Patent Publication Number: US-9425688-B2

Title: Converter circuit and associated method

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of CN application No. 201110233799.5, filed on Aug. 12, 2011, and incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention generally relates to circuit, and more particularly but not exclusively relates to converter circuit and associated method. 
     BACKGROUND 
     Constant on time direct current to direct current (DC-DC) converters are widely applied for their excellent transient response performance and simple internal structure. 
     Conventionally, certain requirements should be satisfied to make the constant on time converter circuit operate in steady status. For an instance, the feedback ripple should be large enough and in-phase with output inductor current. Such requirements result that ceramic capacitor, despite its low price and small size, could not be applied as output capacitor. Rather, solid capacitor with relatively high price is applied as output capacitor. 
     SUMMARY 
     The embodiments of the present invention disclose a converter control circuit and associated method to solve the stability issue existing in the prior constant on time converter, and to obtain a better transient response. 
     The control circuit comprises an error amplifier, coupled to an output voltage or a feedback output signal from the output voltage, and a reference signal, operable to generate an error signal accordingly; a ramp signal generator, generating a first ramp signal and a second ramp signal; a first comparator, coupled to the error signal and the first ramp signal, operable to generate a first comparing signal accordingly; a second comparator, coupled to the error signal and the second ramp signal, operable to generate a second comparing signal accordingly; and a control signal generator, coupled to the first comparing signal and the second comparing signal, operable to generate a control signal to turn switches in the converter circuit ON and OFF accordingly. 
     Some embodiments of the present invention further disclose a method for controlling a converter circuit, comprising amplifying an error between an output voltage or a feedback output voltage of the converter circuit, and a reference signal, configured to obtain an error signal; generating a first ramp signal and a second ramp signal; comparing the error signal with the first ramp signal to obtain a first comparing signal; comparing the sum of the error signal and an offset voltage with the second ramp signal to obtain a second comparing signal; and generating a control signal to control switches of the converter circuit according to the first comparing signal and the second comparing signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments are described with reference to the following drawings. The drawings are not depicted to scale and only for illustration purpose. 
         FIG. 1  illustrates a schematic circuitry diagram of a prior art constant on time converter circuit  50 . 
         FIG. 2  illustrates a schematic circuitry diagram of a converter circuit  60  according to an embodiment of the present invention. 
         FIG. 3  illustrates a schematic circuitry diagram of the ramp signal generator RAMP shown in  FIG. 2  according to an embodiment of the present invention. 
         FIG. 4  illustrates a waveform diagram of the ramp signal VRAMP shown in  FIG. 2  according to an embodiment of the present invention. 
         FIG. 5  illustrates a waveform diagram to indicate the control signal responding from the ramp signal VRAMP in the DC-DC converter circuit  60  according to an embodiment of the present invention. 
         FIG. 6  illustrates another converter circuit  70  according to another embodiment of the present invention. 
         FIG. 7A  and  FIG. 7B  respectively illustrate a schematic circuitry of a first generator RAMP 1  and a second generator RAMP 2  in converter circuit  70  according to another embodiment of the present invention. 
         FIG. 8  A and  FIG. 8B  respectively illustrate the waveform diagrams of the first ramp signal VRAMP 1  and the second ramp signal VRAMP 2  according to another embodiment of the present invention. 
         FIG. 9  illustrates a flowchart diagram of a method for controlling a converter circuit according to an embodiment of the present invention. 
     
    
    
     The use of the same reference label in different drawings indicates the same or like components. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
     The term “on time” hereby and in the following text indicates the duration of the primary switch (high side switch in certain embodiments) in a converter turning on in a single operational cycle. The term “off time” hereby and in the following text indicates the duration of the primary switch turning off in a single operational cycle. 
       FIG. 1  illustrates a schematic circuit diagram of a prior art constant on time converter circuit  50 . 
