Abstract:
The present invention discloses a DC-DC converter with constant on time control. The DC-DC converter includes a transient unit to obtain the transient information of a current flowing through a power switching circuit, to slow down the variation of the output voltage, so as to eliminate the overshot issue.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of Chinese Patent Application No. 201510770698.X, filed Nov. 12, 2015, which is incorporated herein by reference in its entirety. 
       FIELD 
       [0002]    The present invention relates to electronic circuits, more specifically, the present invention relates to DC-DC converter with COT (constant on time) control and the method thereof. 
       BACKGROUND 
       [0003]    Most electric devices such as notebook, desktop computer and PDA need a regulated voltage to power function blocks. DC-DC converters with COT control characterized with fast transient response and simple structure are widely used in the above fields. Typically, DC-DC converters with COT control need slope compensation to stabilize the output voltage. 
         [0004]    A slope compensation signal has a fixed slope in prior art. It is reset to zero when a main power switch in the converter turns on. Then it increases with a fixed slope until the slope compensation signal is reset again when the output voltage of the converter drops to a sum of the slope compensation signal and a reference voltage. This process repeats during the operation of the converter. 
         [0005]    However, if load step occurs (e.g. the load steps to heavy load from light load, or the load steps to light load from heavy load), the output voltage decreases rapidly and falls below the reference voltage in a short time period.  FIG. 1  schematically shows time waveforms of the inductor current I L , the switching control signal PWM, the output voltage V O , the slope compensation signal Vsl, the reference voltage V REF  and the output current I O  in a typical DC-DC converter with COT control when the load suddenly steps to heavy load from light load, wherein the x axis represents time. As shown in  FIG. 1 , the load suddenly steps to heavy load from light load at time point t 1 . Then the slope compensation signal V S l is reset to zero several times and a plurality of switching control signals PWM are generated in short time period (from time point t 1  to time point t 2 ). This plurality of switching control signals PWM causes the inductor current to rise rapidly. Then the inductor would store much more power than needed after the new steady state is reached. The redundant power would charge an output capacitor, which pumps the output voltage V O , and causes an overshoot issue. In some worst situations, voltage ring back may occur. 
       SUMMARY 
       [0006]    It is an object of the present invention to provide an improved DC-DC converter with transient control, which solves the above problems. 
         [0007]    In accomplishing the above and other objects, there has been provided, in accordance with an embodiment of the present invention, a DC-DC converter with transient control, comprising: a power switching circuit, configured to receive an input voltage and to provide an output voltage; a transient unit, configured to generate a transient current signal indicative of the transient information of a current flowing through the power switching circuit; a comparing circuit, configured to generate a set signal based on a reference voltage, a feedback voltage indicative of the output voltage, a slope compensation signal and the transient current signal; and a logical control circuit, configured to generate a switching control signal to control the operation of the power switching circuit based on the set signal. 
         [0008]    In addition, there has been provided, in accordance with an embodiment of the present invention, a DC-DC converter with transient control, comprising: a power switching circuit, configured to receive an input voltage and to provide an output voltage; a resistor, having a first end and a second end, wherein the first end is configured to receive a current sense signal indicative of the current flowing through the power switching circuit; a capacitor, coupled between the second end of the resistor and a reference ground; a comparing circuit, configured to generate a set signal based on a reference voltage, a feedback voltage indicative of the output voltage, a slope compensation signal, the current sense signal and a voltage across the capacitor; and a logical control circuit, configured to generate a switching control signal to control the operation of the power switching circuit based on the set signal. 
         [0009]    Furthermore, there has been provided, in accordance with an embodiment of the present invention, a method used in a DC-DC converter, the DC-DC converter including a power switching circuit configured to receive an input voltage and generate an output voltage, the method comprising: detecting whether the DC-DC converter is in steady state condition or in transient state condition; comparing a sum of a slope compensation signal and a reference voltage with the feedback voltage to generate a set signal when the DC-DC converter is in steady state condition; and generating a transient current signal indicative of the transient information of a current flowing through the power switching circuit, and comparing a sum of the slope compensation signal and the reference voltage with a sum of the feedback voltage and the transient current signal to generate the set signal when the DC-DC converter is in transient state condition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  schematically shows time waveforms of the inductor current I L , the switching control signal PWM, the output voltage V O , the slope compensation signal V SL , the reference voltage V REF  and the output current I O  in a typical DC-DC converter with COT control when the load suddenly steps to heavy load from light load. 
