Abstract:
In one form, a control circuit is adapted for use with a power converter having an inductor and a switch switching the inductor in response to a switching signal to regulate an output voltage of the power converter. The control circuit includes a slow feedback path, a fast feedback path, an integrator, a comparator, and a drive circuit. The slow feedback path provides a ripple signal in response to an average value of the output voltage. The fast feedback path provides a feedback signal in response to the output voltage. The integrator provides an error signal in response to a sum of the feedback signal and the ripple signal. The comparator provides a comparison output signal in response to a comparison of the error signal and a threshold voltage. The driver circuit provides the switching signal in response to the comparison output signal.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    The present disclosure relates generally to power conversion circuits, and more particularly to boost converters. 
       BACKGROUND 
       [0002]    Boost converters are power converters that convert one direct current (DC) voltage into another, higher DC voltage. Boost converters typically regulate the output voltage by switching a transistor connected to an inductor to create a magnetic field across the inductor according to the level of the output voltage. If the switching transistor is connected in series between the inductor and ground, the switching transistor is referred to as a low side switch (LSS). The second terminal of the inductor is connected to a rectifier, and the rectified voltage is smoothed using an output capacitor. The switching of the transistor can be controlled by creating an error voltage that is the difference between the output voltage or some fraction of the output voltage and a reference voltage. 
         [0003]    Boost converters typically operate under a variety of load conditions. Typically boost converters lose efficiency under light load conditions, but light load conditions are becoming more frequent as powered devices adopt so-called “eco-mode” operation. Since the load transitions to and from the light load condition suddenly, the boost mode power supply preferably provides good load transient performance. Moreover since the input voltage can vary over a wide range, which includes being close to or equal to the desired output voltage, it is desirable for the converter to operate in either boost or buck operation. In addition, product cost including the cost of external components associated with the boost converter is an important consideration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings, in which: 
           [0005]      FIG. 1  illustrates in partial block diagram and partial schematic form a boost regulator known in the prior art; 
           [0006]      FIG. 2  illustrates in partial block diagram and partial schematic form another boost regulator known in the prior art; 
           [0007]      FIG. 3  illustrates in partial block diagram and partial schematic form a power converter using a hysteretic boost converter according to an embodiment of the present invention; 
           [0008]      FIG. 4  illustrates a timing diagram associated with a slow feedback path of the boost converter of  FIG. 3 ; and 
           [0009]      FIG. 5  illustrates a timing diagram associated with a fast feedback path of the boost converter of  FIG. 3 . 
       
    
    
       [0010]    The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well. 
       DETAILED DESCRIPTION 
       [0011]      FIG. 1  illustrates in partial block diagram and partial schematic form a boost regulator  100  known in the prior art. Boost regulator  100  is described by Xu, Zhao, and Wu in “On-Chip Boost Regulator with Projected Off- and On-time Control,”  Journal of Zhejiang University,  vol. 10, no. 8, 2009, pages 1223-1230. 
         [0012]    Boost regulator  100  includes an inductor  110 , a diode  120 , an N-channel MOS transistor  130 , a control logic and driver circuit  140 , a control circuit  150 , an output portion  160 , an output capacitor  170 , and a load  180 . Inductor  110  has a first terminal for receiving an input voltage labeled “V IN ”, and a second terminal. Diode  120  has an anode connected to the second terminal of inductor  110 , and a cathode for providing a voltage labeled “V OUT ”. Transistor  130  has a drain connected to the second terminal of inductor  110 , a gate, and a source. Control logic and driver circuit  140  has an input for receiving a control voltage labeled “V OC ”, and an output connected to the gate of transistor  130 . Control circuit  150  includes a resistor  152 , a summing device  154 , and a comparator  156 . Resistor  152  has a first terminal connected to the source of transistor  130  forming a voltage labeled “V IS ”, and a second terminal connected to ground. Summing device  154  has a positive input terminal for receiving a voltage labeled “V P ”, a negative input terminal for receiving voltage V IS , and an output for providing a voltage labeled “V CTRL ”. Comparator  156  has a non-inverting input for receiving a feedback voltage labeled “V FB ”, an inverting input for receiving voltage V IS , and an output for providing voltage V OC . Output portion  160  includes a resistor  162 , a capacitor  164 , and a resistor  166 . Resistor  162  has a first terminal connected to the cathode of diode  120 , and a second terminal for providing voltage V FB . Capacitor  164  has a first terminal connected to the cathode of diode  120 , and a second terminal connected to the second terminal of resistor  162 . Resistor  166  has a first terminal connected to the second terminals of resistor  162  and capacitor  164 , and a second terminal connected to ground. Output capacitor  170  has a first terminal connected to the cathode of diode  120 , and a second terminal connected to ground.  FIG. 1  illustrates load  180  as a resistor  180  having a first terminal connected to the cathode of diode  120 , and a second terminal connected to ground. 