     As shown in  FIG. 1 , through a forward feedback resistor R FEEDFORWARD , a constant on timer U 1  receives an input voltage VIN and an output voltage VOUT from converter circuit  50 . In one embodiment, resistors R 1  and R 2  are coupled in series between the output voltage VOUT and a reference ground to comprise a feedback loop. The feedback loop is configured to obtain a feedback signal VFB (the voltage level on the common node of the resistors R 1  and R 2 ) from the output voltage VOUT. The feedback signal VFB is provided to an inverting input of a comparator U 2 . In another embodiment, the feedback loop may be omitted so that the output voltage VOUT is directly provided to the inverting input of the comparator U 2 . A non-inverting input of the comparator U 2  is coupled to a reference signal VREF. An output of the comparator U 2  is coupled to a first input of an AND gate U 4 . An output of the AND gate U 4  is coupled to a set terminal S of an RS flip-flop U 5 . A reset terminal R of the RS flip-flop U 5  is coupled to an output of the constant on timer U 1 . An output Q of the RS flip-flop U 5  is coupled to an input of a driver U 6 , and further feedbacks to the constant on timer U 1  and a minimum off time circuit U 3 . 
     The minimum off time circuit U 3  receives the output Q of the RS flip-flop U 5 , and the output of minimum off time circuit U 3  is coupled to a second input of AND gate U 4 . The driver U 6  generates two control signals respectively for a high side switch M 1  and a low side switch M 2 . The switches M 1  and M 2  may be bipolar junction transistor (BJT), metal oxide semiconductor field effect transistor (MOSFET) or other suitable device. The two control signals from driver U 6  are coupled to control terminals of switches M 1  and M 2  respectively. Between the common node of switches M 1  and M 2 , and the reference ground, an output inductor L, a resistor ESR, an ideal output capacitor CO are coupled in series, wherein ESR is the practical equivalent series resistor of the ideal output capacitor CO. The output voltage VOUT of converter circuit  50  is obtained between the output inductor L and the resistor ESR. 
     When the converter circuit  50  is operating, if the feedback signal VFB is lower than the reference signal VREF, the output of comparator U 2  is high. At this time, if the output of minimum off time circuit U 3  is also high, the AND gate U 4  provides a high level signal to the set terminal of RS flip-flop U 5 , resulting the “setting” of RS flip-flop U 5 . The voltage level of the output Q is high to turn on the high side switch M 1  and to turn off the low side switch M 2  through the driver circuit U 6 . Thus the output voltage VOUT of converter circuit  50  rises up. Once the output voltage VOUT makes the feedback signal VFB higher than the reference signal VREF, the comparator U 2  generates a low level output, and the voltage level of the set terminal of the RS flip-flop U 5  is low. The output Q of RS flip-flop U 5  is maintained. Meanwhile, the high level output Q triggers the constant on timer U 1  to begin timing. After a predetermined time, for example, 
               N   ×     VIN   VOUT       ,         
the output terminal of constant on timer U 1  provides a high level signal to the reset terminal of the flip-flop U 5  to reset the RS flip-flop U 5 . The voltage level of output Q flops to low level. Through the driver circuit U 6 , the low level output Q turns off the high side switch M 1  and turns on the low side switch M 2 . Consequently the output voltage VOUT of converter circuit  50  declines down.
 
     Moreover, the low level output Q is also provided to the minimum off time circuit U 3 , and makes the minimum off time circuit U 3  generate a low level signal to the AND gate U 4  to indicate to minimum duration of the off time of high side switch M 1 . Therefore, during this minimum off time, the other input of AND gate U 4  is disabled. No matter the output of comparator U 2  is at high level or low level, the output of AND gate U 4  is always low. Once the feedback signal VFB declines down to a level lower than reference signal VREF again, the output of comparator turns to high level. After the minimum off time, the circuit U 3  also provides a high level signal to the input of AND gate U 4 . Accordingly, the output of AND gate U 4  is high and sets the RS flip-flop U 5 . The converter circuit  50  enters into next operational cycle. 
     According to the above description, the constant on timer U 1 , the minimum off time circuit U 3 , the AND gate U 4  and the RS flip-flop U 5  together comprise a constant on time signal generator. And the feedback loop, the comparator U 2 , and the constant on time signal generator further together comprise a control circuit of the converter circuit  50 . 
     One in relevant art may understand that the function of minimum off time circuit U 3  is to prevent the noise and the disruption from making converter circuit  50  intermediately enter the next on time period when a previous on time is just over. 
     One in relevant art may also understand that the minimum off time circuit U 3  is not essential. Without the minimum off time circuit U 3 , the AND gate U 4  is also no longer required. In such occasion, the output of comparator U 2  is directly coupled to the set terminal of the RS flip-flop U 5 . 
