Patent Publication Number: US-8994279-B2

Title: Method and apparatus to control a DC-DC converter

Description:
FIELD 
     Subject matter disclosed herein relates generally to electronic circuits and, more particularly, to control techniques and circuits for use with DC-DC converters. 
     BACKGROUND 
     Light emitting diode (LED) driver circuits are often called upon to drive a number of series connected strings of diodes simultaneously. The strings of diodes (or “LED channels”) may be operated in parallel, with a common voltage node supplying all of the strings. A DC-DC converter (e.g., a boost converter, a buck converter, etc.) may be employed by the LED driver circuit to maintain a regulated voltage level on the various LED channels during operation so that all LED channels have adequate operational power. Feedback from the LED channels may be used to control the DC-DC converter. To reduce unnecessary power consumption, it may be desirable to keep the regulated voltage level on the voltage node to a minimum or near minimum, while still providing adequate power to all channels. 
     Some LED driver circuits are only capable of driving LED channels that are relatively uniform. That is, the driver circuits are only capable of driving channels having the same number of LEDs and the same current levels. In addition, some driver circuits illuminate all driven LEDs using the same dimming duty cycle. These operational constraints simplify the design of the DC-DC converter associated with the LED driver circuit. Newer LED driver circuits are being proposed that will allow more complex illumination functionality. For example, some proposed designs may allow different numbers of diodes and different currents to be used in different LED channels. Some designs may also allow different dimming duty cycles to be specified for different LED channels, in addition, some proposed designs may allow different illumination phasing in different channels (i.e., the LEDs within different channels may be permitted to turn on at different times). 
     As will be appreciated, any increase in the functional complexity of LED driver circuits, and/or the circuitry they drive, can complicate the design of DC-DC converters and/or converter control circuitry for the drivers. Techniques and circuits are needed that are capable of providing DC-DC voltage conversion within LED driver circuits, and/or other similar circuits, that can support this increased complexity. 
     SUMMARY 
     In accordance with one aspect of the concepts, systems, circuits, and techniques described herein, an electronic circuit for use in driving a plurality of loads coupled to a common voltage node, where each load in the plurality of loads including a series-connected string of load devices, comprises: a plurality of current regulators to regulate current through corresponding ones of the plurality of loads; and control circuitry to control a DC-DC converter to generate a regulated voltage on the common voltage node, the control circuitry comprising: (a) a duty cycle control unit to control a duty cycle of the DC-DC converter, the duty cycle control unit being responsive to a duty cycle control signal at a control input thereof that is indicative of a duty cycle to be used by the duty cycle control unit; (b) at least one capacitor to carry a voltage to act as the duty cycle control signal for the duty cycle control unit; and (c) at least one error amplifier to facilitate adjustment of the voltage on the at least one capacitor based on feedback, the at least one error amplifier being configured to generate an error signal based, during first time periods, on the output voltage of the DC-DC converter and, during second time periods, on feedback from the plurality of current regulators, wherein the second time periods are different from the first time periods. 
     In accordance with another aspect of the concepts, systems, circuits, and techniques described herein, a method is provided for operating a duty cycle control unit to generate a switching signal for a DC-DC converter, the DC-De converter to generate an output voltage to power a plurality of light emitting diode (LED) channels coupled to a common voltage node, where each LED channel in the plurality of LED channels includes a series-connected string of LEDs and the duty cycle control unit has an input to receive a duty cycle control signal indicative of a duty cycle to be used for the DC-DC converter. More specifically, the method comprises: generating an error signal for use in adjusting a voltage level on at least one capacitor coupled to the input of the duty cycle control unit based on feedback, wherein generating the error signal includes generating the error signal based on the output voltage of the DC-DC converter during first time periods and generating the error signal based on one or more voltages across one or more current regulators associated with the plurality of LED channels during second time periods, wherein the second time periods are different from the first time periods. 
     In accordance with still another aspect of the concepts, systems, circuits, and techniques described herein, an electronic circuit for use in driving a plurality of loads coupled to a common voltage node, where each load in the plurality of loads including a series-connected string of load devices, comprises: control circuitry for controlling a DC-DC converter to generate a regulated voltage on the common voltage node, the control circuitry comprising: (a) a duty cycle control unit to control a duty cycle of the DC-DC converter, the duty cycle control unit being responsive to a duty cycle control signal at a control input thereof that is indicative of a duty cycle to be used by the duty cycle control unit; (b) a first capacitor to carry a first voltage to act as a duty cycle control signal for the duty cycle control unit; (c) a second capacitor to carry a second voltage to act as a duty cycle control signal for the duty cycle control unit; and (d) a switch circuit to alternately couple the first capacitor and the second capacitor to the control input of the duty cycle control unit in response to one or more control signals. 
     In accordance with a further aspect of the concepts, systems, circuits, and techniques deathbed herein, a method for operating a duty cycle control unit to generate a switching signal for a DC-DC converter, where the duty cycle control unit has an input to receive a duty cycle control signal indicative of a duty cycle to be used for the DC-DC converter, comprises: alternately coupling at least a first capacitor and a second capacitor to the input of the duty cycle control unit, the first and second capacitors each having corresponding voltages across them that act as duty cycle control signals for the duty cycle control unit when the corresponding capacitors are coupled to the duty cycle control unit. 
