitor  305  of the first charge pump; and   Clock signal  1422 =0, indicating the minimum off time timer has timed out.
 
As above, in the C1 state  1630 , buck converter inductor  307   a  is charged and first flying capacitor  305  is discharged through switches Qb  308  and Q 2   a    304   a.  
       

     Case D 
     0,0:D1-- err1&gt;vdeadband, Vout1&lt;(Vin/2), Steer=False, clk=0 
     Controller  1600  can cause the switches to transition from the 0,0 state  1610  to the D1  1640  state responsive to the occurrence of four conditions:
         err1&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout1&lt;½Vin, meaning that the buck converter output voltage is less than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 );   Steer=False, meaning that the buck converter is being instructed to charge flying capacitor  305  of the first charge pump; and   Clock signal  1422 =0, indicating the minimum offtime timer has timed out.
 
In the D1 state  1640 , buck converter inductor  307   a  and first flying capacitor  305  are charged through switches Qa  302  and Q 3   a    306   a.  
       

     D1:D2-- I(L1)&gt;err1, Vout1&lt;(Vin/2), Steer=False 
     If the steer signal remains False, indicating that the buck converter should be operated to charge flying capacitor  305  of the first charge pump, controller  1600  can cause a transition from the D1 state  1650  to the D2 state  1650 . More specifically, the transition from the D1 state to the D2 state can take place on the occurrence of three conditions:
         I(L1)&gt;err1, meaning that the current through buck inductor  307   a  has reached the target value specified by the peak current mode controller for the buck converter;   Vout1&lt;½Vin, meaning that the buck converter output voltage is less than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 ); and   Steer=False, meaning that the buck converter is being instructed to charge flying capacitor  305  of the first charge pump.
 
In the D2 state  1650 , as in the C2 state  1650 , buck converter inductor  307   a  is discharged through switches Q 3   a    306   a  and Qb  308  while the charge on first charge pump flying capacitor  305  remains constant.
       

     D1:C2-- I(L1)&gt;err1, Vout1&lt;(Vin/2), Steer=True 
     If the steer signal transitions from False to True, meaning that the buck converter continues to be instructed to discharge flying capacitor  305  of the first charge pump, controller  1600  can cause a transition from the D1 state  1630  to the C2 state  1650 . (It will be appreciated that the switch positions for the C2 and D2 states are identical, so the actual switching transitions are the same for either transition.) More specifically, the transition from the D1 state  1630  to the C2 state  1650  can occur responsive to the occurrence of three conditions:
         I(L1)&gt;err1, meaning that the current through buck inductor  307   a  has reached the target value specified by the peak current mode controller for the buck converter;   Vout1&lt;½Vin, meaning that the buck converter output voltage is less than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 ); and   Steer=True, meaning that the buck converter is being instructed to discharge flying capacitor  305  of the first charge pump.
 
In the C2 state  1630 , buck converter inductor  307   a  is discharged through switches Q 3   a    306   a  and Qb  308  while the charge on first charge pump flying capacitor  305  remains constant.
       

     D2:0,0 (DCM)-- I(L)&lt;0 
     Although the buck converter made up of switches Q 2   a    304   a  and Q 3   a    306   a  and inductor  307   a  is CCM capable, it may also operate in DCM when it is sufficiently lightly loaded. In such a case, controller  1600  can cause the switches to transition from the D2 state  1650  back to the 0,0 state  1610  responsive to the current through inductor  307   a  reaching zero (i.e., being completely discharged). 
     D2:D1 (CCM)-- err1&gt;vdeadband, Vout1&lt;(Vin/2), Steer=False, clk=0 
     Alternatively, if the buck converter is sufficiently heavily loaded, the buck converter may operate in CCM. In such a case, controller  1600  can cause the switches to transition from the D2 state  1650  back to the D1 state  1640  responsive to the occurrence of four conditions:
         err1&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout&lt;½Vin, meaning that the buck converter output voltage is less than ½ Vin (i.e., the nominal output voltage of the first charge pump, which is the voltage across first flying capacitor  305 );   Steer=False, meaning that the buck converter is being instructed to charge flying capacitor  305  of the first charge pump; and   Clock signal  1422 =0, indicating the minimum off time timer has timed out.
 
