Patent Document

TECHNICAL FIELD  
       [0001]     This disclosure is related to switching regulators, and more particularly to provision of compensation during control of the switching duty cycle.  
       BACKGROUND  
       [0002]     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.  
         [0003]      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.  
         [0004]     The set input is coupled to clock  22 , which may generate pulses in response to an oscillator. 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.  
         [0005]     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 .  
         [0006]     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 compensation 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 (R)  34  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 an oscillator generated ramp signal Vramp. The negative input of amplifier  38  is coupled to the junction between transistor  32  and resistor  34 .  
         [0007]      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 percent duty cycle point Ts 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. Ts thus will be at fifty percent duty cycle. As Vramp continues to increase after point 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 at the oscillator operating frequency, regardless of the actual switch duty cycle. Compensation is provided throughout an operational range of fifty to one hundred percent switch duty cycle.  
         [0008]      FIG. 4  is a circuit diagram of a typical oscillator circuit used for producing the Vramp signal. Constant current source  102  is connected in series with capacitor  104 . Coupled across capacitor  104  is the series arrangement of controlled switch  106 , shown schematically, and constant current source  108 . Switch  106  assumes a closed, or conductive, state in response to a high logic level output of comparator  110 . The positive input of comparator  110  is coupled to the junction between constant current source  102  and capacitor  104 . The negative input of comparator  110  is coupled to the series arrangement of resistor  112  and voltage reference threshold source  114 . Transistor  116  is coupled in parallel with resistor  112  and source  114 .  
         [0009]     With switch  106  in the open state as shown, charge is applied to capacitor  104  to build up its voltage at a constant rate until it exceeds the voltage at the negative input, Vn, of the comparator  110 . At that point, the comparator outputs a signal to activate the switch  106  to a conductive state, thereby coupling the capacitor to constant current source  108  to discharge capacitor  104 . As the current source  108  is much greater than the current source  102 , and the comparator is configured with sufficient hysteresis, the capacitor is quickly discharged to its base minimum level voltage. The voltage at capacitor  104  produces the Vramp signal. In the absence of application of an activation signal to the base of transistor  116 , the circuit operates as a free running oscillator. The charge and discharge cycle is repeated continuously at a constant frequency dependent upon the time necessary for the voltage at capacitor  104  to rise from its base level to its threshold level of reference source  114 . The time required for capacitor discharge is negligible.  
         [0010]     The oscillator may be controlled to operate at a higher frequency by application of a higher frequency synchronous signal to the base of transistor  116 . When a synchronizing pulse is applied to the base of transistor  116 , the negative input to comparator  110  is coupled to ground, causing the immediate closure of switch  106  and discharge of capacitor  104  by current discharge source  108 . Upon discharge of the capacitor to the base voltage level of Vramp, the comparator ceases its output signal, switch  106  again transitions to an open state, and charge is again applied to capacitor  104  to build the Vramp signal. The circuit thus will provide a Vramp signal output at the higher frequency with decreased charging period for capacitor  104 .  
         [0011]     The waveforms of  FIGS. 5A-5D  illustrate operation in both the free running and synchronized oscillator modes. Waveform (a) represents an external voltage signal, Vsync, applied to the base of transistor  116 . Waveform (b) represents the voltage at the negative input to comparator  110 . Waveform (c) represents the Vramp signal. The Vramp signal is applied to the positive input of amplifier  38  of  FIG. 2 . Waveform (d) represents the compensation signal Vcomp. For comparison with the waveform of  FIG. 3 , it is assumed that the voltage threshold source  114  is one volt and that the base line level is zero volt. 100 kHz is taken as an example of the free running oscillator frequency.  
         [0012]     Between time t 0  and t 2 , Vsync (waveform a) is zero, whereby the circuit operates as a free running oscillator at 100 kHz. Vramp (waveform c) exhibits a constant slope from a value of 0.0 volt at t 0  to the threshold 1.0 volt at t 1 . The slope is dependent on the value of capacitor  104  and constant current charge source  102 . Vn (waveform b) drops to 0.0 volt level from 1.0 volt during the brief period of transition of Vramp from its maximum to minimum levels. The compensation signal, Vcomp, is initiated when the Vramp signal attains the voltage VB of the reference source  32 . This point is at fifty percent duty cycle, as described above with respect to  FIGS. 2 and 3 .  
         [0013]     Waveforms (a)-(d) repeat as described until time t 2 , when a Vsync signal having a frequency of 150 kHz is applied to the base of transistor  116 . At that time, the voltage Vn at the negative input to comparator  110  is forced low, the Vramp signal attains the 0.0 volt level and then begins to increase. As there has been no change to the constant current charge source  102  or to the capacitor  104 , the slope of Vramp remains the same. At time t 3 , the next Vsync pulse occurs, again forcing Vn low to terminate the Vramp pulse. As the Vsync frequency of 150 kHz is greater than the 100 kHz frequency at free running operation, the time during which charge can build on capacitor  104 , i.e., between t 2  and t 3 , has decreased. The maximum value of the Vramp signal is 0.66 volt.  