         [0013]    Boost regulator  100  uses projected off- and on-time control. With the projected off-time control, the switch off-time is calculated based on the input and output voltage aiming at quasi fixed frequency operation in continuous conduction mode (CCM) as fixed frequency operation is preferred for ripple control. In consideration of efficiency in discontinuous conduction mode (DCM) operation, the projected on-time combined with modulated off-time enables boost regulator  100  to run in pulse frequency modulation (PFM) operation automatically without additional control circuits. 
         [0014]    While the implementation of boost regulator  100  is simple, it requires current sensing which decreases efficiency. Also the off-time and on-time generators contribute to quiescent current. Moreover the off time for CCM and the on-time for DCM require transition mode management. 
         [0015]      FIG. 2  illustrates in partial block diagram and partial schematic form another boost regulator  200  known in the prior art. Boost regulator  200  is described by Guo, Lin, and Tsai in “A hysteretic Boost Regulator with Emulated-Ramp Feedback (ERF) Current-Sensing Technique for LED Driving Applications,”  IEEE Transactions on Power Electronics,  vol. 26, no. 9, September 2011. 
         [0016]    Boost regulator  200  includes a voltage source  210 , a resistor  212 , an inductor  214 , a switch  216 , a diode  218 , a capacitor  220 , a resistor  222 , a load  230 , a feedback network  240 , a feedback network  250 , a feedback circuit  260 , and a control circuit  270 . Voltage source  210  has a first terminal for providing an input voltage labeled “V I ”, and a second terminal connected to ground. Resistor  212  has a first terminal connected to the first terminal of voltage source  210 , and a second terminal. Inductor  214  has a first terminal connected to the second terminal of resistor  212 , and a second terminal for providing a voltage labeled “V X ”. Switch  216  has a first terminal connected to the second terminal of inductor  214 , a second terminal connected to ground, and a control terminal. Diode  218  has an anode connected to the second terminal of inductor  214 , and a cathode for providing an output voltage labeled “V O ”. Capacitor  220  has a first terminal connected to the cathode of diode  218 , and a second terminal. Resistor  222  has a first terminal connected to the second terminal of capacitor  220 , and a second terminal connected to ground. Load  230  has a first terminal connected the cathode of diode  218 , and a second terminal connected to ground. Feedback network  240  has a first terminal connected to the first terminal of voltage source  210 , a second terminal connected to ground, and an output terminal for providing a signal labeled “β F V I ”. Feedback network  250  includes a divider  252 , a resistor  254 , and a capacitor  256 . Divider  252  has a first terminal connected to the first terminal of voltage source  210 , a second terminal connected to ground, and an output terminal. Resistor  254  has a first terminal connected to the output terminal of divider  252 , and a second terminal for providing a signal labeled “β F V F ”. Capacitor  256  has a first terminal connected to the second terminal of resistor  254 , and a second terminal connected to ground. Feedback network  260  has a first terminal connected to the cathode of diode  218 , a second terminal connected to ground, and an output terminal for providing a signal labeled “β V V O ”. 
         [0017]    Control circuit  270  includes an emulated ramp feedback (ERF) generator  272 , a hysteretic comparator  280 , and non-overlap drivers  290 . ERF generator  272  includes operational transconductance amplifiers (OTAs)  273 - 275 , each labeled “gm”, and a resistor  278 . Transconductance amplifier  273  has an input for receiving signal β V V O , and an output. Transconductance amplifier  274  has an input for receiving signal β F V I , and an output connected to the output of transconductance amplifier  273 . Transconductance amplifier  275  has an input for receiving signal “β F V F ”, and an output connected to the outputs of transconductance amplifiers  273  and  274 . Resistor  278  has a first terminal connected to the outputs of transconductance amplifiers  273 - 275 , and a second terminal connected to ground. Hysteretic comparator  280  has a non-inverting input connected to the outputs of transconductance amplifiers  273 - 275 , an inverting input for receiving a reference voltage labeled “V REF ”, and an output. Non-overlap drivers  290  have an input connected to the output of hysteretic comparator  280 , and an output connected to the control terminal of switch  216 . 
         [0018]    In operation, control circuit  270  of boost regulator  200  only consists of three portions, including an emulated ramp feedback (ERF) generator  272 , a hysteretic comparator  280 , and non-overlap drivers  290 . The goal of the ERF current-sensing technique is to synthesize a ramp which is in-phase with a small signal of inductor current and with a DC level of the output voltage. This current-sensing technique consists of one RC network and one ERF generator. 
