Abstract:
Analog pulse width modulation (PWM) control circuits and techniques are presented for improving output voltage load transient response in controlling DC to DC conversion systems in which a transient detector circuit restarts a PWM carrier ramp waveform to initiate asynchronous injection of a pulse between the regular periodic PWM pulses in a fixed frequency pulse stream to mitigate the effect of output inductor energy depletion on output voltage.

Description:
REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/700,176 that was filed on Sep. 12, 2012 and is entitled CIRCUIT TO IMPROVE OUTPUT VOLTAGE LOAD TRANSIENT RESPONSE IN A FIXED FREQUENCY DC TO DC CONVERTER, the entirety of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to control of DC to DC converters and more particularly to fixed frequency PWM control circuitry for controlling a DC to DC conversion system. 
     BACKGROUND 
     Power supplies for modern electronic circuits are typically required to provide stable regulated supply voltages for proper operation of processors, ASICS, memory, and other components. Computers, smart phones, tablets, and other electronic products typically employ DC to DC converters to provide supply voltages for operation of the internal circuitry. During operation, however, the current required to operate various circuit components changes, and a power supply may need to regulate the supply voltage within a narrow tolerance band (e.g., +/−3% or less) even in the presence of large current draw variations over a short period of time. The variation in current draw is reflected as a load transient to the DC to DC converter that provides the supply voltage, and DC to DC converter controller performs closed loop converter operation to maintain a generally constant output voltage during these load transients. 
     DC to DC converters typically include one or more switching devices operated by pulse width modulated switching control signals, and a common form of pulse width modulation (PWM) employs a generally fixed switching frequency with the controller modifying the width or duration of the switching control signal pulses to regulate the output voltage according to a feedback signal to compensate for changes in the output load requirements. Fixed frequency DC to DC conversion, however, suffers from voltage regulation limitations in the presence of abrupt changes in load current. In particular, many DC to DC converter architectures employ an output inductor providing current to the load, and switching operation of the converter switch or switches selectively connects the inductor to the source of input power during the “on time” of the PWM switching signal pulses. If the load current increases quickly, particularly when the power supply switch is in an “off” state, the inductor energy may become depleted until the next switching interval. As a result, the output voltage may decrease significantly or “dip”. Subsequent PWM switching pulses can replenish the inductor current, but the closed loop control in a fixed frequency control approach may not be able to prevent a significant voltage deviation in the time it takes to react, and subsequent switching operation may cause an overshoot in the output voltage as the circuit returns to regulated control. The initial voltage drop following an increase in the required load current is referred to as the load transient response of the DC to DC converter. The magnitude of the voltage dip may be addressed, to some extent, by addition of output capacitance to source the energy demanded during the load transient, but additional capacitance increases the cost and size of the power conversion system. Variable frequency PWM approaches may be adopted in order to address load transient response issues, including hysteretic constant on-time, constant off-time, and other approaches, which may in certain circumstances closely approximate fixed frequency operation, but variable frequency designs may be difficult to implement for use in systems synchronized with a system clock or in multiphase or stacked converter configurations. The use of hysteretic comparators providing a control loop outside of the fixed frequency inner loop, moreover, requires the controller to multiplex an I/O pin for monitoring the output voltage and the selection of filter components to avoid loop-to-loop oscillations is limited by the use of the comparators. Accordingly, a need remains for improved fixed frequency pulse width modulation control apparatus and techniques for improved regulation and reduced voltage dip magnitude in the presence of abrupt increases in load current requirements. 
     SUMMARY 
     The present disclosure provides improved DC to DC converter control circuitry and PWM controllers by which the above-mentioned difficulties and shortcomings can be mitigated, in which one or more pulses are asynchronously injected into the PWM signal stream in response to detection of increasing load current requirements in order to reduce the effect of depleting inductor energy on the output voltage. These techniques advantageously facilitate fixed frequency operation according to a periodic clock signal, and may be used in systems employing any suitable type of output filter configuration. Moreover, the techniques can be used in both trailing-edge and leading-edge modulation approaches for either voltage mode or current mode control loop operation. Furthermore, the asynchronous pulse induction approaches outlined herein may mitigate the need for additional output capacitance in order to control the amount of voltage dip caused by increases in the output load current. 
     In accordance with one or more aspects of the present disclosure, DC to DC converter control circuits are provided, including an analog carrier waveform generator and a comparison circuit or comparator which compares amplitudes of a periodic carrier signal waveform and an output error control signal and provides a pulse output signal including periodic pulses. The control circuit also includes a driver circuit which provides one or more switching control signals for DC to DC converter switch operation according to the pulse output signal, as well as an error amplifier which generates the output error control signal by comparison of an output voltage feedback signal to a reference voltage signal. An asynchronous pulse injection circuit is provided, which causes injection of one or more pulses into the pulse output signal in response to detection of an output load current transient condition. In this manner, the controller can react asynchronously to the detected load transient without having to wait for the next periodic PWM pulse. This quick reaction, moreover, advantageously allows the DC to DC converter switch to mitigate depletion of the output inductor energy and thereby reduce the amount of voltage dip resulting from the load transient. 