     In accordance with a still further aspect of the concepts, systems, circuits, and techniques described herein, a control circuit for controlling a DC-DC converter to generate a regulated voltage comprises: a duty cycle control unit to control a duty cycle of the DC-DC converter, the duty cycle control unit being responsive to a duty cycle control signal at a control input thereof that is indicative of a duty cycle to be used by the duty cycle control unit; a first capacitor to carry a first voltage to act as a duty cycle control signal for the duty cycle control unit; a second capacitor to carry a second voltage to act as a duty cycle control signal for the duty cycle control unit; and a switch circuit to alternately couple the first capacitor and the second capacitor to the control input of the duty cycle control unit in response to one or more control signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features may be more fully understood from the following description of the drawings in which: 
         FIG. 1  is a schematic diagram illustrating an exemplary system for use in driving light emitting diodes (LEDs), or other similar load devices, in accordance with an embodiment; 
         FIG. 2  is a schematic diagram illustrating exemplary boost control circuitry that may be used in an LED driver circuit in accordance with an embodiment; 
         FIGS. 3A and 3B  are schematic diagrams illustrating exemplary portions of a boost control circuit that may be used in an LED driver circuit that allows variable phasing of LED channels in accordance with an embodiment; 
         FIG. 4  is a schematic diagram illustrating an exemplary boost control circuit that uses multiple feedback paths and dual COMP capacitors in accordance with an embodiment; 
         FIG. 5  is a timing diagram illustrating various exemplary control waveforms that may be used in a boost control circuit in accordance with an embodiment; and 
         FIG. 6  is a flowchart illustrating a method for operating a DC-DC converter in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram illustrating an exemplary system  10  for use in driving light emitting diodes (LEDs), or other similar load devices, in accordance with an embodiment. As shown, system  10  may include LED driver circuitry  12  and a boost converter  14 . The system  10  may drive a plurality of LEDs  16 . As shown, the plurality of LEDs  16  may be arranged in individual, series-connected strings  16   a , . . . ,  16   n  that are each coupled to a common voltage node  20 . These series-connected strings will be referred to herein as LED channels  16   a , . . . ,  16   n . Any number of LED channels  16   a , . . . ,  16   n  may be driven by system  10  in various embodiments. In addition, in some implementations, each LED channel  16   a , . . . ,  16   n  may be allowed to have a different number of LEDs. The LEDs  16  may be intended to provide any of a number of different illumination functions (e.g., backlighting thr a liquid crystal display, LED panel lighting, LED display lighting, and/or others). 
     In some embodiments, LED driver circuitry  12  may be implemented as an integrated circuit (IC) and boost converter  14  may be connected externally to the IC. In other embodiments, an IC may be provided that includes both LED driver circuitry  12  and boost converter  14 . In still other embodiments, system  10  may be realized using discrete circuitry. As will be appreciated, any combination of integrated circuitry and discrete circuitry may be used for system  10  in various implementations. In the discussion that follows, it will be assumed that LED driver circuitry  12  is implemented as an IC. 
     Boost converter  14  is a DC-De voltage converter that is used to convert a direct current (DC) input voltage V IN  to a regulated output voltage on output voltage node  20  for use in driving LEDs  16 . As is well known, a boost converter is a form of switching regulator that utilizes switching techniques and energy storage elements to generate a desired output voltage. Control circuitry for boost converter  14  may be provided within LED driver circuitry  12 . Although illustrated as a boost converter in  FIG. 1 , it should be appreciated that other types of DC-DC converters may be used in other embodiments (e.g., buck converters, boost-buck converters, etc.). 
     As illustrated in  FIG. 1 , LED driver circuitry  12  may include boost control circuitry  22  for use in controlling the operation of boost converter  14 . LED driver circuitry  12  may also include LED dimming logic  24  and a number of current sinks  26   a , . . . ,  26   n . The current sinks  26   a , . . . ,  26   n  are current regulators that may be use to draw a regulated amount of current through the LED channels  16   a , . . . ,  16   n  during LED drive operations. In at least one embodiment, one current sink  26   a , . . . ,  26   n  may be provided for each LED channel  16   a , . . . ,  16   n . LED dimming logic  24  is operative for controlling the brightness of the LEDs in the various channels  16   a , . . . ,  16   n . LED dimming logic  24  may control the brightness of an LED channel by, for example, changing the current and/or the pulse width modulation (PWM) duty cycle (or “dimming” duty cycle) of the channel. In some embodiments, LED dimming logic  24  may be capable of independently controlling both the current level and the dimming duty cycle of each of the LED channels  16   a , . . . ,  16   n  by providing appropriate control signals to corresponding current sinks  26   a , . . . ,  26   n . In some embodiments, LED dimming logic  24  may also be capable of independently adjusting the illumination “turn on” time or phase of the LED channels  16   a , . . . ,  16   n  (i.e., the time when a channel first lights up during a cycle). 
     In at least one embodiment, LED driver circuitry  12  may be user programmable. That is, LED driver circuitry  12  may allow a user to set various operational characteristics of system  10 . One or more data storage locations may be provided within LIED driver circuitry  12  to store user-provided configuration information to set operational parameters such as, for example, dimming duty cycle of different LED channels, current levels of different LED channels, illumination “turn on” times of different LED channels, and/or other parameters. Any type of data storage structures may be used including, for example, flash memory, RAM memory, EEPROMs, and/or others. In some implementations, off-chip storage may be used for user configuration information. Using this approach, one or more pins, terminals, contacts, or leads may be provided on an IC for use in interfacing with external storage during driver operation. An input/output protocol may also be implemented within the IC to control the storage interface. In some implementations, a user may also be able to specify which LED channels are active (i.e., enabled) and which LED channels are inactive disabled). Default values may be used for the different parameters in the absence of user provided values. 
