Abstract:
Controlled compensation for a switching regulator is attained by detecting switching duty cycle of the switching regulator, developing a compensation signal having a time duration that is related to the detected switching duty cycle percentage, and generating a duty cycle control signal for the regulator that is dependent in part on the developed compensation signal. The compensation signal has a slope profile and is initiated during each switching cycle at a set point in the cycle that is related to the switching duty cycle percentage,

Description:
TECHNICAL FIELD 
     This disclosure is related to switching regulators, and more particularly to compensating control of the switching duty cycle. 
     BACKGROUND 
     The use of current mode switching regulators to control a DC output voltage at a level higher than, lower than, or the same as an input voltage is well known. Typically, one or more switches are activated to supply current pulses via an inductor to charge an output capacitor. The output voltage level is maintained at a desired level by adjusting the on and off times of the switching pulses in accordance with output voltage and load conditions. 
       FIG. 1  is a block diagram of a typical current mode switching regulator. Switching control circuit  10  may comprise any of various known controllers that provide pulse width modulated output pulses to regulate a DC output voltage V OUT  at a level that may be greater than, lower than, or the same as a nominal input voltage V IN . Typically, the control circuit includes a latch, having set and reset inputs, coupled to a controlled switch that supplies switched current I SW  to inductor  12 . Capacitor  14  is connected between the output V OUT  and ground. Resistors  16  and  18  are connected in series between V OUT  and ground. A load  20  is supplied from the regulator output. 
     The set input is coupled to clock  22 , which may generate pulses in response to an oscillator, not shown. During normal operation, the latch is activated to initiate a switched current pulse when the set input receives each clock pulse. The switched current pulse is terminated when the reset input receives an input signal, thereby determining the width of the switched current pulse. The reset input is coupled to the output of comparator  24 . An output voltage feedback signal V FB  is taken at the junction of resistors  16  and  18  and coupled to negative input of error amplifier  26 . A voltage reference V REF  is applied to the positive input of error amplifier  26 . Capacitor  28  is coupled between the output of error amplifier  26  and ground. 
     The level of charge of capacitor  28 , and thus its voltage V C , is varied in dependence upon the output of amplifier  26 . As load current increases, the output voltage, and thus V FB , decreases. As the feedback voltage V FB  decreases, V C  increases. Thus, V C  is proportional to load current. V C  is coupled to the inverting input of comparator  24 . The non-inverting input is coupled to adder  30 . Adder  30  combines signal I SW , which is proportional to the sensed switch current, with a compensation signal. Upon switch activation in response to a clock set signal, switch current builds through inductor  12 . When the level of the signal received from adder  30  exceeds V C , comparator  24  generates a reset signal to terminate the switched current pulse. During heavier loads V C  increases and the switched current pulse accordingly increases in length to appropriately regulate the output voltage V OUT . As V C  is an indication of load, it can be monitored by internal circuitry, not shown, to detect light load conditions. In response to V C  reaching a predetermined light load condition threshold, the operation can be changed to a “sleep mode,” in which some circuit elements can be deactivated to consume power. 
     For normal regulator operation at duty cycles of fifty percent or higher, compensation is needed in the switching control to avoid sub-harmonic oscillation. A typical approach is termed “slope compensation,” wherein a signal of increasing magnitude is added to the current signal I SW , or subtracted from the signal V C , during each switching cycle.  FIG. 2  is a circuit diagram of a prior art slope compensation generator that may be input to adder  30  to modify the current signal applied to the non-inverting input of comparator  24 . The output of the circuit is a current signal Sx corresponding to the current in the series circuit path of transistor  32 , resistor  34  (R) and voltage bias (VB) source  36 . The base of transistor  32  is coupled to the output of unity gain buffer amplifier  38 . The positive input of amplifier  38  is coupled to receive a ramp signal Vramp. The negative input of amplifier  38  is coupled to the junction between transistor  32  and resistor  34 . 
       FIG. 3  is a simplified waveform diagram illustrative of the compensation function of the circuit of  FIG. 2 . The Vramp signal is a sawtooth format signal that is generated at the beginning of each clock cycle and extends at linear slope to the end of the cycle, corresponding to one hundred percent duty cycle. As an example, the Vramp magnitude may vary between zero and one volt. Transistor  32  begins conduction at a point Ts in the cycle at which Vramp overtakes the fixed voltage VB. As compensation is needed at fifty percent duty cycle operation or greater, VB typically is arbitrarily chosen at one half the value of the maximum Vramp level, or one half-volt in the present example. As Vramp continues to increase after time Ts, the base signal applied to transistor  32  increases and, thus, the output current Sx increases linearly to a maximum Smax at the end of the switching cycle. Sx is determined by (Vramp−VB)/R. The compensation curve Sx starting point Ts is thus determined by VB, and its slope is determined by R. In this example Ts occurs at fifty percent of the switching cycle, regardless of the actual duty cycle. 