         [0014]     The effect of application of the 150 kHz Vramp signal to the positive input of amplifier  38  on compensation signal Vcomp is as follows. As the voltage bias (VB) source  36  remains at 0.5 volt and the slope of Vramp remains the same, the length of time required to initiate the compensation signal in each cycle remains the same. The percent duty cycle point of Ts is derived as follows: Ts/0.5 volt=100%/0.66 volt; Tx=(0.5/0.66)(100%)=76%. As illustrated in the waveform of  FIG. 5D , Ts has shifted from the fifty percent duty starting point for 100 kHz frequency operation to seventy six percent duty starting point for 150 kHz frequency operation. The regulator loses slope compensation between fifty and seventy six percent duty cycle and thus becomes susceptible to sub-harmonic oscillation in that duty cycle range. If a higher frequency synchronization signal is applied to the oscillator, an even greater shift of Ts will occur. Moreover, as the slope of the compensation signal remains independent of operating frequency, Smax will attain only a small magnitude.  
         [0015]     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 conserve power. At low duty cycles at which no compensation signal is produced, the level of V C  corresponds to the amount of switch and regulator output currents. At higher duty cycles at which compensation signals are produced, the level of V C  corresponds to a load level less than the actual load level. As the compensation signal increases with higher duty cycles, the load level correspondence decreases. For C C  to be a reasonably accurate indicator of load level, the slope compensation Sx should be at the minimum signal magnitude necessary for compensation.  
         [0016]     To obtain adequate compensation, a compensation signal of greater magnitude is required at increased duty cycles. The slope of the linear compensation curve thus is typically set to provide the appropriate magnitude for the maximum duty cycle operation. While this curve satisfies the maximum duty requirement, it over-compensates as duty cycle operation decreases to fifty percent. As the minimum necessary compensation between fifty percent and one hundred percent duty cycle operation is not linear, V C  contains an unnecessary offset component through much of that range.  
         [0017]     The need thus exists for a slope compensation arrangement that provides adequate slope compensation at fifty percent duty cycle and above for all operating frequencies. The need also exists to avoid over-compensation.  
       DISCLOSURE  
       [0018]     The above-described needs of the prior art are fulfilled, at least in part, by developing a compensation signal for a switching regulator that is independent of changes in the switching frequency. The regulator operational frequency is established in accordance with a repetitive ramp signal of constant slope and adjustable frequency. The voltage of the ramp signal is monitored and an offset signal is derived therefrom. A compensation signal is derived based on the ramp signal and the derived offset signal. A duty cycle control signal for the regulator is dependent in part on the developed compensation signal. The peak value of the ramp signal, detected during monitoring, is used to derive the offset signal. Initiation of the compensation occurs at the same percent duty cycle point during each switching cycle and thus is independent of switching frequency. The compensation signal may have a constant slope, or an exponentially increasing slope, and a time duration that is proportional to the difference between the ramp signal and the derived offset signal. Preferably, the derived offset signal is proportional to the detected peak value.  
         [0019]     In an exemplified implementation, a compensation circuit is coupled to an input of a switching controller for terminating a switching pulse during each switching cycle. The compensation circuit is configured to output a compensation signal that varies as a function of changes in regulator switching frequency while maintaining a constant percent duty cycle. A peak detector is coupled to a ramp generator. The ramp generator may have an input coupled to an adjustable frequency synchronization signal, thereby to set the frequency of the repetitive ramp signal to the frequency of the synchronizing signal. A peak voltage hold circuit is coupled to the peak detector. Preferably a voltage divider circuit is coupled between the peak voltage hold circuit and a negative input of an amplifier. A summer, coupled in series with the output circuit, has inputs for receiving a signal from the voltage divider and a voltage reference. An output of the summer is fed to the negative input of the amplifier. A positive input of the amplifier is coupled to the ramp generator. An output circuit comprising a transistor, having a control terminal coupled to the amplifier output, and an impedance coupled in series with the transistor provides the compensation signal to the switching regulator.  
         [0020]     The series arrangement may further include a first multiplier circuit coupled to the transistor and configured to output a signal that is a function of the peak level of the ramp signal and a second multiplier circuit coupled to the first multiplier circuit and configured to output a signal that is proportional to the square of the signal output by the first multiplier circuit.  
         [0021]     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  
       [0022]     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:  
         [0023]      FIG. 1  is a block diagram of a typical current mode switching regulator.  
         [0024]      FIG. 2  is a circuit diagram of a prior art slope compensation generator.  
         [0025]      FIG. 3  is a simplified waveform diagram illustrative of the compensation function of the circuit of  FIG. 2 .  
         [0026]      FIG. 4  is a circuit diagram of a typical oscillator circuit used for producing the Vramp signal.  
         [0027]      FIGS. 5A-5D  are diagrams illustrating waveforms during operation of the circuit of  FIGS. 1 and 2 .  
         [0028]      FIG. 6  is a diagram of a slope compensation implementation in accordance with the present invention.  
         [0029]      FIGS. 7A-7D  is a diagram of waveforms illustrating operation with the compensation arrangement of  FIG. 6 .  
         [0030]      FIG. 8  is a circuit diagram of a peak detector that may be employed in the circuit of  FIG. 6 .  
         [0031]      FIG. 9  is a circuit diagram of another peak detector that may be employed in the circuit of  FIG. 6 .  
         [0032]      FIG. 10  is a partial block diagram of a variation of the slope compensation implementation of  FIG. 6 .  
         [0033]      FIG. 11  is a circuit diagram of multipliers that may be employed for blocks of  FIG. 10 .  
         [0034]      FIG. 12  is a waveform diagram illustrating signals produced by the circuit of  FIG. 11 . 
     