     The pulse injection circuit in certain embodiments includes an offset circuit creating an offset signal by adding an offset voltage to an average or peak of the output error control signal, and a second comparator which compares the offset signal to the original output error control signal. The second comparator provides an output at one of two levels based on whether the offset signal is greater than the output error control signal, and a reset circuit provides an output signal to initiate asynchronous injection of a pulse into the pulse output signal responsive to a transition in the second comparator output indicating the output error control signal exceeds the offset signal. In certain embodiments, the offset circuit includes a low pass filter to provide the offset signal representing the average of the output error current signal offset by the offset voltage amount. In other embodiments, the offset circuit includes a peak detector receiving the output error control signal and providing an output representing a peak voltage of the output error control signal that is offset for comparison with the unmodified output error control signal. In this manner, the offset voltage can be set such that the pulse injection circuit will not react to normal output voltage fluctuations (e.g., ripple) associated with steady state operation or small output load requirement variations, while still reacting quickly to address significant unregulated drops in the output voltage. 
     In certain embodiments, the carrier waveform generator circuit includes a capacitance and a current source to provide an increasing ramp signal waveform to the comparison circuit input, with a switch connected to selectively allow the current source to charge the capacitance or to fully or partially discharge the capacitance in order to reset the signal waveform. For generation of the fixed frequency pulse stream, the switch is operated by a periodic clock signal such that a new ramp signal waveform is begun at periodic intervals, and the reset circuit selectively provides an output signal to the switch to asynchronously reset the capacitor voltage such that the comparison circuit injects an additional pulse independent of the periodic clock signal in response to the detected load transient condition. In this manner, the periodic fixed frequency operation is continued with a new PWM pulse being generated during each clock period, with the pulse injection circuit operating when needed to inject one or more additional pulses between the regularly scheduled (clock-driven) PWM pulses. The control circuit may include an OR gate providing the waveform generator reset switch control signal based on inputs from the periodic clock signal and the reset circuit. In certain implementations, moreover, the comparison circuit or PWM comparator provides a complementary pulse output signal as a PWM reset pulse output to the reset circuit such that the pulse injection circuit can trigger a second or further asynchronous pulse before the next clock signal is received. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The following description and drawings set forth certain illustrative implementations of the disclosure in detail, which are indicative of several exemplary ways in which the various principles of the disclosure may be carried out. The illustrated examples, however, are not exhaustive of the many possible embodiments of the disclosure. Other objects, advantages and novel features of the disclosure will be set forth in the following detailed description when considered in conjunction with the drawings, in which: 
         FIG. 1  is a simplified schematic diagram illustrating an exemplary fixed frequency analog DC to DC converter control circuit with an asynchronous pulse injection (API) circuit for controlling output voltage load transient response in accordance with one or more aspects of the present disclosure; 
         FIGS. 2A-2C  illustrate a detailed schematic showing an exemplary embodiment of the DC to DC converter control circuit having an averaging and offset circuit for detecting increasing output load current transient conditions; 
         FIG. 3  is a graph illustrating various pulse signals and other waveforms in the control circuit of  FIG. 2 ; 
         FIG. 4  is a graph illustrating graphs showing load current transitions and corresponding output voltages for the DC to DC converter control circuit of  FIG. 2  and for a DC to DC converter without the asynchronous pulse injection circuitry; and 
         FIGS. 5A-5C  show a detailed schematic illustrating another exemplary DC to DC converter control circuit embodiment including a peak detect and offset circuit. 
     
    
    
     DETAILED DESCRIPTION 
     One or more embodiments or implementations are hereinafter described in conjunction with the drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the various features are not necessarily drawn to scale. The disclosure provides analog DC to DC converter control circuitry solutions addressing output voltage load transient response problems while still allowing fixed frequency PWM operation for selective introduction of additional PWM signal pulses triggered by detection of unregulated decreases in DC output voltage. As used herein, fixed frequency PWM operation means provision of periodic PWM pulses for use in generating switching control signals for one or more DC to DC converter switching devices, alone or supplemented by selective injection of one or more additional pulses in the PWM signal stream between the regularly occurring (periodic) pulses. The various concepts of the present disclosure find utility in association with any suitable form of DC to DC converter, including without limitation buck converters, boost converters, buck boost converters, cuk converters, etc. Furthermore, the disclosed circuits can be employed in implementing trailing-edge pulse width modulation in which a PWM pulse begins with a clock pulse edge and the trailing edge of the pulse is adjusted according to a feedback error signal, as well as in leading-edge PWM where the pulses end at a particular clock edge, and the pulse start time is determined according to closed loop regulation. Moreover, the PWM control circuitry described herein may be employed in single stage DC to DC converters as well as in operating multi-stage conversion systems, wherein the various aspects of the present disclosure are not limited by the illustrated and described embodiments. 