     Because this slope compensation curve starts at fifty percent of the switching cycle, the Sx builds up to a high level at a maximum duty cycle of one hundred percent. The high level of the compensation signal is disadvantageous at or near maximum duty cycle operation. The voltage V C , the output of feedback amplifier  26  applied to the negative input of comparator  24 , has the same value as the sum of the switch current I SW  and the slope compensation signal Isx at the positive input of comparator  24  when the switched pulse terminates. As a high offset has been introduced, V C  will not accurately indicate true output load current. At the high duty cycle operation, the switching current limit level is reduced. At higher duty cycle operation, the sleep mode threshold, based on V C , will be inaccurate. The need thus exists for an improved compensation scheme that overcomes the drawbacks of the prior art slope compensation. 
     Disclosure 
     The above-described needs of the prior art are fulfilled, at least in part, by detecting switching duty cycle of a switching regulator, developing a compensation signal having a time duration that is related to the detected switching duty cycle, and generating a duty cycle control signal for the regulator that is dependent in part on the developed compensation signal. The compensation signal has a slope profile and is initiated during each switching cycle at a set point in the cycle that is related to the switching duty cycle. 
     The duty cycle may be detected by generating a repetitive pulse signal coordinated with the regulator switching, and integrating the pulse signal. The point in each cycle at which the compensation signal is initiated may be set by generating a ramp signal at the onset of each switching cycle, modifying the duty cycle signal, and comparing the repetitive ramp signal with the modified duty cycle signal. When the ramp signal is equal in magnitude to the modified duty cycle signal, the compensation signal commences. Preferably, the duty cycle signal is modified by offsetting the duty cycle signal by a fixed voltage. 
     In an exemplified implementation, a variable compensation circuit is coupled to an input of a switching controller input for terminating a switching pulse during each switching cycle. In a preferred configuration, an amplifier circuit output is coupled to the controller input. A ramp generator provides a ramp signal to an input of the amplifier circuit, and a variable offset circuit provides a variable offset signal to the amplifier input of the amplifier. The variable offset circuit is coupled in series with the amplifier circuit output. The amplifier output signal is proportional to the difference between the ramp signal and the variable offset signal. The compensation circuit thus outputs a signal that has an offset level that varies as a function of the duty cycle of the regulator switching operation. 
     The amplifier circuit may be configured with an amplifier having a positive input terminal coupled to the ramp generator, a negative input terminal coupled to the variable offset circuit, and an output. A control terminal of a transistor is coupled to the amplifier output. The transistor is coupled between the variable offset circuit and the amplifier circuit output. An impedance is coupled in series with the transistor, thereby determining the slope of the compensation circuit output signal. 
     The variable offset circuit may be exemplified by a duty cycle detection circuit and a constant offset voltage circuit, each coupled to an adder output circuit is coupled to the amplifier input. The duty cycle detection circuit may include an integrator circuit configured to receive a repetitive pulse signal that is coordinated with the regulator switching. 
     Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  is a block diagram of a typical current mode switching regulator. 
         FIG. 2  is a circuit diagram of a prior art slope compensation generator. 
         FIG. 3  is a simplified waveform diagram illustrative of the compensation function of the circuit of  FIG. 2 . 
         FIG. 4  is a block diagram exemplifying slope compensation in accordance with the present invention. 
         FIG. 5  is a circuit diagram of the slope compensation generator of  FIG. 4 . 
         FIG. 6   a  is a waveform diagram illustrative of various signals of the circuit of  FIG. 5 .  FIG. 6   b  is a waveform diagram in which the compensation signals of the circuits of  FIGS. 2 and 5  are compared. 
     
    
    
     DETAILED DESCRIPTION 
     An underlying concept of the present disclosure is based on the realization that slope compensation is needed only at the instant when the regulator switch turns off. If the regulator of  FIG. 2  of the prior art is operating at ninety percent duty cycle, for example, the build up of compensation signal starting at fifty percent is unnecessary until near the ninety percent point. By that time, a substantial offset signal magnitude has been developed.  FIG. 4  is a block diagram of a variable compensation circuit that permits generation of the compensation signal to start in advance of the duty cycle by a fixed small value in each switching cycle. With this circuit, the operating duty cycle is sensed and the slope compensation signal is started only slightly before the regulator switch turns off. The unnecessary slope compensation over lower duty cycles is avoided. Sleep mode level can be detected more accurately. The magnitude of the slope compensation signal at switching pulse termination is at a level that does not significantly lower the switch current limit level. 
     Amplifier circuit  37  outputs a compensation signal Sx, having a constant slope, that may be coupled as an input to the adder  30  of  FIG. 1 . Buffer amplifier  38  receives a ramp signal from ramp generator  48 . The ramp generator may comprise a well known capacitive circuit that is synchronized with a clock signal to provide a sawtooth type waveform having a constant slope. The buffer amplifier  38  receives a second signal from variable offset circuit  41 , which adds, via summer  40 , a fixed offset voltage from source  36  to a signal provided by duty cycle detection circuit  45 . The output of duty cycle detection circuit  45  is proportional to the regulator duty cycle. The output of the circuit  41  is proportional to the regulator duty cycle offset by the fixed level of the source  36 . The output circuit  39  initiates output signal Sx in each cycle when the ramp generator signal exceeds the signal received from variable offset circuit  41 . The output signal Sx commences later in a switching cycle for higher duty cycle operation than for lower duty cycle operation and is thus variable with respect to its phase in the switching cycle. There is fixed phase difference between Sx and the operating duty cycle. 