    
     DETAILED DESCRIPTION  
       [0035]     An underlying concept of the present disclosure is based on the realization that loss of slope compensation when the oscillator frequency is increased can be avoided by maintaining the start of the compensation signal Sx at a constant duty cycle Ts.  FIG. 6  is a diagram of an implementation  80  for regulating the compensation signal accordingly. The output of oscillator  100  is coupled to peak detector  120  as well as to the positive input of amplifier  38 . The negative input of amplifier  38  is coupled to a junction between transistor  32  and resistor  34 . Connected in parallel between the output of peak detector  120  and ground are capacitor  122 , “droop” current source  124 , and the series arrangement of unity gain amplifier buffer  126 , resistor  128  and resistor  130 . Unity gain amplifier buffer  132  is coupled to a junction resistor  128  and resistor  130 . Summer  134  has one input coupled to the buffer  132 , another input coupled to a reference voltage Vtl, and an output coupled to resistor  34 . Peak detector  120  outputs the peak voltage of oscillator  100 , Vhold, which is held temporarily by capacitor  122 . Buffers  126  and  132  avoid loading on the capacitor voltage.  
         [0036]     The compensation signal Sx is initiated, at time Ts, when amplifier  38  outputs a signal to activate transistor  32 . Ts occurs when the Vramp signal at the positive input overtakes the voltage VB applied at the negative input. The voltage VB is a function of the voltage at resistor  130 , and thus of the voltage Vpeak. The voltage at the output of buffer  132  can be calculated as follows: 
 