     Referring initially to  FIGS. 1 and 2A-2C ,  FIG. 1  illustrates a PWM control circuit  10  for a fixed frequency pulse width modulation (PWM) DC-DC converter to provide power to drive a load  20 . The PWM control circuit  10  may be used in any suitable form of fixed frequency DC-DC converter, including without limitation synchronous converters, non-synchronous converters, etc., by which an input DC signal (e.g., from an input voltage source) is converted to provide a DC output to a load  20 . Moreover, the DC-DC converter may be operated in a voltage control mode and/or in a current control mode. The control circuit  10  may be provided in an integrated circuit (IC) product, which may be a PWM controller chip used to provide PWM switching control signals to one or more external power switches (e.g., MOSFETs, IGBTs, etc.), or may be a PWM converter IC including one or more internal power switches, and may further include an onboard output inductor, for example, for buck converter applications. 
     The control circuit  10  in  FIG. 1  includes a sawtooth or ramp waveform generator circuit  14  receiving a clock output signal  32  (CLK) from an oscillator circuit  12 . In certain implementations, the oscillator circuit  12  may be incorporated with the control circuit  10  in a single integrated circuit product, or the control circuit  10  may receive the clock signal  32  from an external oscillator  12  or other clock source. The waveform generator circuit  14  may provide any suitable periodic signal waveform  15  suitable for use as a carrier signal in a carrier-based analog pulse width modulation architecture, and in the illustrated embodiments provides a ramp (sawtooth) output signal waveform  15  which is compared by a PWM comparator or comparison circuit  16  (PWM COMP. in  FIG. 1 ) with an output feedback control signal  23  (CNTL) from a compensated error amplifier circuit  22 . In the illustrated embodiment, a generally increasing ramp signal  15  is provided by the sawtooth generator  14 , and steady state fixed frequency operation begins each cycle with the ramp signal  15  being reset to a starting value, from which it rises. The beginning of the ramp signal  15 , moreover, begins the on or active state of the corresponding PWM pulse output signal  16   a , which continues in that state until the ramp signal  15  is equal to or exceeds the output voltage feedback control signal  23  (trailing edge PWM), at which point the output  16   a  transitions back to an off or inactive state. The PWM pulse output signal  16   a  is provided to a PWM and drive circuit  17  for buffering and/or suitable signal conditioning to generate a corresponding signal or signals to drive control terminals of the DC to DC converter switching devices. 
     Comparison of the sawtooth generator ramp output signal  15  with the control signal  23  thus provides a fixed frequency carrier-based PWM configuration which generates a comparator output signal for operation of the PWM and drive circuitry  17 , which can be used to generate switching control signals for one or more converter switching devices, such as high and low drivers driven in complementary fashion (e.g., MOSFETs S 1  and S 4  in  FIGS. 2A-2C and 5A-5C  below) for a buck converter in one non-limiting example. In such a high/low drive system, the pulse width of the output of the comparator circuit  16  controls the on times of the complementary signals provided to the switching devices, and the circuitry  17  staggers the driver signals such that both devices are not on at the same time. In other possible embodiments, only a single switch may be driven by the PWM output from the drive circuitry  17 , for example, with the signaling being used to control a high side driver switch and a diode is used instead of a low side driver switch (e.g., a diode is substituted for switch S 4  in  FIGS. 2A-2C and 5A-5C  below). The output of the converter switches is provided through an optional output filter circuit  18  to drive the load  20 , and the output voltage is provided as an input to a compensated error amplifier circuit  22 . The error amplifier  22  generates the output error control signal  23  representing the difference between the output voltage supplied to the load  20  and a reference voltage, such as a setpoint voltage, and the control signal  23  is provided to the PWM comparison circuit  16  for comparison with the ramp carrier waveform signal  15 . 
     In accordance with various aspects of the present disclosure, the system  10  further includes an asynchronous pulse injection (API) circuit  24  operative in response to a sensed output load transient to asynchronously inject at least one pulse into the otherwise fixed frequency pulse stream  16   a  provided by operation of the sawtooth generator  14  and the PWM comparator circuit  16 . In certain complementary output systems, the API circuit  24  may be used to selectively modify the pulse stream provided only to the high side driver, although not a strict limitation of all possible embodiments of the present disclosure. As shown in  FIG. 1 , the API circuit  24  includes an optional offset and average or peak detector circuit  26 , as well as a second comparator circuit U 12  (COMP. B), each receiving the output voltage feedback control signal  23  from the error amplifier circuit  22 , with the offset circuit  26  providing an offset signal  27  as an input signal to the comparator circuit U 12 . The offset signal  27  in certain embodiments (e.g.,  FIGS. 2A-2C ) represents an average of the feedback control signal  23 , suitably offset, such as generated using low pass filtering components and an offset current source in the circuit  26 . In other possible embodiments (e.g.,  FIGS. 5A-5C  below), the offset signal  27  is generated using a peak detector circuit (e.g., peak and hold circuit) to represent a most recent peak value of the feedback control signal  23 , offset by a predetermined amount. In such embodiments, for instance, the circuit  26  detects the peak amplitude of the steady state output voltage feedback control signal  23 , and offsets that by a predetermined amount to generate the offset signal  27 . 