     As described above, boost converter  14  is operative for converting a DC input voltage V IN  into a DC output voltage V OUT  that is adequate to supply LED channels  16   a , . . . ,  16   n . In the illustrated embodiment, boost converter  14  includes an inductor  30 , a diode  32 , and a capacitor  34 . Other boost converter architectures may alternatively be used. The operating principles of boost converters are well known in the art. To operate properly, a switching signal having appropriate characteristics must be provided to boost converter  14 . Boost control circuitry  22  of LED driver circuitry  12  is operative for providing this switching signal. As will be described in greater detail, boost control circuitry  22  may draw current from switching node  36  of boost converter  14  at a controlled duty cycle to regulate the output voltage V out  in a desired manner. 
     Boost converter  14  and boost control circuitry  22  are operative for providing an adequate voltage level on voltage node  20  to support operation of all active LED channels  16   a , . . . ,  16   n . To conserve energy, however, it may be desired that the voltage level on voltage node  20  be no higher (or only slightly higher) than a minimum level required to support operation. To achieve this, boost control circuitry  22  may rely, at least in part, on feedback from LED channels  16   a , . . . ,  16   n . Typically, the voltage level required for a particular LED channel will be dictated by the needs of the current sink  26   a  . . . ,  26   n  associated with the channel. That is, each current sink  26   a , . . . ,  26   n  may require a minimal amount of voltage (e.g., an LEDs regulation voltage) to support operation for the corresponding LED channel. 
     In general, the voltage level on each current sink  26   a , . . . ,  26   n  will be equal to the difference between the voltage on voltage node  20  and the voltage drop across the LEDs in the corresponding LED channel  16   a , . . . ,  16   n . Because each LED channel  16   a , . . . ,  16   n  may have a different number of LEDs and a different DC current, different LED channels may require different minimum voltage levels for proper operation. The LED channel that requires the highest voltage level on node  20  for proper operation will be referred to herein as the “dominant” LED channel. As will be appreciated, in some implementations, the dominant LED channel may change with time. The dominant channel will often be the channel that has the most LEDs. If there are multiple channels having the “most” LEDs, than one of the channels may be selected as the dominant channel based on a selection criterion. 
     As shown in  FIG. 1 , in some implementations, optional ballast resistors  40   a , . . . ,  40   n  may be used in one or more of the LED channels  16   a , . . . ,  16   n  to provide balance between the voltage levels on the various current sinks  26   a , . . . ,  26   n . As described above, when no ballast resistor is present, the voltage across a current sink will typically be equal to the difference between the boost output voltage on node  20  and the voltage drop across the LEDs in the corresponding channel. Ballast resistors  40   a , . . . ,  40   n  may be provided, for example, to generate an additional voltage drop in some channels to achieve similar voltages on the various current sinks  26   a , . . . ,  26   n . In this manner, some of the power dissipation that might have occurred on chip within LED driver circuitry  12  can be moved off chip to the ballast resistors  40   a , . . . ,  40   n.    
       FIG. 2  is a schematic diagram illustrating exemplary boost control circuitry  50  in accordance with an embodiment. As will be described in greater detail, boost control circuitry  50  of  FIG. 2  is specially suited for use in LED driver systems that do not allow independent control of illumination “turn on” time (or phase) of different LED channels. That is boost control circuitry  50  is best utilized in systems that turn all enabled LED channels on at the same time (although different LED channels may use different dimming duty cycles). Boost control circuitry  50  may be used within system  10  of  FIG. 1  (e.g., as boost control circuitry  22 ) and/or in other systems. In the discussion that follows, boost control circuitry  50  will be described in the context of system  10  of  FIG. 1 . 
     As shown in  FIG. 2  boost control circuitry  50  may include: a first error amplifier  52 ; a second error amplifier  54 ; a boost duty cycle control unit  56 ; an inverter  58 ; a COMP capacitor  60 ; first, second, and third switches  62 ,  64 ,  66 ; and a sample capacitor  68 . Boost duty cycle control unit  56  is operative for generating a switching signal to be applied at a switching node  70  of a corresponding boost converter (e.g., SW node  36  in boost converter  14  of  FIG. 1 ). During system operation, boost duty cycle control unit  56  may draw current from switching node  70  at a controlled duty cycle in a manner that results in a desired DC voltage level at the boost output (e.g., voltage node  20  in  FIG. 1 ). Boost duty cycle control unit  56  may include an input  72  to receive a duty cycle control signal to set a duty cycle of the boost converter. In the illustrated embodiment, the voltage across COMP capacitor  60  coupled to input  72  of boost duty cycle control unit  56  serves as the duty cycle control signal. Although illustrated as a single capacitor, it should be understood that multiple capacitor combinations may be used to perform this function. 