V132=(Vhold* R 130)/( R 128+ R 130); wherein Vhold=Vpeak.  (1) 
 
 The voltage (VB) at the output of summer  134  is thus: 
 
V B= V132+V tl= (Vpeak* R 130)/( R 128+ R 130)+V tl;   (2) 
 
 wherein Vtl is the base line threshold voltage. In keeping with the earlier described example, the base line voltage for the oscillator Vramp signal is selected to be zero volt; thus Vtl=0.  
                     Ts   ⁢           ⁢     (     in   ⁢           ⁢   percent   ⁢           ⁢   duty   ⁢           ⁢   cycle     )       =       (     VB   -   Vtl     )     /     (     Vpeak   -   Vtl     )                   =         (     Vpeak   *   R   ⁢           ⁢   130     )     /     (       R   ⁢           ⁢   128     +     R   ⁢           ⁢   130       )       ⁢     (   Vpeak   )                   =     R   ⁢           ⁢     130   /       (       R   ⁢           ⁢   128     +     R   ⁢           ⁢   130       )     .                       (   3   )             
 
 Ts is thus a constant, determined by values of the resistors R 128  and R 130 .  
                   Sx   =         (     Vramp   -   VB     )     /   R     ⁢           ⁢   34                 =         Vramp   /   R     ⁢           ⁢   34     -       (     Vpeak   *   R   ⁢           ⁢   130     )     /       (     R   ⁢           ⁢   34   *     (       R   ⁢           ⁢   128     +     R   ⁢           ⁢   130       )       )     .                                             (   4   )                       The   ⁢           ⁢   slope   ⁢           ⁢   of   ⁢           ⁢   Sx     =         ⅆ   Sx     /     ⅆ   t       =         1   /   R     ⁢           ⁢   34   *       ⅆ   Vramp     /     ⅆ   t         -     constant   .                 (   5   )             
 
 Since the charging current of the oscillator is not changed during a change in frequency, dVramp/dt is a constant. From the above formulae, it is evident that Ts and dSx/dt are constant, independent of frequency change. As Vpeak is a measure of operating frequency, the implementation of  FIG. 6  tracks Vpeak and accordingly adjusts the start of Sx during each cycle to maintain constant the duty cycle Ts. 
 