     The feedback control signal  23  is compared with the offset signal  27  to generate a comparator output signal  29  that is used by a sawtooth reset pulse generator circuit  28  to selectively restart the ramp signal  15  provided by the sawtooth generator circuit  14 . In this manner, when the feedback control signal  23  from the error amplifier circuit  22  deviates from the average or peak value by the offset amount, the API circuit  24  starts a new cycle of the ramp signal  15 , thereby injecting a pulse into the PWM pulse stream  16   a  provided by the PWM comparator circuit  16 . In addition, the control signal  23  in such situations will have a voltage amplitude that will control the width of the asynchronously injected pulse, in order to at least partially counteract a corresponding dip in the output voltage provided to the load, thereby facilitating output voltage regulation independent of the condition or state of the clock signal  32 . Once the injected pulse is completed, the PWM comparator circuit  16  provides a PWM reset signal  16   b  to the sawtooth reset pulse generator circuit  28  to enable the circuit  28  to generate a further asynchronous pulse, if still required. In the illustrated examples, moreover, the PWM reset signal  16   b  allows the regularly scheduled PWM pulses triggered by the oscillator circuit  12  to occur at the fixed operating frequency, regardless of whether the API circuit  24  is attempting to insert an asynchronous pulse, whereby the circuit  10  maintains fixed frequency PWM operation. For example, a somewhat lengthy output voltage dip may extend both before and after a regularly scheduled (fixed frequency) PWM pulse according to the oscillator circuit  12 , with the PWM reset signal  16   b  causing the regularly scheduled PWM pulse to be initiated in accordance with the oscillator clock signal, with the potential for asynchronous pulses being inserted both before and after the regularly scheduled pulse, as seen in the example of  FIG. 3  below. In this regard, various implementations are possible in which the API circuit  24  is configured to selectively provide at least one additional pulse in a given oscillator clock period, and certain embodiments may allow more than one API pulse to be provided in a given oscillator clock period. 
     Operation of the API circuit  24 , moreover, is independent of the oscillator circuit  12 , whereby the normal fixed frequency pulse stream continues, and the next occurrence of an oscillator clock pulse will trigger the next regularly scheduled PWM pulse in addition to any injected pulse caused by operation of the API circuit  24 . In addition, the offsetting provided by the circuit  26  is preferably set such that the API circuit  24  will reset the sawtooth generator  14  only when the feedback control signal  23  has undergone an excursion of a predetermined amount indicating a voltage dip in the output. In certain embodiments, for instance, the offsetting provided by the circuitry  26  is such that the API circuit  24  will not inject an asynchronous pulse unless a decreasing output voltage excursion exceeds normal or expected output voltage ripple levels. In this regard, various implementations can be tailored to selectively avoid false triggering based on normally expected output ripple voltage excursions and any associated sources of noise in the circuit  10 , while responding quickly to significant output voltage dips in a timely fashion to mitigate the amount of output voltage drop in response to increasing load current transitions. 
     By this operation, the API circuitry  24  advantageously utilizes the output of the compensated error amplifier  22  for detection of increasing load current transient conditions, without need for a separate integrated circuit pin to monitor output voltage, and the width of the asynchronously injected pulse is controlled to limit the energy delivered to the power converter output inductor thus limiting the amount of current overshoot. Moreover, the addition of the API circuit  24  may be employed in control circuits used in single or multiphase (e.g., stacked) DC-DC conversion architectures. Furthermore, the injected pulse is inserted into the regularly scheduled fixed frequency PWM pulse stream in response to detection of the output load transient, thereby mitigating latency and ultimately reducing the amount of voltage dip resulting from such transient conditions. This operation, moreover, may facilitate potential reduction in the output capacitance requirement of a given DC-DC converter design, thereby reducing system costs and space requirements. 
     Referring also to  FIGS. 2A-4 ,  FIGS. 2A-2C  illustrate further details of an exemplary embodiment of a fixed frequency DC-DC converter control circuit  10 , and  FIGS. 3 and 4  illustrate exemplary waveforms and signals in the circuit  10  for situations involving increasing load current transients. In this embodiment, the pulse width modulation technique involves trailing-edge modulation, but other embodiments are possible using leading-edge modulation circuitry. In addition, the illustrated embodiments provide overall output voltage regulation using a compensated error amplifier circuit  22  providing a voltage output feedback control signal  23 , although other embodiments are possible in which the overall control loop or regulation loop implements a current regulation approach. 