As above, in the D1 state  1640 , buck converter inductor  307   a  and first flying capacitor  305  are charged through switches Qa  302  and Q 3   a    306   a.  
 
D1:C1-- err1&gt;vdeadband, Vout1&lt;Vin/2, Steer=True, clk=0, I(L1)&lt;err1
       

     As discussed briefly above, while operating in Case D, if inductor  307   a  does not sufficiently charge during state D1  1640 , controller  1600  can transition to state C1  1630  when clock signal  1422  transitions to zero, indicating the beginning of the first charge pump&#39;s switching cycle. This can allow output inductor  307   a  to continue charging, now from flying capacitor  305 , to ensure sufficient energy is transferred to the output of the buck converter. As noted above, this prioritizes buck converter output voltage regulation (which is typically more important) over flying capacitor voltage regulation (which is typically less important). This additional state transition may be omitted if, in some application, flying capacitor voltage regulation were more important than buck converter output voltage regulation. Once the buck converter controller has transitioned from state D1  1640  to state C1  1630 , it will either transition to state C2  1650  and so on, as described above (assuming the inductor is eventually fully charged) or it will transition back to state D1  1640  as described below. 
     C1:D1-- err1&gt;vdeadband, Vout1&lt;(Vin/2), Steer=False, clk=0, I(L1)&lt;err1 
     Ordinarily in state C1  1630 , controller  1600  will transition to state C2  1650  based on peak current control of inductor  307   a  as described above. However, in some cases (e.g., when Vout is very close to ½Vin) it may be possible for the inductor to charge sufficiently slowly that it does not reach its current target before both the ‘steer’ signal changes state and the timer times out. In that case, the controller will transition from state C1  1630  to state D1  1640 . 
     As noted above,  FIG. 16B  illustrates a state controller  1601  that may be used to operate the second charge pump (e.g., switches Q 10   1302 , Q 12   1308 , and Q 14   1310  together with second flying capacitor  1355 ) and a buck converter coupled to the output of the second charge pump and able to operate in CCM or DCM (e.g., switches Q 2   c    304   c  and Q 3   c    306   c  together with inductor  307   c ). State controller  1601  is analogous to state controller  1600  discussed above with reference to  FIG. 16A . In this case, the switching states for charge pump high side switches Q 10   1302  and Q 12   1308  correspond to the switching state for Qa  302 ; the switching states for buck converter switches Q 2   c    304   c  and Q 3   c    306   c  correspond to the switching states for buck converter switches Q 2   a    304   a  and Q 3   a    306   a;  and the switching states for charge pump low side switch Q 14   1310  corresponds to the switching state for Qb  308 . As above, in state controller  1601 , there are an all switches off 0,0 state  1610  and eight “active” switching states A1, A2, B1, B2, C1, C2, D1, D2 corresponding to the four Cases A, B, C, and D illustrated in  FIG. 8B  as modified by the states E, F, and G, illustrated in  FIG. 15 . As above, certain of these states correspond to the same switch positions, i.e., states A1 and B1  1621 , states A2 and C1  1631 , states B2 and D1  1641 , and states C2 and D2  1651 , which are consolidated into single “states” in  FIG. 16B . 
     Case A 
     0,0:A1-- err3&gt;vdeadband, Vout3&gt;(Vin/4), Steer2=True, clk=0 
     Controller  1601  can cause the switches to transition from the 0,0 state  1610  to the A1  1621  state responsive to the occurrence of four conditions:
         err3&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband (as discussed above);   Vout3&gt;¼Vin, meaning that the buck converter output voltage is greater than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 );   Steer2=True, meaning that the buck converter is being instructed to discharge flying capacitor  1355  of the second charge pump; and   Clock signal  1432 =0, indicating the minimum off time timer has timed out.
 
In the A1 state  1621 , buck converter inductor  307   c  is being charged from the second charge pump&#39;s high voltage input rail through switch Q 10   1302  or Q 12   1308  (depending on the switching state E or F of the second charge pump) and switch Q 2   c    304   c  while the charge on the flying capacitor  1355  remains constant.
       