     First and second error amplifiers  52 ,  54  are operative for adjusting the voltage across COMP capacitor  60  using error signals. As described above, boost control circuitry  50  of  FIG. 2  is specially suited for use in LED driver systems that do not allow independent control of illumination “on” time. Therefore, during each illumination cycle, there will be a period during which all of the LED channels are on (i.e., illuminated). First error amplifier  52  may be used to set the voltage across COMP capacitor  60  during the periods when all. LED channels are on. During periods when less than all of the LED channels in the system are on, second error amplifier  54  may be used to set the voltage across COMP capacitor  60 . As described previously, in some embodiments, one or more of the LED channels within a system may be controllably disabled by the system (based on, for example, user input or some other reason). In such embodiments, phrases such as “all LED channels” and “less than all LED channels” used herein are referring to all or less than all “enabled” LED channels. 
     First and second switches  62 ,  64  are operative for controllably coupling outputs  76 ,  78  of first and second error amplifiers  52 ,  54  to COMP capacitor  60  at appropriate times. Control signal  74  (i.e., ALLON) may have a first value (e.g., logic one) when all LED channels are on and a second value (e.g., logic zero) when less than all LED channels are on (i.e., one or more LED channels are off). In the illustrated embodiment, control signal  74  is used to control first switch  62  and an inverted version of control signal  74  is used to control second switch  64 . Thus, the output of first error amplifier  52  will be coupled to COMP capacitor  60  when all LED channels are on and the output of second error amplifier  54  will be coupled to COMP capacitor  60  when less than all LED channels are on. As will be appreciated, other switching schemes may be used in other embodiments (e.g., a single switch that couples either output  76  or output  78  to comp cap  60  based on control signal  74  without the need for inverter  58 , etc.). 
     In the illustrated embodiment, first and second error amplifiers  52 ,  54  are trans-conductance amplifiers that each generate an output current error signal that is proportional in magnitude to a difference between two corresponding input voltages. When coupled to COMP capacitor  60 , these error currents will act to adjust the voltage across the capacitor in a controlled manner. Other types of error amplifiers may be used in other embodiments. As shown in  FIG. 2 , first error amplifier  52  has an LED regulation voltage VLED_Reg) coupled to a non-inverting input thereof and voltage feedback signals from LED channels (i.e., VLED&lt;1:n&gt;) coupled to an inverting input thereof. The LED regulation voltage may represent a minimum voltage level required on a current sink (e.g., current sinks  26   a , . . .  26   n  of  FIG. 1 ) to ensure proper operation. The voltage feedback signals may represent voltages across the current sinks of the driver circuitry (e.g., the voltages on the LED pins  42   a , . . . ,  42   n  of  FIG. 1 ). 
     First error amplifier  52  may generate its output error signal based on a difference between a feedback voltage and the regulation voltage. In some implementations, the feedback voltage that is used may be associated with the present dominant LED channel. In other implementations, an average or mean of the feedback signals of all channels (or some other combination) may be used. Therefore, during periods when all LED channels are on, the voltage on COMP capacitor  60  will be adjusted to ensure that the voltage level on all LED pins equals or exceeds the LED regulation voltage. 
     During periods when less than all of the LED channels are on, boost control circuitry  50  will simply maintain the voltage on COMP capacitor  60  at the level it had when all channels were on. This may be achieved using second error amplifier  54 . As illustrated in  FIG. 2 , a sample capacitor  68  may be coupled to a non-inverting input of second error amplifier  54  and the present boost output voltage (VOUT) may be coupled to the inverting input. As shown, sample capacitor  68  may be controllably coupled to the boost output VOUT through switch  66 , which is controlled by control signal  74 . Thus, when all LED channels are on, switch  66  is closed and the output voltage of the boost converter appears across sample capacitor  68 . When one or more LED channels are turned off; third switch  66  is opened and the voltage on capacitor  68  is held at its present value. Second error amplifier  54  will thereafter generate an output error signal based on a difference between the present output voltage of the boost converter and the previous value when all channels were on. The resulting error current is coupled to COMP capacitor  60  through second switch  64  to adjust the voltage thereon. The boost control circuitry  50  of  FIG. 2  can be used to control the boost converter associated with LED driver circuitry even in implementations where the dimming duty cycles of the LED channels and/or the current levels of the LED channels are independently controllable. The boost control circuitry  50  can also be used in implementations where the LED channels being driven have different numbers of LEDs. 
     In some embodiments, the illumination turn on time or phase of the various LED channels may be independently controlled. In these embodiments, there may not always be a period during which all LED channels are concurrently illuminated and boost control circuitry  50  of  FIG. 2  may be of limited effectiveness.  FIGS. 3A and 3B  are schematic diagrams illustrating exemplary portions of a boost control circuit that may be used when variable phasing of LED channels is permitted.  FIG. 3A  illustrates a control circuit $0 that may be provided for each LED channel in an LED driver system to provide a feedback signal for the channel.  FIG. 3B  illustrates as control circuit  110  that may be used to process the feedback signals associated with the various channels to generate a switching signal for a corresponding boost converter. The boost control circuit of  FIGS. 3A and 3B  may be used in system  10  of  FIG. 1  (e.g., as boost control circuitry  22 ) and/or in other systems. 