         [0019]    Boost regulator  200  does not require current sensing or any timing generation. However boost regulator  200  requires three OTAs, causing a large quiescent current, and has been implemented using a switching speed of 566 kilohertz (kHz). 
         [0020]      FIG. 3  illustrates in partial block diagram and partial schematic form a power converter  300  using a hysteretic boost converter according to an embodiment of the present invention. Power converter  300  includes generally an inductor  310 , a low side switch  312 , a diode  314 , an output capacitor  316 , a load  318 , and a control circuit  320 . Inductor  310  has a first terminal for receiving an input voltage labeled “V IN ”, and a second terminal. Low side switch  312  has a first terminal connected to the second terminal of inductor  310 , a second terminal connected to ground, and a control terminal. Diode  314  has an anode connected to the second terminal of inductor  310 , and a cathode for providing an output voltage labeled “V OUT ”. Capacitor  316  has a first terminal connected to the cathode of diode  314 , and a second terminal connected to ground. Load  318  is shown as a purely resistive load having a first terminal connected to the cathode of diode  314 , and a second terminal connected to ground. 
         [0021]    Control circuit  320  includes a feedback network  330 , an integrator  340 , a ripple emulator  350 , a hysteresis comparator  360 , and a set of non-overlap drivers  370 . Feedback network  330  includes a capacitor  332 , a resistor  334 , and a resistor  336 . Capacitor  332  has a first terminal connected to the cathode of diode  314 , and a second terminal. Resistor  334  has a first terminal connected to the cathode of diode  314 , and a second terminal connected to the second terminal of capacitor  332 . Resistor  336  has a first terminal connected to the second terminals of capacitor  332  and resistor  334 , and a second terminal connected to ground. 
         [0022]    Integrator  340  includes an operational amplifier  342  and a capacitor  344 . Operational amplifier  342  has an inverting input connected to the second terminals of capacitor  332  and resistor  334 , a non-inverting input terminal for receiving a reference voltage labeled “V REF ”, an output terminal for providing a signal labeled “V ERROR ”. Capacitor  344  has a first terminal connected to the inverting input of operational amplifier  342 , and a second terminal connected to the output of operational amplifier  342 . 
         [0023]    Ripple emulator  350  includes a resistor  352  and a capacitor  354 . Resistor  352  has first and second terminals. Capacitor  354  has a first terminal connected to the second terminal of resistor  352 , and a second terminal connected to the inverting input of operational amplifier. 
         [0024]    Hysteresis comparator  360  has an inverting input connected to the output of operational amplifier  342  for receiving the V ERROR  signal, a non-inverting input for receiving high and low hysteretic thresholds labeled “V H ” and “V L ”, respectively, and an output terminal for providing a signal labeled “COMP_OUT”. 
         [0025]    Non-overlap drivers  370  have an input connected to the output of hysteresis comparator  360 , a first output connected to the control input of low side switch  312  for providing a switching signal labeled “LSS”, and a second output connected to the first terminal of resistor  352  for providing a signal labeled “V R ” which is a voltage representative of switching signal LSS. 
         [0026]    Power converter  300  implements a step-up DC-DC (i.e. boost) converter controlled by integrator  340  monitoring V OUT  through feedback network  330  combined with ripple emulator  350 , and a hysteresis comparator  360 . Current into integrator  340  (I C ) is equal to the sum of the current from feedback network  330  (I FB ) and current from ripple emulator  350  (I R ): 
         [0000]        I   C   =I   R   +I   FB    [1]
 
         [0027]    Power converter  300  is controlled based on a slow feedback path (DC regulation) and a fast feedback path (load transient response). The slow feedback path is provided by ripple emulator  350 , while the fast feedback path is provided by feedback network  330  using capacitor  332  operating as a feedforward capacitor. 
         [0028]      FIG. 4  illustrates a timing diagram  400  associated with a slow feedback path of power converter  300  of  FIG. 3 . In  FIG. 4 , the horizontal axis represents time in nanoseconds (ns), and the vertical axis amplitude of various signals in volts or amperes as the case may be. Timing diagram  400  shows five signals of interest, including V R , I L , V ERROR , COMP_OUT, and V FB . 