         [0037]      FIGS. 7A-7D  are diagrams of waveforms illustrating operation with the compensation arrangement of  FIG. 6 . The voltage threshold levels and charging rate are taken to be the same as the earlier described example for purpose of comparison. The Vsync and Vramp waveforms are the same as those of  FIGS. 5A-5D . The peak oscillator output voltage Vpeak changes with changes in frequency, i.e., 1.0 volt at 100 kHz and 0.66 volt at 150 kHz. As shown in the Vcomp waveform, the start Ts of the compensation signal in each cycle, at both frequencies is fifty percent. Compensation is thus provided at every percent duty cycle above fifty percent at all frequencies.  
         [0038]      FIG. 8  is a circuit diagram of a peak detector  120  that may be employed in the circuit of Fig.  6 . Current source  140  is coupled in series with PNP transistor  142 . NPN transistor  144  is coupled in series with current source  146 . The oscillator Vramp signal is applied to the base of transistor  142 . The emitter of transistor  142  is coupled to the base of transistor  144 . The emitter of transistor  144  is coupled in series with controlled switch  148  and the Vhold terminal of capacitor  122 . A positive input of comparator  150  is supplied by the Vramp signal. A negative input of comparator  150  is coupled to a junction between switch  148  and capacitor  122 .  
         [0039]     The voltage at the emitter of transistor  142  is Vramp plus the base-emitter voltage. The voltage at the emitter of transistor  144  is Vramp plus the base-emitter voltage of transistor  142  minus the base-emitter voltage of transistor  144 , i.e., substantially equal to Vramp. The transistors  142  and  144  are buffers for level shift. When switch  148  is closed, Vhold will be forced to equal Vramp. When switch  148  is open, Vhold is isolated from Vramp and is held by capacitor  122 . Switch  148  is activated when the voltage at the positive input of comparator  150  exceeds the voltage at the negative input. Vhold will then follow the increase in Vramp. When Vramp goes lower than Vhold, comparator  148  will turn off switch  148 .  
         [0040]     Vhold thus maintains the peak of the Vramp signal, Vpeak, until a higher peak is reached. If, for example, the oscillator reverts from synchronized operation at 150 kHz to free running 100 kHz operation, the increase in Vpeak will be detected and the compensation signal Sx adjusted to maintain Ts at fifty percent duty cycle. If frequency is increased, Vpeak will decrease. The provision of the “droop” current source  124  ( FIG. 6 ) in parallel with capacitor  122  permits discharge of the capacitor at an appropriate rate to detect a lower Vpeak. In response to the lower value of Vhold, Sx will be adjusted to maintain the percent duty cycle, Ts, constant.  
         [0041]      FIG. 9  is a circuit diagram of another peak detector  120  that may be employed in the circuit of  FIG. 6 . The Vramp signal is applied to a positive input of unity gain buffer amplifier  150 . Coupled in series with the output of amplifier  150  are diode  152  and the Vhold terminal of capacitor  122 . The Vhold terminal is coupled to the negative input of amplifier  150 . Blocking diode  152  allows flow of amplifier output current only when Vramp is higher than Vhold. When Vramp is higher than Vhold, the diode will be forward biased and Vhold will follow Vramp. When Vramp goes lower than Vhold, the diode will be reversed biased and Vpeak will be held until a higher peak is produced or until the discharge of capacitor  112  by “droop” current source  124  brings Vhold lower than Vramp.  
         [0042]      FIG. 10  is a diagram of a variation of the slope compensation arrangement of  FIG. 6 . The  FIG. 6  implementation  80  is shown by the elements surrounded by a dashed outline. The linear slope signal Sx output therefrom is not directly applied as the compensation signal input to adder  30 . A first multiplier  160  receives the signal Sx and multiplies that signal by the factor Vth/Vpeak to compensate the reduction in Vpeak that occurs with increased frequency. Thus, while block  160  is designated a multiplier in  FIG. 10 , it performs the function of dividing Vth by Vpeak and multiplying the result by Sx. Sx 1 , the output of block  160  is Sx * (Vth/Vpeak). Sx 1  is applied to a second multiplier  180  to produce an output Sx 2 . The function of block  180  is multiply Sxl by itself, the result divided by a constant Iconst. The output Sx 2  is (Sx * Vth/Vpeak) 2 / Iconst. Sx 2  is applied as the compensation signal input to adder  30 .  
         [0043]     Circuits that may be utilized in the multipliers  160  and  180  are illustrated in  FIG. 11 . The output Sx of the compensation circuit  80  of  FIG. 10 , which has a linear slope characteristic, is mirrored by transistors  162  and  164 . Connected in series between Vcc and ground is the series path including transistors  164  and  166 . A parallel circuit path, comprising transistor  168  and current source  170  is also connected between Vcc and ground. Current source  170  is proportional to Vth. The base of transistor  168  is connected to the junction of transistors  164  and  166 . The base of transistor  166  is connected to the junction of transistor  168  and current source  170 . Transistor  172  is connected between Vcc and current source  174 . Current source  174  is connected to the buffer  126  of  FIG. 10  and thus is proportional to Vpeak. The base of transistor  172  is also connected to the junction of transistors  164  and  166 . Transistor  178  and  176  are connected in series across Vcc and ground. The base of transistor  176  is connected to the junction of transistor  172  and current source  174 . The current through transistor  178  is the output Sx I of multiplier circuit  160 .  
         [0044]     Sx 1  is mirrored by transistor  182  in multiplier circuit  180 . Connected in series between Vcc and ground are transistors  182 ,  184  and  186 . The base and collector of each of transistors  184  and  186  are connected together. Connected in series between Vcc and ground are transistor  188  and constant current source  190 . The base of transistor  188  is connected to the junction of transistors  182  and  184 . The junction of transistor  188  and current source  190  is coupled to the base of transistor  192 . The current through transistor  192  is the output Sx 2  of multiplier circuit  180  that is applied to the adder  30  as a compensation signal.  
         [0045]     Circuits  160  and  180  operate as follows, wherein VBE represents base to emitter voltage; Vt is the thermal voltage of a bipolar resistor; Ic is the collector current of a bipolar transistor; Is is the saturation current of a bipolar transistor and proportional to transistor size; Ie is emitter current; and Rx is an arbitrarily assigned resistor, to convert voltage to current. The functional operation of multiplier  160  is performed by transistors  166 ,  168 ,  172  and  176 . The voltage at the collector node of transistor  166 , is represented as follows: 
 