       FIG. 5  is a partial circuit diagram of the variable compensation circuit of  FIG. 4 . Transistor  32  is coupled in series with resistor  34  and adder  40 . These elements form, in part, the output circuit  39  that produces the compensation signal Sx. A first input of adder  40  is coupled to fixed offset voltage  36 . The second input of adder  40  is coupled to buffer  42 . The input of buffer  42  is coupled to a junction between resistor  44  and capacitor  46 . Buffer  42 , resistor  44  and capacitor  46  form the duty cycle detection circuit  45 . Voltage signal V 1  is coupled to resistor  44 . The positive input of amplifier  38  is coupled to receive a ramp signal Vramp. The negative input of amplifier  38  is coupled to the junction between transistor  32  and resistor  34 . 
     Reference is made to the waveforms of  FIG. 6   a  in describing the operation of the circuit of  FIG. 5 . For clarity of explanation, an example is taken in which the regulator is operating at a ninety percent duty cycle and in which the Vramp and V 1  signals have upper and lower limits of one volt and zero volt, respectively. It should be understood that the regulator is capable of operation throughout the complete duty cycle range and that the voltage parameter ranges are subject to selection. 
     The V 1  signal is a square signal that coincides with the regulator switching. As shown, V 1  is at a level of one volt for ninety percent of the cycle and at zero the last ten percent. This signal is averaged by the integrator formed by resistor  44  and capacitor  46 . The voltage at the junction of these two circuit elements thus is 0.9 volt at ninety percent duty cycle. This averaged voltage is buffered by buffer  42  and input to the adder  40  where it is added to the fixed voltage offset of voltage source  36  to provide a voltage Vx at the adder output. In this example, the fixed voltage offset has been selected at −0.1 volt. Vx is thus 0.8 volt. Prior to conduction of transistor  32 , the voltages at both the transistor emitter and the negative input of amplifier  38  are at the level of Vx. 
     The Vramp signal is generated at the start of each clock cycle and increases linearly until the next clock. The slope of this signal is selected such that its magnitude corresponds in number with the duty cycle. That is, for example, Vramp at 0.5 volt occurs at fifty percent duty cycle. When the Vramp signal rises to the level of Vx, amplifier  38  will drive transistor  32  to conduction. As Vramp continues to increase, the voltage applied to the base of transistor  32  continues to increase and its conduction current, Sx, increases. In the illustrated example, the fixed offset voltage of source  36  has been chosen to be −0.1 volt, whereby Vx becomes 0.8 volt. As both voltage inputs to amplifier are equal at 0.8 v, the compensation signal is initiated at eighty percent duty cycle, as illustrated in  FIG. 6   a . The particular voltage offset selection has been made so that the slope compensation curve starts at only a small percent duty cycle ahead of the regulator operating duty cycle, ten percent in this example. If the duty cycle were to change to seventy percent, Vx would change to 0.6 volt and the compensation signal would commence at sixty percent duty cycle. When the transistor conducts, a voltage drop occurs across resistor  34 . The value of resistor  34  is selected to set a slope of Sx so that it is suitable for compensation without reaching a high level having the drawbacks of the prior art. 
     The waveform diagram of  FIG. 6   b  is illustrative of the compensation signals produced by the prior art circuit of  FIG. 2  and the circuit of  FIG. 5  for ninety percent duty cycle operation, corresponding to the above described example. Compensation signal Sx 1  of the  FIG. 2  circuit commences at fifty percent duty cycle and builds up to a high level at ninety percent duty cycle at which point switch cutoff occurs. At this point, the output current of adder  30  is equal to V C  and comprises a large Sx 1  component. At the illustrated compensation signal slope, the Sx 1  component appears to be approximately eighty percent of the magnitude of the Vramp signal and a significantly large value in comparison with the V C  level. In contrast, compensation signal Sx 2  of the circuit of  FIG. 5  commences at eighty percent duty cycle with the same slope. At the ninety percent duty cycle cutoff, the Sx 2  component of the output of adder  30  appears to be approximately twenty percent of the magnitude of the Vramp signal. As a result, the accuracy of V C  as an indicator of output load current and the switching current limit level are greatly improved. 
     In this disclosure there are shown and described only preferred embodiments of the invention and but a few examples of its versatility. It is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. The principles of the invention are applicable to a variety of voltage regulators, including buck, boost, and buck-boost regulators. By appropriate selection of the parameters of the resistor  34 , the duty cycle detection elements, and the operating voltage levels, the slope of Sx and its onset in relation to duty cycle operation can be adjusted to desired levels.