     As seen in  FIGS. 2A-2C , an oscillator circuit  12  is illustrated, which may be incorporated into the control circuit  10 , or a clock signal  32  may be provided from an external source. For example, an integrated circuit implementation is possible including an on-board oscillator circuit  12 , or such integrated circuit may include a pin for receipt of the clock signal  32 . As shown, the oscillator circuit  12  includes a capacitor C 3  with a first or lower terminal connected to a voltage source V 3 , and a second or upper terminal connected to a current source I 3  at node N 3 . A switch S 2  is connected in parallel across the capacitor C 3  and is operated according to a switching control signal to be in either an open (high impedance) state allowing the current source I 3  to charge the capacitor C 3  or a closed (low impedance) state to discharge the capacitor C 3 . When discharged, the upper terminal of the capacitor C 3  will be at a voltage approximately equal to the voltage source V 3 , and opening the switch S 2  allows the capacitor C 3  to charge, thereby increasing the voltage at the upper capacitor terminal in a linear (e.g., ramp) fashion. The oscillator circuit  12  further includes comparators U 1  and U 3  as well as voltage sources V 2  and V 4  and a set/reset (S/R) flip-flop U 2  connected as shown, with the non-inverting (+) input of U 1  being connected to a voltage equal to the sum of V 2  and V 4 , and the inverting (−) input of U 3  being connected to a voltage V 4 . In addition, the upper terminal of capacitor C 3  is connected to the inverting input of U 1  and the non-inverting input of U 3  to provide the increasing ramp capacitor voltage as an input thereto. As connected, the inverting output of U 1  provides an input to a set (S) input of the flip-flop U 2 , and the flip-flop reset (R) input is connected to the inverting output of U 3 . The clock signal  32  (CLK) is provided by the data output “Q” of the flip-flop U 2 , and the signal  32  is also provided as the switch control signal to switch S 2 , whereby the circuit  12  oscillates at a fixed frequency dictated by the capacitance of C 3 , the current provided by I 3 , and the relative voltages V 2 -V 4 . In addition, the complementary output “Q′” of the flip-flop U 2  is provided to the driver circuit  17  at node N 1  as described further below. 
     The control circuit  10  in  FIGS. 2A-2C  further includes an analog carrier waveform generator circuit  14  which provides a periodic carrier signal waveform  15  according to the clock input signal  32 . In the illustrated implementation, the waveform generator circuit  14  includes a capacitor C 4  connected in parallel with a current source I 2 , and the capacitor C 4  is also connected in parallel with a series combination of a switch S 3  and a voltage source V 5 . In operation, the current source I 2  charges the capacitor C 4  while the switch S 3  is open, whereby the voltage of the carrier signal waveform  15  is a generally linearly increasing or rising ramp voltage. Closure of the switch S 3  discharges the capacitor C 4  to the voltage V 5 , after which subsequent opening of the switch S 3  starts a new cycle of the increasing ramp signal waveform  15 . The switch S 3  is operated by a reset control signal  33  provided by an OR gate U 7  having a first input receiving the oscillator clock output signal  32  and a second input receiving an output signal  25  from a reset circuit  28  for asynchronous injection of one or more pulses into the PWM output stream as described further below. In addition, independent of any assertion of the signal  25  from the reset circuit  28 , the periodic restarting or resetting of the carrier waveform generator circuit output ramp waveform  15  is controlled by the clock signal  32 . 
     The periodic carrier signal waveform  15  is provided as a voltage signal to the non-inverting input (+) of the PWM comparator circuit  16 , and the comparator  16  receives the output error control signal  23  at an inverting input (−). The comparator  16  has a non-inverting output providing a pulse output signal  16   a  to a reset (R) input of a driver circuit flip-flop U 5 , whose set (S) input is connected to receive the reset signal  33  from the OR gate U 7  as shown. The data (Q) output of the driver circuit flip-flop U 5  provides a first input to an AND gate U 4  via node N 6 , and the complementary data output (Q′) of the oscillator circuit flip-flop U 2  is connected as the second input to the gate U 4 . The output of U 4  provides a PWM input to a driver circuit U 6 , and the ramp voltage across C 3  of the oscillator circuit  12  provides a peak voltage input to the driver U 6 . The driver circuit U 6  includes suitable signal conditioning circuitry as is known for creating complementary high and low switch driver output signals including a high drive output signal  42  (HDRV) and a complementary low drive output signal LDRV. 
     In operation, the driver circuit  17  provides the switching control signal  42  to operate a corresponding DC to DC converter switching device Si for selective conversion of power from a DC input VIN to provide DC output power to drive the load (load  20  in  FIG. 1 ) in whole or in part according to the pulse output signal  16   a  from the comparator  16 . In the illustrated buck converter configuration, moreover, a low side driver switch S 4  is connected between the lower terminal of S 1  and a circuit ground node, with fly back diodes D 1  and D 2  connected in parallel with the switches S 1  and S 4 , respectively. In addition, the converter output is provided via an output inductor L 1  for conduction of an inductor current IL, with an output filter  18  including a capacitor CC 1  being connected between the output terminal and the circuit ground node, where the output voltage VOUT appears across the output capacitor CC 1 . In the illustrated embodiment, moreover, the low side driver switch S 4  is driven by the output of an AND gate U 9  having a first input receiving the LDRV signal from the driver U 6  and a second input provided by a comparator U 10 . The comparator U 10  compares the output error control signal  23  to the voltage value V 5  at node N 5  and selectively allows the low drive signal to operate the switch S 4  when the control signal  23  exceeds V 5 . In operation, therefore, U 10  turns OFF the synchronous rectifier when the output is above the regulation voltage to facilitate reduced output voltage overshoot during an abrupt load decrease. 