     A1:A2-- I(L3)&gt;err3, Steer2=True 
     Controller  1601  can cause the switches to transition from the A1 state  1621  to the A2 state  1631  responsive to the occurrence of two conditions:
         I(L3)&gt;err3, meaning that the current through buck inductor  307   c  has reached the target value specified by the peak current mode controller for the buck converter; and   Steer2=True, meaning that the buck converter is being instructed to discharge flying capacitor  1355  of the second charge pump.
 
In the A2 state  1631 , buck converter inductor  307   c  and second flying capacitor  1355  are discharged through switches Q 14   1310  and Q 2   c    304   c.  
       

     A2:0,0 (SCM)-- I(L)&lt;0 
     Although the buck converter made up of switches Q 2   c    304   c  and Q 3   c    306   c  and inductor  307   c  is CCM capable, it may also operate in DCM when it is sufficiently lightly loaded. In such a case, controller  1601  can cause the switches to transition from the A2 state  1631  back to the 0,0 state  1611  responsive to the current through inductor  307   c  reaching zero (i.e., being completely discharged). 
     A2:A1 (CCM)-- err3&gt;vdeadband, Vout3&gt;(Vin/4), Steer2=True, clk=0, I(L3)&gt;0 
     Alternatively, if the buck converter is sufficiently heavily loaded, the buck converter may operate in CCM. In such a case, controller  1601  can cause the switches to transition from the A2 state  1631  back to the A1 state  1621  responsive to the occurrence of four conditions:
         err3&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout3&gt;¼Vin, meaning that the buck converter output voltage is greater than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 );   Steer2=True, meaning that the buck converter is being instructed to discharge flying capacitor  1355  of the second charge pump;   Clock signal  1432 =0, indicating the minimum off time timer has timed out; and   I(L3)&gt;0, meaning that the current through inductor  307   c  has not reached zero.
 
As above, in the A1 state  1621 , buck converter inductor  307   c  is being charged from the second charge pump&#39;s high voltage input rail through switch Q 10   1302  or Q 12   1308  (depending on the switching state E or F of the second charge pump) and switch Q 2   c    304   c  while the charge on the flying capacitor  1355  remains constant.
 
A2:B 1 (CCM)-- err3&gt;vdeadband, Vout3&gt;(Vin/4), Steer2=False, clk=0, I(L3)&gt;0
       

     While the buck converter is operating in CCM, it may be that the steer signal changes from True to False, indicating that the buck converter should be operated so as to charge flying capacitor  1355  of the second charge pump. In such a case, controller  1601  can cause a transition from the A2 state  1631  to the B1 state  1621 . (Recall that the switch positions for the A1 state and B1 state are the same, so the actual switching transitions are the same for either transition in the CCM mode of operation in Case A.) More specifically, the A2:B1 transition will occur responsive to the occurrence of five conditions:
         err3&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout3&gt;¼Vin, meaning that the buck converter output voltage is greater than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 );   Steer2=False, meaning that the buck converter is now being instructed to operate so as to charge flying capacitor  1355  of the second charge pump;   Clock signal  1432 =0, indicating the minimum off time timer has timed out; and   I(L3)&gt;0; meaning that the current through inductor  307   c  has not reached zero.
 
In the B1 state  1621 , buck converter inductor  307   c  is being charged from the second charge pump&#39;s high voltage input rail through switch Q 10   1302  or Q 12   1308  (depending on the switching state E or F of the second charge pump) and switch Q 2   c    304   c  while the charge on the flying capacitor  1355  remains constant.
       

     Case B 
     0,0:B1-- err3&gt;vdeadband, Vout3&gt;(Vin/4), Steer2=False, clk=0 
     Controller  1601  can cause the switches to transition from the 0,0 state  1611  to the B1 state  1621  responsive to the occurrence of four conditions:
         err3&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband (as discussed above);   Vout3&gt;¼Vin, meaning that the buck converter output voltage is greater than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 );   Steer2=False, meaning that the buck converter is being instructed to charge flying capacitor  1355  of the second charge pump; and   Clock signal  1432 =0, indicating the beginning of the second charge pump&#39;s switching cycle.
 