     As illustrated in  FIG. 3A , control circuit  80  may include: an error amplifier  82 ; an inverter  84 ; a sample capacitor  86 ; and first, second, third, and fourth switches  88 ,  90 ,  92 ,  94 . As described above, in some implementations, a control circuit  80  may be provided for each LED channel in an LED driver system, in the discussion that follows, control circuitry  80  of  FIG. 3A  will be described in the context of a first LED channel (LED 1). Error amplifier  82  is operative for generating a feedback signal for the first LED channel based on a difference between two input voltages. In addition, the sources of the two input voltages of error amplifier  62  may vary during system operation based on, for example, a dimming duty cycle of the first LED channel. In the illustrated embodiment, switches  88 ,  90 ,  92 ,  94  may be used to change the inputs applied to error amplifier  82  based on, for example, a pulse width modulation (PWM) signal  96  associated with the first LED channel. When PWM signal  96  has a first value (e.g., logic one) corresponding to an “on” portion of the dimming duty cycle of the first LED channel, switches  88  and  90  may be closed and switches  92  and  94  may be open. During this time, an LED regulation voltage may be applied to a non-inverting input of error amplifier  82  and an LED pin voltage VLED&lt;1&gt; (or other feedback) associated with the first LED channel may be applied to the inverting input of error amplifier  82 . 
     When PWM signal  96  has a second value (e.g., logic zero) corresponding to an “off” portion of the dimming duty cycle of the first LED channel, switches  88  and  90  may be open and switches  92  and  94  may be closed. During this time, a voltage across sample capacitor  86  may be applied to the non-inverting input of error amplifier  82  and a present output voltage of the boost converter (VOUT) may be applied to the inverting input of error amplifier  82 . As shown, sample capacitor  98  may be coupled to the boost output through switch  98 , which is controlled by PWM signal  96 . Thus, when the first LED channel is on, switch  98  is closed and the output voltage of the boost converter appears across sample capacitor  86 . When the first LED channel is turned off, switch  98  is opened and the voltage on capacitor  86  is held at its present value. Thus, when the first LED channel is tuned off, error amplifier  82  will generate an error signal based on the difference between the present boost output voltage and the prior boost output voltage when the first LED channel was on. 
     As illustrated in  FIG. 3B , control circuit  110  may include: an error amplifier  112 , a COMP capacitor  114 , and a boost duty cycle control unit  120 . As in the previously described embodiment, boost duty cycle control unit  120  is operative for generating a switching signal to be applied at a switching node  122  of a corresponding boost converter (e.g., SW node  36  in boost converter  14  of  FIG. 1 ). The voltage across COMP capacitor  114  serves as a duty cycle control signal for boost duty cycle control unit  120 . Error amplifier  112  generates an error signal at an output thereof that adjusts the voltage across COMP capacitor  114  based on, for example, a difference between two input voltages. In the illustrated embodiment, error amplifier  112  is a trans-conductance amplifier that generates an output current signal (although other types of amplifiers may be used in other implementations). 
     In at least one implementation, the voltage applied to the non-inverting input of error amplifier  112  is an average or mean of the feedback voltages of all of the LED channels (i.e., an average of the outputs of controller  80  of each channel or VFB). In some embodiments, the feedback voltages of all of the LED channels may be applied to error amplifier  112  and the averaging (or other processing) may be performed internal to amplifier  112 . In at least one implementation, the control circuits  80  of all of the LED channels will be providing feedback all of the time to error amplifier  112 . A reference voltage (VREF) may be applied to the inverting input of error amplifier  112 . When one or more of the LED channels needs a higher voltage, the value of VFB will be greater than the reference voltage and error amplifier  112  will increase the voltage on. COMP capacitor  114 . When the LED channels are being overdriven, the value of VFB will be less than the reference voltage and error amplifier  112  will reduce the voltage on COMP capacitor  114 . In either case, boost duty cycle control unit  120  will change the duty cycle of the boost converter accordingly. The boost control circuit of  FIGS. 3A and 3B  may allow a corresponding boost converter to regulate an LED pin for minimum dimming duty cycle without generating significant ripple at the boost output. 
       FIG. 4  is a schematic diagram illustrating an exemplary boost control circuit  130  that uses multiple feedback paths and dual COMP capacitors in accordance with an embodiment. As will be described in greater detail, boost control circuit  130  of  FIG. 4  is designed to maintain the output of the boost converter at a level required by a dominant LED channel, while also allowing the boost duty cycle to be rapidly changed with a sudden change in load. The boost control circuit  130  of  FIG. 4  may be used in system  10  of  FIG. 1  (e.g., as boost control circuitry  22 ) and/or in other systems. 
     As illustrated in  FIG. 4 , boost control circuit  130  may include: first, second, and third error amplifiers  132 ,  134 ,  136 ; a boost duty cycle control unit  138 ; first and second COMP capacitors  140 ,  142 ; an inverter  146 ; a sample capacitor  148 ; a number of switches  150 ,  152 ,  154 ,  156 ,  158 ,  160 ,  162 ,  166 ,  168  and a unity gain buffer  164 . As in the previously described embodiments, boost duty cycle control unit  138  is operative for generating a switching signal to be applied at a switching node  170  of a corresponding boost converter. A duty cycle control signal is applied to an input  174  of boost duty cycle control unit  138  to set a duty cycle of the boost converter. Unlike the previous embodiments, however, the duty cycle control signal is not derived from a single COMP capacitor, but from two COMP capacitors  140 ,  142  that are alternately coupled to an active COMP node  172  during, for example, successive odd and even PWM dimming cycles. In the illustrated embodiment, switches  156 ,  158  are used to alternately couple the two COMP capacitors  140 ,  142  to active COMP node  172 . The operation of all switches  150 ,  152 ,  154 ,  156 ,  158 ,  160 ,  162 ,  166 ,  168  in the illustrated embodiment will be described in detail below. 