     The control circuit  10  further includes optional current mode control components, including an RC network with a resistor R 2  and a capacitor C 2  connected to provide a low pass filter for measurement of the voltage across the inductor L 1  to provide a differential signal to an amplifier E 1 . The amplifier E 1 , in turn, provides a differential output voltage through another RC filter circuit including resistor R 1  and capacitor C 1  as an input to a transconductance amplifier G 1 . The transconductance amplifier G 1  is powered from a voltage source V 7  via node N 4 , and provides an offset current output connected to the output error control signal  23  at the inverting input of the comparison circuit  16 . In this embodiment, therefore, the control circuit  10  can be operated in a current mode in which the error signal is offset according to the average inductor current IL, although other embodiments are possible in which the transconductance amplifier output current is not used to offset the control signal  23  for voltage mode operation. In another possible implementation, the offset current output of G 1  could be used to provide an offset to the non-inverting input to the PWM comparator  16  (instead of the inverting input as shown). In other possible embodiments, the current measurement and offsetting components R 1 , R 2 , C 1 , C 2 , E 1  and G 1  can be omitted, with the circuit  10  operating in voltage mode according to the voltage error feedback control signal  23 . 
     As seen in  FIGS. 2A-2C , moreover, the DC to DC converter control circuit  10  includes an error amplifier circuit  22  which provides the output error control signal  23  to the comparison circuit  16  representing an output voltage error. In particular, the output terminal provides a voltage VOUT across the output capacitor CC 1 , and the output voltage signal is provided as an input to the error amplifier circuit  22  at a junction of a resistor R 6  and a capacitor C 6 . C 6  and a second resistor R 4  are connected in parallel across the resistor R 6  to provide an input to an inverting input (−) of an operational amplifier (op amp) X 1 , which is connected through a divider resistor R 9  to the circuit ground. The resistors R 6  and R 9  form a resistive voltage divider circuit to create the inverting input to X 1 , and the non-inverting (+) input to X 1  is connected to a reference voltage VREF. In practice, the reference voltage VREF can be a fixed voltage source in the circuit  10 , or the reference voltage signal may be provided as a setpoint input for regulation of the DC to DC converter, for example, from another circuit or other suitable source (not shown). The error amplifier  22  further includes a feedback network with feedback capacitors C 5  connected between the inverting input and the output of the amplifier X 1 , as well as a resistor R 5  and another feedback capacitor C 7  connected in series with one another in parallel with C 5  as shown. 
     In accordance with one or more aspects of the present disclosure, moreover, the control circuit  10  includes a pulse injection circuit, referred to herein as an asynchronous pulse injection (API) circuit  24 , which is coupled with the error amplifier  22  to receive the output error control signal  23 . In operation, the API circuit  24  detects an output load current transient condition indicating an increased output load current according to the output error control signal  23 . The pulse injection circuit  24  operates to selectively provide at least one signal  25  to cause the comparison circuit  16  to asynchronously inject at least one pulse into the pulse output signal  16   a  in response to detection of the output load current transient condition. The API circuit embodiment  24  in  FIGS. 2A-2C  is connected through a resistor R 8  to the output of the error amplifier  22  to receive the output error control signal  23  (CNTL). In general, the circuit  24  operates to cause the comparison circuit  16  to asynchronously inject at least one pulse into the pulse output signal  16   a  in response to detection of an output load current transient condition indicating an unregulated drop in the output voltage VOUT, thus representing an increase in the output load current requirement. 
     As seen in  FIGS. 2A-2C , the API circuit  24  includes an offset circuit  26  which creates an offset signal  27  by adding an offset voltage to the output error control signal  23 . In the embodiment of  FIGS. 2A-2C , the offset circuit  26  provides the offset signal  27  to represent an average of the output error control signal  23 , offset or shifted by the offset voltage amount. In the illustrated implementation, for example, the offsetting effectively raises the voltage of the offset signal  27  relative to that of the original output error control signal  23 , in order to provide a tolerance band to prevent false triggering of the asynchronous pulse injection, such that the output voltage VOUT can be regulated using the periodic PWM pulses in the signal stream  16   a  including a certain expected amount of ripple voltage. The API circuit  24  responds when the voltage drops below this normal expected regulation range to introduce one or more asynchronous pulses into the signal  16   a . Moreover, the offset circuit embodiment  26  of  FIGS. 2A-2C  provides low pass filtering or averaging such that the offset signal  27  represents the average of the control signal  23 , shifted by the offset voltage amount. The circuit  26  in this case includes a resistor R 7  to receive the output error control signal  23 , which is connected between R 8  and an inverting input (−) of a second comparator U 12 , as well as a capacitor CC 4  connected between the inverting input and the circuit ground, whereby resistor R 7  and capacitor CC 4  create an RC low pass filter circuit to present the offset signal  27  as the inverting input based on the average of the control signal  23 , where the values of R 7  and CC 4  are set to provide a cutoff frequency below the expected output voltage ripple frequency (e.g., below the frequency of the clock signal  32 ). In addition, the offset circuit  26  includes an offset current source  14  injecting current into the inverting input of U 12 , where the amount of injected offset current from  14  is preferably set to increase the voltage at the inverting input by an amount greater than the expected ripple voltage of the control signal  23  to avoid false triggering of the API circuit  24 . In this manner, the offset signal  27  provided to the inverting input of U 12  represents the average of the output error control signal  23 , offset or shifted by the offset voltage amount. The offset voltage amount is preferably greater than the peak to peak ripple voltage on the control signal  23  plus some margin, but also preferably low enough to allow for a small delay from the time the signal  23  begins to move to the time U 12  outputs a “high” signal in order to dominate the latency of the circuit  10 . 