In the B1 state  1621 , buck converter inductor  307   c  is being charged from the second charge pump&#39;s high voltage input rail through switch Q 10   1302  or Q 12   1308  (depending on the switching state E or F of the second charge pump) and switch Q 2   c    304   c  while the charge on the flying capacitor  1355  remains constant.
       

     B1:B2-- I(L3)&gt;err, Steer2=False 
     Controller  1601  can cause the switches to transition from the B1 state  1621  to the B2 state  1641  responsive to the occurrence of two conditions:
         I(L3)&gt;err3, meaning that the current through buck inductor  307   c  has reached the target value specified by the peak current mode controller for the buck converter; and   Steer2=False, meaning that the buck converter is being instructed to charge flying capacitor  355  of the second charge pump.
 
In the B2 state  1641 , buck converter inductor  307   c  is discharged and second flying capacitor  1355  is charged through switch Q 10   1302  or Q 12   1308  (depending on the switching state E or F of the second charge pump) and switch Q 3   c    306   c.  
       

     B2:0,0 (DCM)-- I(L)&lt;0 
     Although the buck converter made up of switches Q 2   c    304   c  and Q 3   c    306   c  and inductor  307   c  is CCM capable, it may also operate in DCM when it is sufficiently lightly loaded. In such a case, controller  1601  can cause the switches to transition from the B2 state  1641  back to the 0,0 state  1611  responsive to the current through inductor  307   c  reaching zero (i.e., being completely discharged). 
     B2:B1 (CCM)-- err3&gt;vdeadband, Vout3&gt;(Vin/4), Steer2=False, clk=0, I(L3)&gt;0 
     Alternatively, if the buck converter is sufficiently heavily loaded, the buck converter may operate in CCM. In such a case, controller  1601  can cause the switches to transition from the B2 state  1641  back to the B1 state  1621  responsive to the occurrence of four conditions:
         err3&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout3&gt;¼Vin, meaning that the buck converter output voltage is greater than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 );   Steer2=False, meaning that the buck converter is being instructed to charge flying capacitor  1355  of the second charge pump; and   Clock signal  1432 =0, indicating the minimum off time timer has timed out; and   I(L3)&gt;0, meaning that the current through inductor  307   c  has not reached zero.
 
As above, in the B1 state  1621 , buck converter inductor  307   c  is being charged from the second charge pump&#39;s high voltage input rail through switch Q 10   1302  or Q 12   1308  (depending on the switching state E or F of the second charge pump) and switch Q 2   c    304   c  while the charge on the flying capacitor  1355  remains constant.
 
B2:A1 (CCM)-- err3&gt;vdeadband, Vout3&gt;(Vin/4), Steer2=True, clk=0, I(L3)&gt;0
       

     As discussed above, while the buck converter is operating in CCM, it may be that the steer signal changes from False to True, indicating that the buck converter should be operated so as to discharge flying capacitor  1355  of the second charge pump. In such a case, controller  1601  can cause a transition from the B2 state  1641  to the A1 state  1621 . (Recall that the switch positions for the A1 state and B1 state are the same, so the actual switching transitions are the same for either transition in the CCM mode of operation in Case B.) More specifically, the B2:A1 transition will occur responsive to the occurrence of five conditions:
         err3&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout3&gt;¼Vin, meaning that the buck converter output voltage is greater than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 );   Steer2=True, meaning that the buck converter is being instructed to discharge flying capacitor  1355  of the second charge pump;   Clock signal  1432 =0, indicating the minimum off time timer has timed out; and   I(L3)&gt;0; meaning that the current through inductor  307   c  has not reached zero.
 
As above, in the A1 state  1621 , buck converter inductor  307   c  is being charged from the second charge pump&#39;s high voltage input rail through switch Q 10   1302  or Q 12   1308  (depending on the switching state E or F of the second charge pump) and switch Q 2   c    304   c  while the charge on the flying capacitor  1355  remains constant.
       

     Case C 
     0,0:C1-- err3&gt;vdeadband, Vout3&lt;(Vin/4), Steer2=True, clk=0 
     Controller  16010  can cause the switches to transition from the 0,0 state  1611  to the C1  1631  state responsive to the occurrence of four conditions:
         err3&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout3&lt;¼Vin, meaning that the buck converter output voltage is less than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 );   Steer2=True, meaning that the buck converter is being instructed to discharge flying capacitor  1355  of the first charge pump; and   Clock signal  1432 =0, indicating the minimum off time timer has timed out.
 