           [0011]      FIG. 2  schematically shows a DC-DC converter  100  in accordance with an embodiment of the present invention. 
           [0012]      FIG. 3  schematically shows a circuit configuration of the transient unit  104  in the DC-DC converter  100  in  FIG. 2  in accordance with an embodiment of the present invention. 
           [0013]      FIG. 4  schematically shows a circuit configuration of the transient unit  104  in the DC-DC converter  100  in  FIG. 2  in accordance with an embodiment of the present invention. 
           [0014]      FIG. 5  schematically shows a circuit configuration of the comparing circuit  105  in the DC-DC converter  100  in  FIG. 2  in accordance with an embodiment of the present invention. 
           [0015]      FIG. 6  schematically shows a DC-DC converter  200  in accordance with an embodiment of the present invention. 
           [0016]      FIG. 7  schematically shows a circuit configuration of the comparing circuit  105  in the DC-DC converter  200  in  FIG. 6  in accordance with an embodiment of the present invention. 
           [0017]      FIG. 8  schematically shows a circuit configuration of the power switching circuit  103  in accordance with an embodiment of the present invention. 
           [0018]      FIG. 9  schematically shows a flow chart  300  of a method used in a DC-DC converter in accordance with an embodiment of the present invention. 
       
    
    
       [0019]    The use of the similar reference label in different drawings indicates the same of like components. 
       DETAILED DESCRIPTION 
       [0020]    Embodiments of circuits for DC-DC converter are described in detail herein. In the following description, some specific details, such as example circuits for these circuit components, are included to provide a thorough understanding of embodiments of the invention. One skilled in relevant art will recognize, however, that the invention can be practiced without one or more specific details, or with other methods, components, materials, etc. 
         [0021]    The following embodiments and aspects are illustrated in conjunction with circuits and methods that are meant to be exemplary and illustrative. In various embodiments, the above problem has been reduced or eliminated, while other embodiments are directed to other improvements. 
         [0022]      FIG. 2  schematically shows a DC-DC converter  100  in accordance with an embodiment of the present invention. In the example of  FIG. 2 , the DC-DC converter  100  comprises: an input port  101 , configured to receive an input voltage Vin; an output port  102 , configured to provide an output voltage V O ; a power switching circuit  103 , coupled between the input port  101  and the output port  102 ; a control circuit, configured to provide a switching control signal PWM, to control the operation of the power switching circuit  103 , the control circuit including: a transient unit  104 , configured to receive a current sense signal I CS  indicative of a current flowing through the power switching circuit  103 , to generate a transient current signal V tra ; a comparing circuit  105 , configured to receive a reference voltage V REF , a feedback voltage V FB  indicative of the output voltage V O , a slope compensation signal V SL  and the transient current signal V tra , to generate a set signal S; and a logical control circuit  106 , configured to receive the set signal S, to generate a switching control signal PWM; wherein when a sum of the feedback voltage V FB  and the transient current signal V tra  is less than a sum of the reference voltage V REF  and the slope compensation signal V SL  (i.e. below expression (1)), the set signal S goes high, and the switching control signal PWM is triggered by the set signal S to turn on the power switching circuit  103 . 
         [0000]        V   FB   +V   tra   &lt;V   REF   +V   SL   (1)
 
         [0023]    In one embodiment, the current sense signal I CS  may be a current signal or a voltage signal. 
         [0024]    In one embodiment, the slope compensation signal V SL  is reset to zero and increases with a fixed slope when a main power switch in the power switching circuit  103  turns on. 
         [0025]    In one embodiment, the transient current signal V tra  generated by the transient unit  104  is indicative of the transient information (i.e. the AC component) of the current sense signal I CS . 
         [0026]    During the operation of the DC-DC converter, when the load step occurs (e.g. the load jumps to heavy load from light load very fast and within a short period of time), the output voltage V O  decreases. The frequency of the switching control signal PWM increases, and the current flowing through the power switching circuit  103  increases, i.e. the current sense signal I CS  increases. The transient current signal V tra  also increases. Thus the variation of the output voltage V O  slows down according to expression (1). As a result, the set signal postpones triggering the switching control signal PWM, which reduces the pulses of the switching control signal PWM. So the energy stored in the inductor is reduced during the load step, and the overshoot is eliminated, which ensures the system stability. 