     The non-inverting (+) input of the API circuit comparator U 12  is connected to receive the output error control signal  23 , and the output of U 12  provides a binary comparator voltage output signal  29  at a first voltage level (low) indicating regulated operation when the offset signal  27  exceeds the present value of the output error control signal  23 , and at a second voltage level (high) when the output error control signal  23  is greater than the offset signal  27  indicating an unregulated drop in the output voltage. 
     The API circuit  24  in this example also includes a reset circuit  28  which provides an output signal  25  to selectively restart the periodic carrier signal waveform  15  via a reset OR gate U 7  to cause the comparator  16  to generate an asynchronous pulse in response to the comparator voltage output signal  29  from the API comparator U 12  transitioning from the first voltage level to the second voltage level. The illustrated reset circuit  28  includes a flip-flop U 11  receiving the binary comparator voltage output signal  29  at a set (S) input, and receiving a PWM reset signal  16   b  from the PWM comparator  16  at a reset (R) input. The flip-flop U 11  provides an output signal  25  from the data output “Q” through an AND gate U 14  to U 7  in order to cause the comparison circuit  16  to asynchronously inject one or more pulses into the pulse output signal  16   a  in response to the binary comparator voltage output signal  29  transitioning from the first voltage level to the second voltage level (e.g., when the output voltage dips below the normal regulation range in response to an increased output current demand). 
     The PWM comparison circuit  16  includes an inverting second output providing the PWM reset pulse output signal  16   b  complementary to the primary PWM pulse output signal  16   a,  which allows the reset circuit  28  to potentially inject more than one asynchronous pulse into the pulse stream  16   a  between clock pulses, if needed to address a detected output voltage dip. In this example, moreover, the AND gate U 14  receives the voltage input V 8 , and the output of a buffer amplifier U 15  receiving the output at node N 2  from a comparator U 16  comparing the reference voltage (setpoint) input VREF to a voltage source V 10 . In operation, U 15  effectively deactivates the API circuit  24  during startup and delays initialization until the reset signal  27  (average) has stabilized. 
     Referring also to  FIGS. 3 and 4 ,  FIG. 3  illustrates various waveforms in the control circuit embodiment  10  of  FIG. 2 , including a graph  30  showing the clock signal pulses  32  and a graph  40  showing the high drive PWM signal pulses  42  associated with the clock pulses  32 , as well as examples of asynchronously inserted pulses  25   a  and  25   b  initiated by the API circuit  24  asserting the output signal  25  to reset the waveform generator  14 . As seen in  FIGS. 2A-2C , the OR gate U 7  allows either the clock signal  32  or the API output signal  25  to restart the ramp generator circuit  14 , thereby initiating the beginning of a PWM pulse. In the example of  FIG. 3 , moreover, a graph  50  illustrates the output load current requirement  52  (ILOAD) for the DC to DC converter, which undergoes an exemplary fast transient increase as shown. 
     The graph  60  in  FIG. 3  further illustrates the periodic carrier signal waveform  15  that provides an increasing ramp (sawtooth) waveform for which each cycle begins in response to a negative edge of the clock signal  32  (e.g., by operation of the switch S 3  in the waveform generator circuit  14  of  FIG. 2 ). Graph  70  in  FIG. 3  further illustrates the output error control signal  23  along with the offset signal  27 , where the transition of the control signal  23  above the offset signal  27  causes the output of U 12  to transition from low to high, thereby triggering the flip-flop U 11  to generate the reset circuit output signal  25  to begin a new cycle of the ramp signal  15  (graph  60 ), resulting in creation of the inserted asynchronous pulse  25   a  (graph  40 ). At the next negative edge of the clock signal  32 , the OR gate U 7  initiates another reset signal  33  to again restart the ramp waveform to create the next scheduled (periodic) PWM pulse  42  (graph  40 ). The width of this periodic pulse  42 , moreover, is greater than that of the preceding periodic pulse  42 , since the control error signal  23  is now at a higher level due to the output voltage dropping below the normal regulation range (e.g., the voltage error is increased). After the periodic pulse  42 , the control signal  23  in this example remains above the offset signal  27  (e.g., due to a large increase in the output load current demand  52  (graph  50 )), and thus the API circuit  24  again asserts the signal  25  to initiate another reset signal  33 , resulting in another restart to the ramp signal  15  (graph  60 ), causing the comparator circuit  16  to insert a further asynchronous pulse  25   b  (graph  40 ) into the high drive PWM pulse stream. 