         [0029]      FIG. 4  also illustrates various time points of interest, including times labeled “t 0 ”, “t 1 ”, “t 2 ”, “t 3 ”, “t 4 ”, and “t 5 ”, which delineate ON and OFF phases corresponding to ON and OFF times of low side switch  312 , respectively. During the ON phase, for example between times t 0  and t 1 , t 2  and t 3 , t 4  and t 5 , non-overlap drivers  370  provide signal LSS at a high voltage to close low side switch  312 , and also provide signal V R  at a high level to the first terminal of ripple emulator  350 . During the ON phase, inductor current I L  rises, currents I R  and I C  are positive, and signal V ERROR  decreases until it reaches the low threshold level V L . When V ERROR  becomes less than V L , hysteresis comparator  360  switches its output to a logic high, causing non-overlap drivers  370  to deactivate signal LSS, and power converter  300  begins the OFF phase. 
         [0030]    During the OFF phase, for example between times t 1  and t 2 , t 3  and t 4 , and after t 5 , non-overlap drivers  370  provide signal LSS at a logic low voltage to open low side switch  312 , and also provide signal V R  at a logic low to the first input of ripple emulator  350 . Inductor current I L  falls during the OFF phase, currents I R  and I C  are negative, and signal V ERROR  increases until it reaches the high threshold V H . When V ERROR  exceeds the high threshold V H , power converter  300  again switches to the ON period. 
         [0031]    During the ON phase, when the low side switch  312  is ON, the current in inductor  310  is increasing, thus a voltage V R  is applied to ripple emulator  350  causing an increase current in capacitor  344  and a decrease of signal V ERROR . 
         [0032]    When the error voltage crosses the threshold low, V L , of hysteresis comparator  360 , the comparator triggers and the LSS is turned OFF, meaning the end of the T ON  phase, and if a high side switch (HSS) is used in place of diode  314 , the HSS is turned ON, meaning the start of a T OFF  phase. 
         [0033]      FIG. 5  illustrates a timing diagram  500  associated with a fast feedback path of power converter  300  of  FIG. 3 . In  FIG. 5 , the horizontal axis represents time in ns, and the vertical axis amplitude of various signals in volts. Timing diagram  500  shows four signals of interest, including LSS, V ERROR , COMP_OUT, and V FB . 
         [0034]      FIG. 5  also illustrates the same time points of interest, including times t 0 , t 1 , t 2 , t 3 , t 4 , and t 5 , which delineate ON and OFF phases corresponding to ON phases of low side switch  312  and OFF phases of low side switch  312 , respectively. During the ON phase, for example between times t 0  and t 1 , t 2  and t 3 , t 4  and t 5 , non-overlap drivers  370  provide signal LSS at a logic high to close low side switch  312 . As low side switch  312  operates to increase I L , it also decreases V OUT  and also V FB  and I FB . This reduction in I FB  decreases V ERROR  until, at time t 1 , V ERROR  is less than low threshold level V L . 
         [0035]    During an increase of the load current, the drop in signal V OUT  accelerates the current drop in capacitor  344  through capacitor  332 . Thus the ON phase will be extended while the OFF phase will be shortened, causing more energy to be stored in the inductor. During the next cycle, signal V OUT  will be restored. When V ERROR  again exceeds the high threshold V H , power converter  300  switches to the ON phase. 
         [0036]    In response to a load transient, for example when the load switches from a light-load condition to a full load condition, signal V OUT  decreases suddenly. The sudden decrease in V OUT  causes in increase in current I FB , which causes V ERROR  to decrease more slowly, and hence to lengthen the ON time such that the new ON time is between t0 and a new time labeled “t 1 ′”. Thus the fast feedback path improves the load transient response of power converter  300 . 
         [0037]    Power converter  300  also provides an “eco mode”. During light load operation, diode  314  prevents a negative current from inductor  310  (i.e. a current flowing in the reverse direction from the second terminal to the first terminal thereof). In an alternate embodiment, diode  314  may be replaced by a synchronous rectifier. In the synchronous version, a zero crossing detector prevents reverse current flow using zero crossing detection to turn off the synchronous rectifier (i.e. the HSS). In either case, the energy stored during the ON phase is not completely absorbed by the load, and the output voltage increases during the OFF phase. The rise in the output voltage increases the current in capacitor  344  so the next ON phase will shorten while the OFF will lengthen. When power converter  300  reduces the ON phase to a minimum, it continues to increase the OFF phase in order to maintain output regulation, and the operating frequency will decrease naturally. Thus, power converter  300  improves light load efficiency by reducing switching losses. 
         [0038]    Thus power converter  300  includes a current ripple emulator to avoid the need for current sensing, improving converter efficiency. It uses only one operational amplifier, further reducing quiescent current. Moreover power converter  300  has an embedded “eco-mode” using a single loop system with good load transient response. 
         [0039]    The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments that fall within the true scope of the claims. For example in other embodiments, diode  314  could be replaced by a synchronous rectifier. Moreover a boost DC-DC converter could be formed using drivers for both a high side switch and a low side switch. 
         [0040]    Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.