         [0027]      FIG. 3  schematically shows a circuit configuration of the transient unit  104  in the DC-DC converter  100  in  FIG. 2  in accordance with an embodiment of the present invention. In the example of  FIG. 3 , the transient unit  104  comprises: a resistor  41 , having a first end and a second end, wherein the first end is configured to receive the current sense signal I CS ; a capacitor  42 , coupled between the second end of the resistor  41  and a reference ground; and an operation amplifier  43 , coupled crossover the resistor  41  to receive a voltage across the resistor  41 , to generate the transient current signal V tra . 
         [0028]    When the system is in operation, if the load step occurs (e.g. the load jumps to heavy load from light load), the current sense signal I CS  increases. Then the voltage across the resistor  41  also increases, i.e. the transient current signal V tra  increases. According to expression (1), the variation of the output voltage V O  slows down, and the set signal postpones triggering the switching control signal PWM, which reduces the pulses of the switching control signal PWM. So the energy stored in the inductor is reduced during the load step, and the overshoot is eliminated as discussed above. In steady state, due to the existence of the capacitor  42 , the average voltage across the resistor  41  is zero. So the transient current signal V tra  provided by the transient unit  104  reflects the transient information of the current sense signal I CS . That is, the transient current signal V tra  reflects the transient information (the alternating information) of the current flowing through the power switching circuit  103 . 
         [0029]      FIG. 4  schematically shows a circuit configuration of the transient unit  104  in the DC-DC converter  100  in  FIG. 2  in accordance with an embodiment of the present invention. The transient unit  104  in  FIG. 4  is similar to that in  FIG. 3 , with a difference that the transient unit  104  in  FIG. 4  further comprises: a current-voltage converter  44 , wherein the resistor  41  is configured to receive the current sense signal I CS  via the current-voltage converter  44 . When the current sense signal I CS  is a current signal, the current-voltage converter  44  converts the current form into voltage form; and then it delivers the voltage form to the first end of the resistor  41 . 
         [0030]      FIG. 5  schematically shows a circuit configuration of the comparing circuit  105  in the DC-DC converter  100  in  FIG. 2  in accordance with an embodiment of the present invention. In the example of  FIG. 5 , the comparing circuit  105  comprises: a comparator  51 , having a first input terminal, a second input terminal and an output terminal; a first adder  52 , configured to receive the transient current signal V tra  and the feedback voltage V FB , to execute add operation on the transient current signal V tra  and the feedback voltage V FB , and deliver the add result to the first input terminal of the comparator  51 ; and a second adder  53 , configured to receive the reference voltage V REF  and the slope compensation signal V SL , to execute add operation on the reference voltage V REF  and the slope compensation signal V SL , and deliver the add result to the second input terminal of the comparator  51 ; wherein the comparator  51  compares the signal at its first input terminal with that at its second input terminal to generate the set signal S. 
         [0031]      FIG. 6  schematically shows a DC-DC converter  200  in accordance with an embodiment of the present invention. In the example of  FIG. 6 , the DC-DC converter  200  comprises: an input port  101 , configured to receive an input voltage Vin; an output port  102 , configured to provide an output voltage V O ; a power switching circuit  103 , coupled between the input port  101  and the output port  102 ; a control circuit, configured to provide a switching control signal PWM, to control the operation of the power switching circuit  103 , the control circuit including: a resistor  41 , having a first end and a second end, wherein the first end is configured to receive the current sense signal I CS ; a capacitor  42 , coupled between the second end of the resistor  41  and a reference ground, the voltage across the capacitor  42  being a steady current signal V ste ; a comparing circuit  105 , configured to receive a reference voltage V REF , a feedback voltage V FB  indicative of the output voltage V O , a slope compensation signal V SL  and the steady current signal V ste , to generate a set signal S; and a logical control circuit  106 , configured to receive the set signal S, to generate the switching control signal PWM, to control the operation of the power switching circuit  103 . 