     A graph  80  in  FIG. 3  shows the corresponding output voltage signal  82  (VOUT) which initially dips as a result of the increased output current load transition  52  (graph  50 ). As seen in the graph  80 , moreover, the voltage output curve  82  then recovers fairly quickly as a result of the asynchronously injected pulses  25   a  and  25   b  in addition to the intervening regularly scheduled pulse  42 . In this regard, the additional pulses  25   a  and  25   b  serve to replenish the energy in the output inductor L 1  ( FIGS. 2A-2C ), thereby minimizing or in any event reducing the amount of output voltage dip during the transient. The graph  80  further illustrates a corresponding output voltage curve  84  showing operation of the PWM controller  10  without use of the API circuit  24 , wherein the voltage output  84  drops significantly following the increasing load current transition  52 . Moreover, the response of the control circuit  10  without the asynchronous pulse injection aspects of the present disclosure results in a larger voltage amplitude drop  84 , as well as a longer response time (e.g., several cycles of the clock signal  32  in this example). Also, the graph  80  in  FIG. 3  illustrates the output voltage curve  84  undergoing an overshoot condition (indicated at  86 ), which overshoot result is avoided or mitigated by use of the API circuit  24  as seen in the curve  82 .  FIG. 3  further illustrates a graph  90  showing the output inductor current  92  (IL) which generally increases periodically in response to the regularly scheduled PWM pulses  42 , and then provides increased levels in response to the asynchronous pulses  25   a  and  25   b  as shown, whereby the average inductor current value generally tracks the load requirement transient  52  (ILOAD in graph  50 ). The graph  90  further illustrates a curve  94  showing the inductor current for the case in which no API circuit  24  is used, wherein it is seen that the current  94  follows a lower trajectory immediately following the output load transient increase  52 , and results in higher output current for the following several clock cycles  32 , whereby it is seen that the graph  92  involving use of the API circuit  24  successfully mitigates or avoids output current ringing or oscillation or overshoot compared with the approach in which no asynchronous pulses are injected. 
       FIG. 4  further illustrates operation of the circuit  10  with and without use of the API circuit  24 . The graph  100  in  FIG. 4  shows the output load current requirement curve  52  for a longer time interval then was shown in the graph  50  of  FIG. 3 , undergoing an initial increase, followed by a subsequent decrease. The graph  110  in  FIG. 4  shows the output voltage curve  82  (similar to that of graph  80  in  FIG. 3 ), in which the output voltage dip following the load current requirement transition has a magnitude  112 , and the output voltage curve  82  resumes normal regulation fairly quickly with no overshoot. The graph  120  shows the output voltage curve  84  where no API pulses are injected into the PWM pulse stream  16   a  (similar to that of graph  80  in  FIG. 3 ) which exhibits a voltage dip magnitude  122  greater than the magnitude  112  experienced when one or more API pulses are injected. In addition, as seen in  FIG. 4 , the output voltage undergoes a significant overshoot condition at  86 , which is not seen in the curve  82  of the graph  110 . 
     The asynchronous pulse injection circuitry  24  thus advantageously operates to improve the load transient response of the fixed frequency trailing-edge PWM DC to DC converter, and may also be implemented in leading-edge PWM configurations. Moreover, as discussed above, the API circuitry  24  can be employed in fixed frequency PWM DC to DC conversion applications implemented for a current mode control architecture. In certain examples, for instance, the saw-tooth ramp signal  15  can be used as a slope compensation signal, and the control signal  23  is again used as the inverting input to the comparator  16 . 
       FIGS. 5A-5C  illustrate another circuit embodiment  10  generally as described above in connection with  FIGS. 2A-2C . In this example, however, the offset circuit  26  includes a peak detector circuit U 17  with an input connected to receive the output error control signal  23 , as well as an output connected to provide a peak voltage signal representing the peak of the output error control signal  23  as the second (inverting) input of the comparator U 12 . In addition, the offset circuit  26  includes the offset current source  14  which provides an offset current to increase the voltage at the inverting input of U 12 . In this manner, the offset circuit  27  provides the offset signal  27  to the comparator U 12  to represent the peak voltage of the output error control signal  23  offset or shifted up by the offset voltage amount. As in the above described implementations, the offset signal  27  in  FIGS. 5A-5C  is compared by the comparator U 12  with the unmodified control signal  23  for selectively detecting upward load current transient conditions and corresponding triggering of asynchronous pulse injection into the pulse stream  16   a  as described above. Any suitable peak and hold circuit U 17  can be used which quantifies the peak output of the input signal for subsequent offsetting using the current source  14 . 
     The above examples are merely illustrative of several possible embodiments of various aspects of the present disclosure, wherein equivalent alterations and/or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. In addition, although a particular feature of the disclosure may have been disclosed with respect to only one of multiple implementations, such feature may be combined with one or more other features of other embodiments as may be desired and advantageous for any given or particular application. Also, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the detailed description and/or in the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.