In the C1 state  1631 , buck converter inductor  307   c  is charged and second flying capacitor  1355  is discharged through switches Q 14   1310  and Q 2   c    304   c.  
       

     C1:C2-- I(L3)&gt;err3, Vout3&lt;(Vin/4), Steer2=True 
     If the steer signal remains true, meaning that the buck converter continues to be instructed to discharge flying capacitor  1355  of the second charge pump, controller  1601  can cause a transition from the C1 state  1631  to the C2 state  1651  responsive to the occurrence of three conditions:
         I(L3)&gt;err3, meaning that the current through buck inductor  307   c  has reached the target value specified by the peak current mode controller for the buck converter;   Vout3&lt;¼Vin, meaning that the buck converter output voltage is less than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 ); and   Steer2=True, meaning that the buck converter is being instructed to discharge flying capacitor  1355  of the second charge pump.
 
In the C2 state  1631 , buck converter inductor  307   c  is discharged through switches Q 3   c    306   c  and Q 14   1310  while the charge on second charge pump flying capacitor  1355  remains constant.
       

     C1:D2-- I(L3)&gt;err3, Vout3&lt;(Vin/4), Steer2=False 
     Alternatively, if the steer2 signal transitions from True to False, indicating that the buck converter should be operated to charge flying capacitor  1355  of the second charge pump, controller  1601  can cause a transition from the C1 state  1651  to the D2 state  1651 . (It will be appreciated that the switch positions for the C2 and D2 states are identical, so the actual switching transitions are the same for either transition.) More specifically, the transition from the C1 state to the D2 state can take place on the occurrence of three conditions:
         I(L3)&gt;err3, meaning that the current through buck inductor  307   a  has reached the target value specified by the peak current mode controller for the buck converter;   Vout3&lt;¼Vin, meaning that the buck converter output voltage is less than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 ); and   Steer2=False, meaning that the buck converter is being instructed to charge flying capacitor  1355  of the second charge pump.
 
In the D2 state  1631 , as in the C2 state  1651 , buck converter inductor  307   c  is discharged through switches Q 3   a    306   a  and Q 14   1310  while the charge on second charge pump flying capacitor  1355  remains constant.
       

     C2:0,0 (DCM)-- I(L)&lt;0 
     Although the buck converter made up of switches Q 2   c    304   c  and Q 3   c    306   c  and inductor  307   c  is CCM capable, it may also operate in DCM when it is sufficiently lightly loaded. In such a case, controller  1601  can cause the switches to transition from the C2 state  1651  back to the 0,0 state  1611  responsive to the current through inductor  307   c  reaching zero (i.e., being completely discharged). 
     C2:C1 (CCM)-- err3&gt;vdeadband, Vout3&lt;(Vin/4), Steer2=True, clk=0 
     Alternatively, if the buck converter is sufficiently heavily loaded, the buck converter may operate in CCM. In such a case, controller  1601  can cause the switches to transition from the C2 state  1651  back to the C1 state  1631  responsive to the occurrence of four conditions:
         err3&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout3&lt;¼Vin, meaning that the buck converter output voltage is less than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 );   Steer2=True, meaning that the buck converter is being instructed to discharge flying capacitor  1355  of the second charge pump; and   Clock signal  1432 =0, indicating the minimum off time timer has timed out.
 
As above, in the C1 state  1631 , buck converter inductor  307   c  is charged and second flying capacitor  1355  is discharged through switches Q 14   1310  and Q 2   c    304   c.  
       

     Case D 
     0,0:D1-- err3&gt;vdeadband, Vout3&lt;(Vin/4), Steer2=False, clk=0 
     Controller  1601  can cause the switches to transition from the 0,0 state  1611  to the D1  1641  state responsive to the occurrence of four conditions:
         err3&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout3&lt;¼Vin, meaning that the buck converter output voltage is less than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 );   Steer2=False, meaning that the buck converter is being instructed to charge flying capacitor  1355  of the second charge pump; and   Clock signal  1422 =0, indicating the minimum off time timer has timed out.
 