         [0032]    In one embodiment, if the current sense signal I CS  is in current form, the DC-DC converter  200  further comprises a current-voltage converter as shown in  FIG. 4 , and the resistor  41  is configured to receive the current sense signal I CS  via the current-voltage converter. 
         [0033]      FIG. 7  schematically shows a circuit configuration of the comparing circuit  105  in the DC-DC converter  200  in  FIG. 6  in accordance with an embodiment of the present invention. In the example of  FIG. 7 , the comparing circuit  105  comprises: a comparator  51 , having a first input terminal, a second input terminal and an output terminal; a first adder  52 , configured to receive the current sense signal I CS  and the feedback voltage V FB , to execute add operation on the current sense signal I CS  and the feedback voltage V FB , and deliver the add result to the first input terminal of the comparator  51 ; and a second adder  53 , configured to receive the reference voltage V REF , the slope compensation signal V SL  and the steady current signal V ste , to execute add operation on the reference voltage V REF , the slope compensation signal V SL  and the steady current signal V ste , and deliver the add result to the second input terminal of the comparator  51 ; wherein the comparator  51  compares the signal at its first input terminal with that at its second input terminal to generate the set signal S. 
         [0034]    When the system is in operation, the current sense signal I CS  is delivered to the first input terminal of the comparator  53  via the first adder  51 , and is delivered to the second input terminal of the comparator  53  via the resistor  41 , the capacitor  42  and the second adder  52 . So the current sense signal I CS  is counteracted in steady state condition. But in transient state condition, e.g. when the load suddenly jumps to heavy load from light load, the current sense signal I CS  increases rapidly. Because of the existence of the capacitor, the voltage across the capacitor  42  (i.e. the transient current signal V ste ) cannot change so fast, so the transient information of the current sense signal I CS  is delivered to the first input terminal of the comparator  53 . As a result, the variation of the output voltage V O  slows down, and the set signal postpones triggering the switching control signal PWM, which reduces the pulses of the switching control signal PWM. Thus the energy stored in the inductor is reduced during the load step, and the overshoot is eliminated. 
         [0035]      FIG. 8  schematically shows a circuit configuration of the power switching circuit  103  in accordance with an embodiment of the present invention. In the example of  FIG. 8 , the power switching circuit  103  comprises a typical buck circuit. That is, the power switching circuit  103  comprises: a high side power switch  31 , a low side power switch  32 , an inductor  33  and an output capacitor  34  connected as shown. The configuration of buck circuit is well known in the art, and will not be discussed in detail for brief illustration, 
         [0036]      FIG. 9  schematically shows a flow chart  300  of a method used in a DC-DC converter in accordance of the present invention. The DC-DC converter including a power switching circuit configured to receive an input voltage and generate an output voltage, the method comprises: 
         [0037]    Step  301 , deriving a feedback signal indicative of the output voltage, and a current sense signal indicative of a current flowing through the power switching circuit. 
         [0038]    Step  302 , detecting whether the DC-DC converter is in steady state condition or not, if the DC-DC converter is in steady state condition, go to step  303 ; and if the DC-DC converter is in transient state condition, go to step  304 . 
         [0039]    Step  303 , comparing a sum of a slope compensation signal V SL  and a reference voltage V REF  with the feedback voltage V FB  to generate a set signal. 
         [0040]    Step  304 , generating a transient current signal V tra  indicative of the transient information of the current sense signal. 
         [0041]    Step  305 , comparing a sum of the slope compensation signal V SL  and the reference voltage V REF  with a sum of the feedback voltage V FB  and the transient current signal to generate the set signal. 
         [0042]    Step  306 , generating a switching control signal in response to the set signal, to control the operation of the power switching circuit. 
         [0043]    It is to be understood in these letters patent that the meaning of “A” is coupled to “B” is that either A and B are connected to each other as described below, or that, although A and B may not be connected to each other as described above, there is nevertheless a device or circuit that is connected to both A and B. This device or circuit may include active or passive circuit elements, where the passive circuit elements may be distributed or lumped-parameter in nature. For example, A may be connected to a circuit element that in turn is connected to B. 
         [0044]    This written description uses examples to disclose the invention, including the best mode, and also to enable a person skilled in the art to make and use the invention. The patentable scope of the invention may include other examples that occur to those skilled in the art.