       FIG. 2  illustrates a schematic circuitry diagram of a converter circuit  60  according to an embodiment of the present invention. Converter circuit  60  incorporates some features of converter circuit  50 , and these features will not be referred to in explaining converter circuit  60 . 
     The control circuit of converter circuit  60  is different from the control circuit of converter circuit  50 . Specifically, the control circuit of converter  60  further comprises an error amplifier EA, an adder ADD, a ramp signal generator RAMP, and one additional comparator. Besides, a control signal generator comprising the RS flip-flop U 5  is applied instead of the constant on time signal generator. Therefore the on time of converter circuit  60  is no longer constant. One in relevant art may understand that in certain embodiments, other suitable logic devices may be applied to replace RS flip-flop U 5  for obtaining the same function in the control signal generator. 
     More specifically, an inverting input of error amplifier EA receives the feedback signal VFB, and a non-inverting input of error amplifier EA receives the reference signal VREF. An output of EA provides an error signal COMP to an input of the adder ADD, and to a non-inverting input of a first comparator U 21 . The other input of the adder ADD is coupled to a DC voltage source V to receive an offset voltage VW. An output of the adder ADD is coupled to an inverting input of a second comparator U 22 . A non-inverting input of the second comparator U 22  and the inverting input of the first comparator U 21  are together coupled to the ramp signal generator RAMP to receive the ramp signal VRAMP. Thus, the second comparator compares the sum of error signal COMP and the offset voltage VW with the ramp signal VRAMP. While the first comparator compares the error signal COMP with the ramp signal VRAMP. An output of the first comparator U 21  is coupled to the set terminal of the RS flip-flop U 5 , and an output of the second comparator U 22  is coupled to the reset terminal of the RS flip-flop U 5 . The output Q of the RS flip-flop U 5  is coupled to the input of the driver circuit U 6 . Furthermore, as well-known by these skilled in relevant art, the error amplifier EA also comprises a resistor REA which is coupled between the inverting input and the output of the error amplifier EA. 
     When the ramp signal VRAMP is lower than the error signal COMP, a first comparing signal from the output of first comparator U 21  steps up to high level. The RS flip-flop U 5  is set and the voltage level of output Q is high. Through the driver circuit U 6 , this high level output Q turns on the high side switch M 1  and turns off the low side switch M 2 . 
     During the on time of converter circuit  60 , the ramp signal is rising. The speed of rising, in other word the rising slope of the ramp signal VRAMP, is proportional to the output voltage VOUT and inversely proportional to the input voltage VIN of the converter circuit  60 . One skilled in relevant art may understand that the rising slope of ramp signal VRAMP may relate to other parameters. Further, in other embodiments, the rising slope of ramp signal VRAMP may be proportional to the output voltage VOUT of the converter circuit  60  only, but no longer relates to the input voltage VIN. 
     Once the ramp signal VRAMP reaches to the error signal COMP, the first comparing signal from the first comparator U 21  steps down to low level. The voltage level on set terminal S of the RS flip-flop U 5  also flops to low level. The voltage level of output Q of U 5  is maintained. And therefore the high side switch M 1  of converter circuit  60  keeps on, and the low side switch M 2  keeps off. 
     As the ramp signal VRAMP is continuously rising up, it is finally higher that the sum of the error signal COMP and the offset voltage VW. At this time, the second comparing signal from the second comparator U 22  step up to high level. Then the RS flip-flop U 5  is reset and the output Q of RS flip-flop U 5  is turned to low level. Through the driver circuit U 6 , this low level output Q turns off the high side switch M 1  and turns on the low side switch M 2 . The converter circuit  60  enters into off time. 
     During the off time, the ramp signal VRAMP is gradually declining down. The speed of declining, in other word, the absolute value of the declining slope of the ramp signal VRAMP, is proportional to the amplitude of the error signal COMP. 
     When the ramp signal VRAMP flops down to a level lower than the sum of the error signal COMP and the offset voltage VW, the second comparing signal from the second comparator U 22  steps down. The voltage level on the reset terminal of the RS flip-flop U 5  declines to low level. The output Q of U 5  is maintained to low level. Thus in the converter circuit  60 , the high side switch M 1  keeps off and the low side switch M 2  keeps off. 