In the D1 state  1641 , buck converter inductor  307   c  and second flying capacitor  1355  are charged through switch Q 10   1302  or Q 12   1308  (depending on the switching state E or F of the second charge pump) and switch Q 3   c    306   c.  
       

     D1:D2-- I(L3)&gt;err3, Vout3&lt;(Vin/4), Steer2=False 
     If the steer signal remains False, indicating that the buck converter should be operated to charge flying capacitor  1355  of the second charge pump, controller  1601  can cause a transition from the D1 state  1651  to the D2 state  1651 . More specifically, the transition from the D1 state to the D2 state can take place on the occurrence of three conditions:
         I(L3)&gt;err3, meaning that the current through buck inductor  307   c  has reached the target value specified by the peak current mode controller for the buck converter;   Vout3&lt;¼Vin, meaning that the buck converter output voltage is less than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 ); and   Steer2=False, meaning that the buck converter is being instructed to charge flying capacitor  1355  of the second charge pump.
 
In the D2 state  1651 , as in the C2 state  1651 , buck converter inductor  307   c  is discharged through switches Q 3   c    306   c  and Q 14   1310  while the charge on second charge pump flying capacitor  1355  remains constant.
       

     D1:C2-- I(L3)&gt;err3, Vout3&lt;(Vin/4), Steer2=True 
     If the steer signal transitions from False to True, meaning that the buck converter continues to be instructed to discharge flying capacitor  305  of the first charge pump, controller  1600  can cause a transition from the D1 state  1631  to the C2 state  1651 . (It will be appreciated that the switch positions for the C2 and D2 states are identical, so the actual switching transitions are the same for either transition.) More specifically, the transition from the D1 state  1631  to the C2 state  1651  can occur responsive to the occurrence of three conditions:
         I(L3)&gt;err3, meaning that the current through buck inductor  307   c  has reached the target value specified by the peak current mode controller for the buck converter;   Vout3&lt;¼Vin, meaning that the buck converter output voltage is less than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  1355 ); and   Steer2=True, meaning that the buck converter is being instructed to discharge flying capacitor  1355  of the second charge pump.
 
In the C2 state  1631 , buck converter inductor  307   c  is discharged through switches Q 3   c    306   c  and Q 14   1310  while the charge on second charge pump flying capacitor  1355  remains constant.
       

     D2:0,0 (DCM)-- I(L)&lt;0 
     Although the buck converter made up of switches Q 2   c    304   c  and Q 3   c    306   c  and inductor  307   c  is CCM capable, it may also operate in DCM when it is sufficiently lightly loaded. In such a case, controller  1601  can cause the switches to transition from the D2 state  1651  back to the 0,0 state  1611  responsive to the current through inductor  307   c  reaching zero (i.e., being completely discharged). 
     D2:D1 (CCM)-- err3&gt;vdeadband, Vout3&lt;(Vin/4), Steer2=False, clk=0 
     Alternatively, if the buck converter is sufficiently heavily loaded, the buck converter may operate in CCM. In such a case, controller  1601  can cause the switches to transition from the D2 state  1651  back to the D1 state  1641  responsive to the occurrence of four conditions:
         err3&gt;vdeadband, meaning that the buck converter output voltage has dropped below the setpoint and associated deadband;   Vout3&lt;¼Vin, meaning that the buck converter output voltage is less than ¼Vin (i.e., the nominal output voltage of the second charge pump, which is the voltage across second flying capacitor  355 );   Steer2=False, meaning that the buck converter is being instructed to charge flying capacitor  1355  of the second charge pump; and   Clock signal  1432 =0, indicating the minimum off time timer has timed out.
 
As above, in the D1 state  1641 , buck converter inductor  307   c  and second flying capacitor  1355  are charged through switch Q 10   1302  or Q 12   1308  (depending on the switching state E or F of the second charge pump) and switch Q 3   c    306   c.  
       