     When first COMP capacitor  140  is coupled to active COMP node  172 , second COMP capacitor  142  is used to sample the maximum voltage on lint COMP capacitor  140  the voltage corresponding to the “on” period of the dominant LED channel) for use during the next PWM cycle, and vice versa. When the next PWM cycle starts, the second COMP capacitor  142  is coupled to active COMP node  172  and first COMP capacitor  140  is decoupled from node  172 . Because second COMP capacitor  142  sampled the highest voltage across the first COMP capacitor from the previous PWM cycle, the boost duty cycle control unit  138  can adjust almost instantaneously to the correct duty cycle for the dominant LED channel. Because of this rapid adjustment, control circuit  130  is capable of supporting very high dimming ratios (i.e., dimming duty cycles that generate a large amount of dimming, with very short “on” periods) without negatively effecting system stability. 
     Third error amplifier  136  is operative for generating an error signal at an output thereof to adjust the voltage across the COMP capacitor that is currently coupled to active COMP node  172 . The error signal is generated based on a difference between two input signals. Based on the current state of the dominant LED channel (i.e., on or off), the non-inverting input of error amplifier  136  will be received from either first error amplifier  132  or second error amplifier  134  (each corresponding to a different feedback path from the LED channels). The inverting input of third error amplifier  136  may be coupled to a fixed reference voltage (e.g. 12 volts in the illustrated embodiment). The fixed reference voltage used with third error amplifier  136  may be the common mode voltage for the differential input, if the first and second error amplifiers  132 ,  134  do not have errors, then the outputs of both amplifiers may be the same as reference voltage. In this regard, the absolute do value of the fixed reference voltage can vary from implementation to implementation. In the illustrated embodiment, third error amplifier  136  comprises a trans-conductance amplifier and first and second error amplifiers  132 ,  134  comprise differential amplifiers, although other types of amplifiers can be used in other implementations. 
     When the dominant LED channel is on, switch  150  is closed (and switch  152  is open) and the output of first error amplifier  132  is coupled to non-inverting input of error amplifier  136 . During this time period, the boost output voltage be regulated to the highest level required by the corresponding system. First error amplifier  132  generates an output error signal based on a voltage difference between feedback from the LED channels (e.g., one or more LED pin voltages, VFB_LED&lt;6:1&gt;) and a reference voltage (VREF) (e.g., the LED pin regulation voltage). In at least one embodiment, first error amplifier  132  uses the feedback voltage of the dominant LED channel to generate the output error signal. In other embodiments, other feedback signals may be used (e.g., an average or mean of all LED pin voltages, etc). 
     When the dominant LED channel is off switch  152  is closed (and switch  150  is open) and the output of second error amplifier  134  is coupled to the non-inverting input of error amplifier  136 . Second error amplifier  134  generates an output error signal based on a voltage difference between a current value of the boost output voltage and the value of the boost output voltage during the most recent PWM on period of the dominant channel. Switch  154  will be closed during the on portion of the diming duty cycle of the dominant channel, allowing sample capacitor  148  to sample the corresponding boost output voltage. When the off period of the dimming duty cycle of the dominant channel starts, switch  154  opens and the voltage on sample capacitor  148  is held at its current value. The action of second error amplifier  134  is designed to maintain the output voltage of the boost converter at the value it had during the on portion of the dimming duty cycle of the dominant LED channel. 
     As described above, active COME node  172  may be alternately switched between first and second COMP capacitors  140 ,  142  during odd and even PWM cycles. In the illustrated embodiment, switches  156  and  158  are used to effect this result. During odd cycles, switch  156  will be closed and switch  158  will be open. During even cycles, switch  158  will be closed and switch  156  will be open. When first COMP capacitor  140  is coupled to active COMP node  172 , second COMP capacitor  142  may be used to sample the maximum voltage on first COMP capacitor  140 . In the illustrated embodiment, this sampling is realized using unity gain buffer  164  and switches  160 ,  162 ,  166 ,  168 . Unity gain buffer  164  may be used to charge the capacitor that is currently sampling. During odd PWM cycles, switch  162  will be closed coupling unity gain buffer  164  to second COMP capacitor  142 . During even PWM cycles, switch  160  will be closed coupling unity gain buffer  164  to first COMP capacitor  140 . 
     As shown in  FIG. 4 , switch  166  is coupled between first COMP capacitor  140  and an input of unity gain buffer  164 . Likewise, switch  168  is coupled between second COMP capacitor  142  and the input of unity gain buffer  164 . Switch  166  is controlled by a signal S 1 sub that closes the switch during the “on” period of the dominant LED channel in odd PWM cycles (and opens switch  166  otherwise). When switch  166  is closed, the voltage across first COMP capacitor  140  is applied to the input of unity gain buffer  164 . The input node of unity gain buffer  164  behaves as a holding node that holds the voltage level applied thereto, even after switch  166  subsequently opens. Based on the voltage at the input node, unity gain buffer  164  sets the voltage of second COMP capacitor  142  to equal the voltage that was on first COMP capacitor  140  dining the most recent “on” period of the dominant LED channel (i.e., the maximum value of the voltage on first capacitor  140  during the odd PWM cycle). Because the input node of unity gain buffer  164  behaves as a holding node, second COMP capacitor  142  does not have to charge up to the maximum voltage value of first COMP capacitor  140  within the limited time period that switch  166  is closed (which can be very short when high dimming ratios are used by the dominant channel). Instead, second COMP capacitor  142  can continue to charge even after switch  166  is closed, until it readies the maximum voltage value of first COMP capacitor  140 . 