V c 166=V BE 168+V BE 166=V BE 172+V BE 176 
 
 As the base to emitter voltage (VBE)=Vt In (Ic/Is), the above relationship becomes: 
 
V tln ( Ic 168/ Ic 168)+V tln ( Ic 166/ Ic 166)=V tln ( Ic 172/ Ic 172)+V tln ( Ic 176/ Ic 176) 
 
 The transistors  166 ,  168 ,  172  and  176  may be chosen to be of the same size so that Is of all of these transistors are equal. Thus: 
 
( Ic 168)*( Ic 166)=( Ic 172)*( Ic 176); and 
 
 Ic 176=[( Ic 168)*( Ic 166)]/ Ic 172 
 
 Since Ic 166 =Sx, Ic 168 =Vth/Rx, and Ic 172 =Vpeak/Rx, and Ic 176 =Sx 1 , then: 
 
 Sx 1= Sx* (V th /Vpeak) 
 
 The functional operation of multiplier  180  is performed by transistors  184 ,  186 ,  188  and  192 . Using the same analysis applied above for multiplier  160 , the current of transistor  192  is: 
 
 Ic 192=[( Ic 184)*( Ic 186)]/ Ic 188 
 
 Since Ic 184 =Ic 186 =Sx 1 , Ic 188 =Iconst, and Ic 192 =Sx 2 , then: 
 
 Sx 2=( Sx 1) 2   /I const 
 
         [0046]      FIG. 12  is a waveform diagram illustrating the signals Vramp, Sx, Sx 1  and Sx 2  for the free running oscillator mode and the synchronized oscillator mode, comparable to the conditions illustrated in  FIGS. 5A-5D . The left hand portion of the waveform, designated by “a”, depicts a free running 100 kHz frequency operation with Vpeak at one volt. The right hand portion, designated by “b”, depicts a synchronized 150 kHz frequency operation with Vpeak at 0.66 volt.  
         [0047]     At 100 kHz operation, Vpeak is equal to Vth and the signals Sx a  and Sx 1   a  are equal with linear slope. Sx 2   a  has an exponential characteristic instead of a linear slope. Ts a  is at fifty per cent duty cycle. At 150 kHz operation, Vpeak is no longer equal to Vth. Sx b  and Sx 1   b  have linear, but unequal, slopes. Sx 2   b  has an exponential characteristic. Ts b  is at fifty per cent duty cycle. As evident from these waveforms for the compensation circuit of  FIGS. 10 and 11 , the start, of the compensation signal Ts in each cycle is maintained at fifty percent. Compensation is thus provided at every percent duty cycle above fifty percent at all frequencies. As the slope of the compensation signal Sx 2  is non-linear, it can satisfy maximum duty cycle requirements without over-compensating at duty cycles closer to fifty percent. The signal V C  is thus a reliable indicator of load current at all duty cycles.  
         [0048]     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 circuit elements of the compensation circuit and the oscillator circuit, and the operating voltage levels, the slope of Sx and its onset at a constant duty cycle can be defined. If, for example, the use of a particular regulator would find more advantageous use with a compensation signal of a different slope characteristic, or at a constant onset Tx percent duty cycle level other than fifty percent, these ends are attainable within the concepts of the present disclosure.

Technology Category: h