     As the ramp signal VRAMP is continuously declining down, it is finally lower than the error signal COMP. At this time, the first comparing signal from the first comparator U 21  steps up. The RS flip-flop U 5  is set, and the output Q turns to high level. Through the driver circuit U 6 , this high level Q output turns on the high side switch M 1  and turns off the low side switch M 2 . Thus the converter circuit  60  enters into a next operational cycle. 
     According to the above text, the on time of the converter circuit  60  TON=VW/RUP, wherein RUP is the rising slope of the ramp signal VRAMP. As described above, in one embodiment, the rising slope of the ramp signal VRAMP is proportional to the output voltage VOUT and is inversely proportional to the input voltage VIN of the converter circuit  60 . Consequently, the on time TON is inversely proportional to the output voltage VOUT and proportional to the input voltage VIN. Meanwhile, the off time of the converter circuit  60  TOFF=VW/RDOWN, wherein RDOWN is the absolute value of the declining slope of the ramp signal VRAMP. As described above, the absolute value of the declining slope of the ramp signal VRAMP is proportional to the amplitude of the error signal COMP. So the off time TOFF is reversely proportional to the amplitude of the error signal COMP. The larger amplitude the error signal COMP is, the shorter the off time TOFF is. 
       FIG. 3  illustrates a schematic circuitry diagram of the ramp signal generator RAMP shown in  FIG. 2  according to an embodiment of the present invention. 
       FIG. 4  illustrates a waveform diagram of the ramp signal VRAMP shown in  FIG. 2  according to an embodiment of the present invention. 
     As shown in  FIG. 3 , the ramp signal generator RAMP comprises a first current source I 1 , a first switch SW 1 , a first capacitor C 1 , a second switch SW 2 , and a second current source I 2 . Wherein, a negative end of the first current source I 1  is coupled to the reference ground, and a positive end of the first current source I 1  is coupled to a first end of the first switch SW 1 . A second end of the first switch SW 1  is coupled to a first end of the second switch SW 2  at a conjunction node C. A control end of the switch SW 1  is coupled to a first switch control signal. When the ramp signal VRAMP is lower than the error signal, the first switch control signal turns the first switch on and maintains until the ramp signal VRAMP is higher than the sum of the error signal COMP and the offset voltage VW. In one embodiment, the control end of the first switch SW 1  is coupled to the output Q of the RS flip-flop U 5  to receive the first switch control signal. When the voltage level of the output Q is high, the first switch SW 1  is ON, and when the voltage level of the output Q is low, the first switch SW 1  is OFF. 
     A second end of the second switch SW 2  is coupled to the negative end of the second current source I 2 . A control end of the second switch SW 2  is coupled to a second switch control signal. When the ramp signal VRAMP is higher than the sum of the error signal COMP and the offset voltage VW, the second switch control signal turns the second switch SW 2  on and maintains on until the ramp signal VRAMP is smaller than the error signal COMP. In one embodiment, the control end of the second switch SW 2  is coupled to an inverse output  Q  of the RS flip-flop U 5 . When the voltage level of output Q is low, the second switch SW 2  is ON, and when the voltage level of the output Q is high, the second switch SW 2  is OFF. 
     A positive end of second current source I 2  is coupled to the reference ground. The first capacitor C 1  is coupled between the conjunction node C and the reference ground. The conjunction node C is also applied as the output of the ramp signal VRAMP. The output current of the first current source is proportional to the output voltage VOUT of the converter circuit  60 , and inversely proportional to the input voltage VIN. The output current of the second current source is proportional to the amplitude of the error signal COMP. 
     When the ramp signal VRAMP is lower than the error signal COMP, as described above, the voltage level of output Q of the RS flip-flop is high, and the voltage level of inverse output  Q  is low. The first switch is ON and the second switch is OFF. The first current begins charging the first capacitor C 1 . The charging slope of the voltage across the first capacitor C 1  is Ii 1 /Cc 1 , wherein Ii 1  is the output current of the first current source I 1 , and Cc 1  is the capacitance of the first capacitor C 1 . As being charged, the voltage across the first capacitor C 1  is gradually increasing. The increasing speed, in other word the rising slope of the ramp signal VRAMP, is proportional to the charging slope Ii 1 /Cc 1 . 
     Since the output current Ii 1  is proportional to the output voltage VOUT of the converter circuit  60 , and inversely proportional to the input voltage VIN, the rising slope of the ramp signal VRAMP is proportional to the output voltage VOUT and inversely proportional to the input voltage VIN. 