     In a similar fashion, switch  168  is controlled by a signal S 2 sub that closes the switch during the “on” period of the dominant LED channel in even PWM cycles (and opens switch  168  otherwise). When switch  168  is closed, the voltage across second COMP capacitor  142  is applied to the input of unity gain buffer  164  which then sets the voltage of first COMP capacitor  140  accordingly. Once again, the input node unity gain buffer  164  will act as a holding node so that first COMP capacitor  140  can continue to charge even after switch  168  opens. In some embodiments, the input node of unity gain buffer  164  may not act as a holding node. For example, in some implementations, dimming ratios may only be used that will allow the appropriate capacitor to be fully charged during the “on” period of the dominant LED channel. 
     As described previously, because first COMP capacitor  140  and second COMP capacitor  142  each sample the highest voltage of the other capacitor, when they are subsequently connected to active COMP node  172 , they immediately apply the correct voltage to the node  172  for the dominant LED channel. This allows boost duty cycle control unit  138  to rapidly adjust the duty cycle of the boost converter to the necessary value. Because of this rapid adjustment, control circuit  130  is capable of supporting very high dimming ratios without negatively effecting system stability. 
     In the embodiments described above, various switching arrangements are shown for use in swathing components into and out of an active circuit. It should be appreciated that many alternative switching schemes may be used to achieve these switching functions and the particular switching schemes shown are merely illustrative of example arrangements. For example, as described above, switches  156  and  158  of  FIG. 4  may be switched in a manner that alternately couples COMP capacitor  140  and COMP capacitor  142  to active COMP node  172 . Instead of using two single pole, single throw (SPST) switches as illustrated, one single pole, double throw (SPDT) switch (or some other switching arrangement) may be used to achieve substantially the same function. Similar substitutions may be used in other circuits described herein. Any types of switches may be used in various embodiments. In some embodiments, for example, the switches may include semiconductor based switches implemented on or off chip using transistors (e.g., field effect transistors (FETS), bipolar junction transistors (BJTs), etc.) or other semiconductor switching devices. In an alternative approach, electro-mechanical switches implemented off-chip or on a secondary chip of a multi-chip module may be used. A combination of semiconductor based switches and electro-mechanical switches may also be used in some embodiments. 
       FIG. 5  is a timing diagram illustrating various exemplary waveforms that may be used as control signals to control switches in boost control circuit  130  of  FIG. 4  in accordance with an embodiment. Waveform  180  (PWM DDM ) represents a possible dimming duty cycle signal for the dominant channel in an LED driver system. As shown in  FIG. 4 , this signal may be used to control switch  150  and switch  154 . Waveform  182  (  PWM DOM   ) represents an inverse of waveform  180 . As shown in  FIG. 4 , this signal may be used to control switch  152 . Waveform  184  represents control, signal S 1  that may be used to identify odd PWM cycles in an LED driver system. With reference to  FIG. 4 , this signal may be used to control switch  156  and switch  162  in the illustrated embodiment. Waveform  186  represents control signal S 2  that may be used to identify even PWM cycles in the LED driver system. With reference to  FIG. 4 , this signal may be used to control switch  158  and switch  160  in an embodiment. 
     Waveform  188  (S 1   sub ) represents an “on” portion of a duty cycle of the dominant LED channel of the LED driver system during the odd PWM cycles. As shown in  FIG. 5 , waveform  188  only includes “on” pulses during corresponding pulses in waveform  184  (but not during pulses of waveform  186 ). With reference to  FIG. 4 , this signal may be used to control switch  166  in an embodiment. Waveform  190  (S 2   sub ) represents an “on” portion of a duty cycle of the dominant LED channel of the LED driver system during the even PWM cycles. As shown in  FIG. 5 , waveform  190  only includes “on” pulses during corresponding pulses in waveform  186  (but not during pulses of waveform  184 ). With reference to  FIG. 4 , this signal may be used to control switch  168  in an embodiment. 
       FIG. 6  is a flowchart illustrating a method  200  for operating a DC-DC converter in accordance with an embodiment. The method  200  may be used to control DC-DC converters associated with LED driver circuits and/or other types of circuits and systems. A duty cycle control unit may be provided to generate a switching signal for use by the DC-DC converter to set an output voltage thereof. The duty cycle control unit may have an input to receive a duty cycle control signal that is indicative of the duty cycle to be used. First and second capacitors may be provided to carry voltage levels to be used as the duty cycle control signal. The first and second capacitors may be alternately coupled to the input of the duty cycle control circuit to provide the duty cycle control signal (block  202 ). In some implementations, the first capacitor may be coupled to the input of the duty cycle control circuit during, for example, odd PWM dimming cycles and the second capacitor may be coupled to the input of the duty cycle control circuit during even PWM dimming cycles. The second capacitor may be used to sample the maximum voltage on the first capacitor during periods when the first capacitor is coupled to the duty cycle control unit (block  204 ). Likewise, the first capacitor may be used to sample the maximum voltage on the second capacitor during periods when the second capacitor is coupled to the duty cycle control unit (block  206 ). In this manner, when the capacitors are switched, the new capacitor will always have the correct voltage value for use with the dominant LED channel. The maximum voltage on each capacitor may be the voltage that exists on the capacitor when the dominant channel is illuminated. The voltage of the capacitor that is currently coupled to the duty cycle control circuit will be adjusted based on feedback. As described above, in some implementations, two different feedback paths may be used to adjust the voltage on the capacitor, in some embodiments, three or more capacitors may be used in block  202 . In addition, as described previously, a multi-capacitor combination may be used to store a single voltage in some implementations. 