     D1:C1-- err3&gt;vdeadband, Vout3&lt;(Vin/4), Steer2=True, clk=0, I(L3)&lt;err3 
     As discussed briefly above, while operating in Case D, if inductor  307   c  does not sufficiently charge during state D1  1641  (e.g., when Vout is very close to ½Vin), controller  1601  can transition to state C1  1631  when both the steer2 signal changes state, and the clock signal  1432  transitions to zero, indicating the timer has timed out. This can allow output inductor  307   c  to continue charging, now from flying capacitor  1355 , to ensure sufficient energy is transferred to the output of the buck converter. Once the buck converter controller has transitioned from state D1  1641  to state C1  1631 , it will either transition to state C2  1651  and so on, as described above (assuming the inductor is eventually fully charged) or it will transition back to state D1  1641  as described below. 
     C1:D1-- err3&gt;vdeadband, Vout3&lt;(Vin/4), Steer2=False, clk=0, I(L3)&lt;err3 
     Ordinarily in state C2  1631 , controller  1601  will transition to state C2  1651  based on peak current control of inductor  307   c  as described above. However, in some cases (e.g., when Vout is very close to ¼Vin) it may be possible for the inductor to charge sufficiently slowly that it does not reach its current target before both the ‘steer’ signal changes state and the timer times out. In that case, the controller will transition from state C2  1631  to state D1  1641 . 
       FIG. 17  depicts a series of logic circuits  1701 - 1709  that may be used in conjunction with the state controllers illustrated in  FIGS. 16A and 16B  to trigger the gates of the respective switching devices of the multi output three level buck converter with multiple charge pumps and one CCM capable buck converter per charge pump. 
     Logic circuit  1701  may be used to trigger a gate of first charge pump high side switch Qa  302  ( FIG. 13 ). The logic circuit comprises a NOT or inverter  1701   a  that receives the steer signal discussed above. The inverted steer signal and a signal indicating that state controller  1600  ( FIG. 16A ) is in the 0,0 state  1610  ( FIG. 16A ) are input into AND gate  1701   b.  The output of AND gate  1701   b  is input into OR gate  1701   c,  which has two other inputs indicating that state controller  1600  is in the A1,B1 state ( 1620 ) or the B2,D1 state  1640 . If any of these conditions is true, the output of OR gate will transition high, which may be used to trigger the gate of first charge pump high side switch Qa  302 . 
     Logic circuit  1702  may be used to trigger a gate of first charge pump low side switch Qb  308  ( FIG. 13 ). The logic circuit comprises an AND gate  1702   a  that receives two input signals: (1) the steer signal discussed above and (2) a signal indicating that state controller  1600  ( FIG. 16A ) is in the 0,0 state  1610 . The output of AND gate  1702 A is fed as an input to OR gate  1702   b,  which also receives two other input signals: (1) a signal indicating that state controller  1600  is in the A2,C1 state  1630  and (2) a signal indicating that state controller  1600  is in the C2,D2 state  1650 . If any of these conditions is true, the output of OR gate  1702   b  will transition high, which may be used to trigger the gate of first charge pump low side switch Qb  308 . 
     Logic circuit  1703  may be used to trigger a gate of high side switch Q 2   a    304   a  ( FIG. 13 ) of a CCM-capable buck converter coupled to the output of the first charge pump. Logic circuit  1703  includes an OR gate  1703   a  that receives two input signals (1) an indication that state controller  1600  ( FIG. 16A ) is in the A1,B1 state  1620  and (2) an indication that state controller  1600  is in the A2,C1 state  1630 . If either of these conditions is true, the output of OR gate  1703   a  will transition high, which may be used to trigger the gate of CCM capable buck high side switch Q 2   a    304   a.    
     Logic circuit  1704  may be used to trigger a gate of low side switch Q 3   a    306   a  ( FIG. 13 ) of a CCM-capable buck converter coupled to the output of the first charge pump. Logic circuit  1704  includes an OR gate  1704   a  that receives two input signals (1) an indication that state controller  1600  ( FIG. 16A ) is in the B2,D1 state  1640  and (2) an indication that state controller  1600  is in the C2,D2 state  1650 . If either of these conditions is true, the output of OR gate  1704   a  will transition high, which may be used to trigger the gate of CCM capable buck low side switch Q 3   a    306   a.    