     When the ramp signal VRAMP is larger than the sum of the error signal COMP and the offset voltage VW, as described above, the voltage level of the output Q of the RS flip-flop U 5  is low, while the voltage level of the inverse output  Q  of RS flip-flop is high. Thus the first switch SW 1  is OFF and the second switch SW 2  is ON. The second current source I 2  begins discharging the first capacitor C 1 . The discharging slope of the voltage across the capacitor C 1  is Ii 2 /Cc 1 , wherein Ii 2  is the output current of the second current source I 2 . As being discharged, the voltage across the capacitor C 1 , in other word the ramp signal VRAMP, is gradually declining down. The speed of declining, which means the absolute value of the declining slope, is proportional to the discharging slope Ii 2 /Cc 1 . 
     Since the output current of the second current source I 2  is proportional to the error signal COMP, the declining slope of the ramp signal VRAMP is proportional to the error signal COMP. 
     As shown in  FIG. 4 , the amplitude of the ramp signal VRAMP fluctuates from COMP to COMP+VW. Once the amplitude of the ramp signal VRAMP declines down to a level lower than the error signal COMP, it begins rising up. And once it rises up to a level higher than the sum of the error signal COMP and the offset voltage VW, it begins declining again. 
     As described above, the declining slope of the ramp signal VRAMP is proportional to the error signal COMP. The larger the error signal COMP is, the faster the ramp signal declines down to a level lower than the error signal COMP, so that the on time may come earlier. As a result, the switch M 1  is turned on earlier, and the output voltage VOUT rises up faster to result in the feedback signal VFB approaching the reference signal VREF more quickly. 
     Meanwhile, the rising slope of the ramp signal VRAMP is proportional to the output voltage VOUT of the converter circuit  60 , and inversely proportional to the input voltage VIN. Once the output voltage VOUT is continuously declining down, and the error signal keeps increasing, the rising slope of the ramp signal VRAMP is continuously decreasing. Thus the time for the ramp signal VRAMP approaching the sum of the error signal COMP and the offset voltage VW is prolonged, and the on-time of the converter circuit  60  is also prolonged. As the high side switch M 1  keeps on for a longer time, the feedback signal VFB approaches the reference signal VREF more quickly. 
       FIG. 5  illustrates a waveform diagram to indicate the control signal responding from the ramp signal VRAMP in the DC-DC converter circuit  60  according to an embodiment of the present invention. As shown in  FIG. 5 , the upper portion of the diagram illustrates the change of the rising slope and the declining slope of the ramp signal VRAMP, and the lower portion of the diagram illustrates the corresponding generated on-time of the converter circuit  60 . It is indicated that the faster the ramp signal VRAMP declines down, the earlier the on-time is generated. The slower the ramp signal rises up, the longer the on-time lasts. 
     According to the above analysis, it shows that the performance of the transient response of the presented embodiment is excellent. When a load is coupled into the converter circuit, the output voltage VOUT and also the feedback signal VFB step down, so that the amplitude of the error signal COMP gets larger, which will make the on time come earlier. The operational frequency of the high side switch of the converter circuit  60  is correspondingly increases to provide more energy to the load in an unit-time. Thus the output voltage VOUT quickly returns to steady state. In additional, the step down of the output voltage VOUT also makes the on time of the converter circuit lasts longer. This effect also helps to provide more energy to the load in a single unit time and to make the output voltage VOUT return to steady state quickly. 
     Moreover, inasmuch as the utilizing of error amplifier, the stability issue existing in prior art constant on time converter circuit is also solved. 
       FIG. 6  illustrates another DC-DC converter circuit  70  according to another embodiment of the present invention. The primary difference between the converter circuit  70  and the converter circuit  60  is that two ramp signal generator RAMP 1  and RAMP 2  are adopted to replace the ramp signal generator RAMP. 
     Wherein, a first ramp signal generator RAMP 1  generates a first ramp signal VRAMP 1  and provides it to the inverting input of the first comparator U 21 . And a second ramp signal generator RAMP 2  generates a second ramp signal VRAMP 2  and provides it to the non-inverting input of the second comparator U 22 . 