     As described above, in some implementations, the dominant LED channel may change with time. For example, in some implementations, a user may be permitted to disable one or more LED channels during system operation. If one of the disabled channels is the current dominant channel, a new dominant channel needs to be identified. In some implementations, it may be possible to add one or more LEDs to a channel after system deployment. This can also affect the dominant LED channel. In addition, during system operation, it may be discovered that one or more of the non-dominant LED channels is not receiving enough power. In this case, the underpowered channel may be made the dominant channel. In some embodiments, one or more components or controllers may be provided within LED driver circuitry for identifying and tracking a dominant LED channel. As used herein, the term “controller” is meant to include both digital and analog controllers and may include, for example, programmable or reconfigurable processors, embedded processors, ASICs, and/or digital or analog circuits. Controllers may be implemented either on or off chip in different embodiments. 
     Referring back to  FIG. 1 , in some implementations, a priority queue  38  may be maintained that tracks the various LED channels in order of priority. A highest priority channel  44  in the queue  38  may represent the dominant LED channel. Digital memory may be provided within LED driver circuitry  12  to store priority queue  38 . Priority queue  38  may be continually updated during system operation so that the dominant LED channel is always known. Priority queue  38  may provide the updated dominant LED channel information to LED dimming logic  24  and/or boost control circuitry  22 . LED dimming logic  24  may need this information to provide the appropriate dimming duty cycle information to boost control circuitry  22  for use in controlling boost converter  14 . 
     In some implementations, a queue manager  46  may be provided for maintaining and updating priority queue  38 . Queue manager  46  may, for example, include a digital or analog controller that is capable of identifying the occurrence of certain events and/or conditions that may require a change in LED channel priority. In some implementations, fir example, queue manager  46  may receive feedback from LED channels  16   a , . . . ,  16   n . This feedback may include, for example, voltage levels on the LED pins  42   a , . . . ,  42   n  of the LED driver circuitry  12 , or some other feedback. If queue manager  46  detects, based on the feedback, that one of the LED channels requires more voltage (e.g., the pin voltage for the channel is below a specified regulation voltage), it may move that channel to the top of priority queue  38 . When the LED channel is moved, all of the other channels may be moved down in priority. Queue manager  46  may also have access to information describing which LED channels have been disabled by a user. If the highest priority LED channel in the queue  38  is disabled, queue manager  46  may move that channel to the lowest priority position in queue  38 . Other LED channels may then be moved up in priority to accommodate the new lowest priority channel. In one possible approach, the LED channels may initially be listed in a default order within priority queue  38  (e.g., by channel number, etc.). The action of queue manager  46  may they rearrange and maintain the order of the channels so that the channel in the highest priority position  44  is the dominant LED channel. 
     In at least one embodiment, instead of a queue, one or more storage locations may be provided within LED driver circuitry  12  to record and track the identity of the current dominant LED channel. A controller may be provided to continually update the identity of the dominant channel stored in the storage location(s) based on events and conditions. Other techniques for identifying and tracking a dominant LED channel being driven by LED driver circuitry may alternatively be used. 
     As described previously, in some implementations, LED driver circuitry  12  of  FIG. 1  may be partially or fully implemented as an IC or as a multi-chip module (MCM). In such embodiments, the various boost control circuits described herein (e.g., boost control circuitry  50  of  FIG. 2 , etc.) may be fully implemented on-chip or one or more elements thereof (e.g., one or more capacitors or other components) may be implemented off chip. In addition, it should be understood that the elements of the boost control circuitry will not necessarily be located in close proximity to one another within a realized circuit, which may be an integrated circuit or a multi-chip circuit in some embodiments. That is, in some implementations, the elements may be spread out within a larger system and coupled together using appropriate interconnect structures. 
     In the embodiments described above, features are described in the context of LED driver systems that utilize current sinks to draw current down through LED channels. It should be appreciated that other types of current regulation devices may be used in other embodiments. For example, in some embodiments, current sources are used that may be located near the boost output (e.g., near node  20  in  FIG. 1 ) and which drive current downward through the LED channels, in these embodiments, feedback signals for use in boost control may be derived based on, for example, voltage drops across the various current sources. In some embodiments, these current sources may be located external to an LED driver integrated circuit (e.g., outside LED driver circuitry  12  of  FIG. 1 , etc.). As will be appreciated, the use of external current sources can allow higher maximum currents to be used in some implementations. In some embodiments, external current sinks may be used. For example, with reference to  FIG. 1 , in some implementations, the current sinks  26   a - 26   n  may be located outside the LED driver circuitry  12 . In each of these different scenarios, the boost control circuits and techniques described above can still be implemented. 
     In the description above, techniques and circuits for providing control for a boost converter or other DC-DC converter have been discussed in the context of LED drives circuitry. It should be appreciated, however, that these techniques and circuits may also be used in other applications. For example, in some implementations, the described techniques and circuits may be used in driver circuits that drive load devices other than LEDs. The described techniques and circuits may also have application in other types of systems, components, and devices that require the generation of a regulated voltage level. 
     Having described exemplary embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.