     Logic circuit  1705  may be used to trigger a gate of second charge pump first side switch Q 10   1302  ( FIG. 13 ). Logic circuit  1705  includes OR gate  1705   a  that receives two input signals: (1) a signal indicating that state controller  1602  ( FIG. 16B ) is in the A1,B1 state  1621  and (2) a signal indicating that state controller  1602  is in the B2,D1 state  1641 . If either of these conditions are true, a high signal is passed to one input of AND gate  1705   b.  AND gate  1705   b  receives as its other input the Qb ( 308 ) drive signal that is the output of logic circuit  1702  discussed above. If both of these signals are high, AND gate  1705   b  will output a signal triggering the gate of second charge pump high side switch Q 10   1302 . 
     Logic circuit  1706  may be used to trigger a gate of second charge pump second high side switch Q 12   1308  ( FIG. 13 ). Logic circuit  1706  includes OR gate  1706   a  that receives two input signals: (1) a signal indicating that state controller  1602  ( FIG. 16B ) is in the A1,B1 state  1621  and (2) a signal indicating that state controller  1602  is in the B2,D1 state  1641 . If either of these conditions are true, a high signal is passed to one input of AND gate  1706   b.  AND gate  1706   b  receives as its other input the Qa ( 302 ) drive signal that is the output of logic circuit  1701  discussed above. If both of these signals are high, AND gate  1706   b  will output a signal triggering the gate of second charge pump second high side switch Q 11   1308 . 
     Logic circuit  1707  may be used to trigger a gate of second charge pump low side switch Q 14   1310  ( FIG. 13 ). Logic circuit  1707  includes OR gate  1707   a  that receives two input signals: (1) a signal indicating that state controller  1602  ( FIG. 16B ) is in the A2,C1 state  1631  and (2) a signal indicating that state controller  1602  is in the C2,D2 state  1651 . If either of these conditions are true, OR gate  1707   a  will output a signal triggering the gate of second charge pump low side switch Q 14   1310 . 
     Logic circuit  1708  may be used to trigger a gate of high side switch Q 2   c    304   c  ( FIG. 13 ) of a CCM-capable buck converter coupled to the output of the second charge pump. Logic circuit  1708  includes an OR gate  1708   a  that receives two input signals (1) an indication that state controller  1601  ( FIG. 16B ) is in the A1,B1 state  1621  and (2) an indication that state controller  1601  is in the A2,C1 state  1631 . If either of these conditions is true, the output of OR gate  1708   a  will transition high, which may be used to trigger the gate of CCM capable buck high side switch Q 2   c    304   c.    
     Logic circuit  1709  may be used to trigger a gate of low side switch Q 3   c    306   c  ( FIG. 13 ) of a CCM-capable buck converter coupled to the output of the first charge pump. Logic circuit  1709  includes an OR gate  1709   a  that receives two input signals (1) an indication that state controller  1601  ( FIG. 16B ) is in the B2,D1 state  1641  and (2) an indication that state controller  1601  is in the C2,D2 state  1651 . If either of these conditions is true, the output of OR gate  1709   a  will transition high, which may be used to trigger the gate of CCM capable buck low side switch Q 3   c    306   c.    
     Described above are various features and embodiments relating to multi output three level buck converters. Such converters may be used in a variety of applications, but may be particular advantageous when used in conjunction with portable electronic devices such as mobile telephones, smart phones, tablet computers, laptop computers, media players, and the like, as well as the peripherals associated therewith. Such associated peripherals can include input devices (such as keyboards, mice, touchpads, tablets, microphones and the like), output devices (such as headphones or speakers), combination input/output devices (such as combined headphones and microphones), storage devices, or any other peripheral. Other applications can include on-chip point of load regulators. 
     Additionally, although numerous specific features and various embodiments have been described, it is to be understood that, unless otherwise noted as being mutually exclusive, the various features and embodiments may be combined in any of the various permutations in a particular implementation. Thus, the various embodiments described above are provided by way of illustration only and should not be constructed to limit the scope of the disclosure. Various modifications and changes can be made to the principles and embodiments herein without departing from the scope of the disclosure and without departing from the scope of the claims.