     When the first ramp signal VRAMP 1  is lower than the error signal COMP, the first comparing signal from first comparator U 21  steps up to high level. The RS flip-flop U 5  is set, and the voltage level of the output Q is high. This high level output Q turns on the high side switch M 1  and turns off the low side switch M 2  through the driver circuit U 6 . 
     Once the first ramp signal VRAMP 1  is lower than the error signal COMP, the first ramp signal VRAMP 1  is pulled up to the level equaling the sum of the error signal COMP and the offset voltage VW. Then the first ramp signal gradually declines down. The declining slope of the first ramp signal is proportional to the amplitude of the error signal COMP. 
     As the stepping up of the first ramp signal VRAMP 1 , it becomes higher than the error signal COMP. Thus the first comparing signal from first comparator U 21  steps down to low level, and the voltage level on the set terminal S of the RS flip-flop U 5  declines to low level. The output Q of U 5  is maintained so that the high side switch M 1  keeps on and the low side switch M 2  keeps off. 
     As the first ramp signal VRAMP 1  is pulled up, the second ramp signal VRAMP 2  simultaneously begins to rising gradually. When the second ramp signal VRAMP 2  reaches the sum of the error signal COMP and the offset voltage VW, the second comparing signal from the second comparator U 22  steps up. Thus the RS flip-flop U 5  is reset, so that the voltage level of the output Q is turned to low. Through the driver circuit U 6 , this low level output Q turns off the high side switch M 1  and turns on the low side switch M 2 . 
     When the second ramp signal VRAMP 2  is higher than the sum of the error signal COMP and the offset voltage VW, it is pushed down to the level equaling the error signal COMP and is maintained until the next time when the first ramp signal VRAMP 1  is pulled up to the level equaling the sum of the error signal COMP and the offset voltage VW. The rising slope of the second ramp signal is proportional to the output voltage VOUT and inversely proportional to the input voltage VIN. 
     As the second ramp signal VRAMP 2  is pushed down, the second comparing signal from the second comparator U 22  steps down to low level. Then the voltage level on set terminal of the RS flip-flop U 5  turns to low level. The output Q is maintained, so the high side switch M 1  keeps ON and the low side switch keeps OFF. 
     Then the first ramp signal VRAMP 1  is continuously declining down. When the first ramp signal VRAMP 1  is lower than the error signal COMP, the first comparing signal from the first comparator U 21  steps up to high level. The RS flip-flop is set and the output Q is turned to high level. Thus the high level output Q turns on the high side switch M 1  turns off the low side switch M 2 . The converter circuit  70  enters into next operational cycle. 
       FIG. 7A  and  FIG. 7B  respectively illustrate a schematic circuitry of a first ramp signal generator RAMP 1  and a second ramp signal generator RAMP 2  in DC-DC converter circuit  70  according to another embodiment of the present invention. 
       FIG. 8A  and  FIG. 8B  respectively illustrate the waveform diagrams of the first ramp signal VRAMP 1  and the second ramp signal VRAMP 2  according to another embodiment of the present invention. 
     As shown in  FIG. 7A , the first ramp signal generator RAMP 1  comprises the offset voltage source V, an adder ADD, a third switch SW 3 , a second capacitor C 2  and a third current source I 3 . 
     Wherein, the inputs of the adder ADD are respectively coupled to the output of the error amplifier EA and a positive end of the offset voltage source V, and the output of the adder ADD is coupled to a first end of the third switch SW 3 . A second end of the third switch SW 3  is coupled to a negative end of the third current source I 3 . A negative end of the voltage source V and a positive end of the third current source I 3  is connected to the reference ground. The second capacitor C 2  is coupled between the second end of the third switch SW 3  and the reference ground. A control end of the third switch SW 3  receives a third switch control signal. When the first ramp signal is lower than the error signal COMP, the third switch control signal turns the third switch SW 3  on. The on time of the third switch SW 3 , defined as a first period, is relatively short, for example, 30 ns. In one embodiment, the on time of the third switch may be constant. The second end of the third switch SW 3  is utilized as the output of the first ramp signal generator RAMP 1 , configured to provide the first ramp signal VRAMP 1 . Wherein, the output current of the third current source I 3  is proportional to the amplitude of the error signal COMP. 
     When the first ramp signal VRAMP 1  is lower than the error signal COMP, the third switch SW 3  is turned on according to the third switch control signal. The first ramp signal VRAMP 1  is immediately pulled up to the level equaling to the amplitude of the output of the adder ADD which is the sum of the error signal COMP and the offset voltage VW. Hence, once the third switch is turned on, the voltage across the second capacitor C 2  is immediately pull up to the sum of the error signal COMP and the offset voltage VW. 
     Controlled by the third switch control signal, the third switch SW 3  is turned off after being on for a first period, e.g. 30 ns. The third current source I 3  begins to discharging the second capacitor. The discharging speed, meaning the slope of the discharging, is Ii 3 /Cc 2 , wherein Ii 3  is the output current of the third current source, and Cc 2  is the capacitance of the capacitor C 2 . As the discharging continues, the voltage across the capacitor C 2 , meaning the first ramp signal VRAMP 1  gradually declines down. The declining slope is proportional to the discharging slope Ii 3 /Cc 2 . 
     Since the output current of the third current source I 3  is proportional to the amplitude of the error signal COMP, the declining slope of the first ramp signal is proportional to the amplitude of the error signal COMP. 
     As shown in  FIG. 7B , the second ramp signal generator RAMP 2  comprises a fourth current source I 4 , a fourth switch SW 4 , a fifth switch SW 5  and a third capacitor C 3 . 
     Wherein, an output of the fourth current source I 4  is coupled to a first end of the fifth switch SW 5 , a first end of the fourth switch SW 4 , and a first end of the third capacitor C 3 . The output of the fourth current source I 4  is also utilized as the output of the second ramp signal generator RAMP 2  to provide the second ramp signal VRAMP 2 . A negative end of the fourth current source, a second end of the fourth switch SW 4 , and a second end of the capacitor C 3  are connected to reference ground. A second end of the fifth switch receives the error signal COMP. Control ends of the fourth switch SW 4  and the fifth switch SW 5  are coupled to a fourth switch control signal. When the second ramp signal VRAMP 2  is higher than the sum of the error signal COMP and the offset voltage VW, the fourth switch control signal turns off the fourth switch SW 4  and the fifth switch SW 5  until the first ramp signal VRAMP 1  is lower than the error signal COMP. In one embodiment, the control ends of the fourth switch SW 4  and the fifth switch SW 5  are coupled to the inverse output  Q  of the RS flip-flop U 5 . When the output Q of the RS flip-flop is low, the fourth switch SW 4  and the fifth switch SW 5  are turned on. And when the output Q is high, the fourth switch SW 4  and the fifth switch SW 5  are turned off. Wherein, the output current of the fourth current source is proportional to the output voltage VOUT and inversely proportional to the input voltage VIN 
     Once the output Q of the RS flip-flop U 5  is high, the switches SW 4  and SW 5  are turned off. The fourth current source I 4  begins charging the third capacitor C 3 . The charging slope is Ii 4 /Cc 3 , wherein Ii 4  is the output current of the fourth current source, and Cc 3  is the capacitance of the capacitor C 3 . As the charging continues, the voltage across the capacitor C 3 , meaning the second ramp signal VRAMP 2 , gradually rises up. The rising slope is proportional to the charging slope Ii 4 /Cc 3 . 
     Since the output current of the fourth current source is proportional to the output voltage VOUT and inversely proportional to the input voltage VIN, the rising slope of the second ramp signal VRAMP 2  is also proportional to the output voltage VOUT and inversely proportional to the input voltage VIN. 
     When the second ramp signal VRAMP 2  is higher than sum of the error signal COMP and the offset voltage VW, as described above, the second comparing signal from the second comparator U 22  steps up to a high level. The RS flip-flop U 5  is reset, and the output Q is low. The switches SW 4  and SW 5  are turned on. The voltage across the capacitor C 3 , meaning the second ramp signal VRAMP 2 , is pushed down to the level equaling the amplitude of the error signal COMP. Until the first ramp signal VRAMP 1  is lower than the error signal COMP, and the first comparator U 21  generates a high level comparing signal, the output Q of the RS flip-flop could be turned to high again. The switches SW 4  and SW 5  are turned off and the second ramp signal VRAMP 2  gradually rises up. 
     Seen in  FIG. 8A , the amplitude of the first ramp signal VRAMP 1  gradually declines down from the sum of error signal COMP and the offset voltage VW. If the first ramp signal VRAMP 1  is lower than the error signal COMP, it will be immediately pulled up to the sum of error signal COMP and the offset voltage VW again, and once more begins declining down. Wherein the declining slope of the first ramp signal RAMP 1  is proportional to the amplitude of the error signal COMP. 
     As shown in  FIG. 8B , the amplitude of the second ramp signal VRAMP 2  gradually rises up from the error signal COMP. Once it rises above the sum of the error signal COMP and the offset voltage VW, it is immediately pushed down to the level of the error signal COMP and maintained at this level until the first ramp signal VRAMP 1  gradually declines to a level lower than the error signal COMP. Then the second ramp signal VRAMP 2  rises up again. Wherein, the rising slope of the second ramp signal VRAMP 2  is proportional to the output voltage VOUT and reversely proportional to the input voltage VIN. 
     Compared with the converter circuit  60  shown in  FIG. 2 , the converter circuit  70  in  FIG. 6  may obtain a better transient response performance. For example, when the load current is step up, if the declining slopes of VRAMP and VRAMP 1  are the same, and if the rising slopes of the VRAMP and VRAMP 2  are the same, the off time of the converter circuit  70  is shorter than the converter circuit  60 . It means that the converter circuit  70  may turns on the high side switch and turns off the low side switch more quickly than the converter circuit  60 , so that the feedback signal VFB of the converter circuit  70  also increases to approach the reference signal VREF more quickly. The reasons of the better transient response performance for converter circuit  70  may comprises that once the second ramp signal VRAMP 2  rises from the error signal COMP, the first ramp signal VRAMP 1  declines from the sum of the error signal COMP and the offset voltage VW. The off time TOFF is changed from VW/RDOWN of the converter circuit  60  to VW/RDOWN-VW/RUP of the converter circuit  70 , which is reduced for VW/RUP. 
       FIG. 9  illustrates a flowchart diagram of a method  900  for controlling a DC-DC converter according to an embodiment of the present invention. 
     Seen in  FIG. 9 , the method  900  comprises: step S 910 , amplifying an error between an output voltage or a feedback output voltage of the converter circuit, and a reference signal, configured to obtain an error signal; step  920 , generating a first ramp signal and a second ramp signal; step  930 , comparing the error signal with the first ramp signal to obtain a first comparing signal; step S 940 , comparing the sum of the error signal and a offset voltage with the second ramp signal to obtain a second comparing signal; and step S 950 , generating a control signal to control switches of the converter circuit according to the first comparing signal and the second comparing signal. 
     In one embodiment, the switches of the converter circuit comprises a high side switch and a low side switch, when the first ramp signal is lower than the error signal, a high level first comparing signal is generated to make the control signal generate an on time of the converter circuit, wherein during the on time, the high side switch is turned on and the low side switch is turned off; when the second ramp signal is higher than the sum of the error signal and the offset voltage, a low level second comparing signal is generated to make the control signal generates an off time of the converter circuit, wherein during the off time, the high side switch is turned off and the low side switch is turned on. 
     In one embodiment, the first ramp signal and the second ramp signal are identical ramp signals. 
     In one embodiment, wherein, the ramp signal comprises the following features: rising up gradually if the ramp signal is lower than the error signal; declining down gradually if the ramp signal is higher than the sum of the error signal and the reference signal; wherein the declining slope of the ramp signal is proportional to the error signal, and wherein the rising slope of the ramp signal is proportional to the output voltage of the converter circuit and inversely proportional to an input voltage of the converter circuit. 
     In another embodiment, the first ramp signal and the second ramp signal may be different from each other. 
     wherein, the first ramp signal is pulled up to a level equaling the sum of the error signal and the offset voltage if it is lower than the error signal, and then gradually declines down from this level, wherein the declining slope of the first ramp signal is proportional to the error signal. 
     And wherein, the second ramp signal is pushed down to a level equaling the error signal if it is higher than the sum of the error signal and the offset voltage, and then gradually rises up from this level, wherein the rising slope is proportional to the output voltage of the converter circuit and inversely proportional to the input voltage of the converter circuit. 
     The above description and discussion about specific embodiments of the present invention is for purposes of illustration. However, one with ordinary skill in the relevant art should know that the invention is not limited by the specific examples disclosed herein. Variations and modifications can be made on the apparatus, methods and technical design described above. Accordingly, the invention should be viewed as limited solely by the scope and spirit of the appended claims.