Abstract:
A current-mode control type DC-DC converter includes a switching transistor turned on with a clock signal output in predetermined cycles, an inductor supplied with electric current when the switching transistor is turned on, an error amplifier circuit to output an error voltage that is an amplified difference between a predetermined reference voltage and a divided output voltage of the DC-DC converter, a slope voltage generation circuit to generate a slope voltage by performing slope compensation on an inductor current, a PWN comparator to compare the slope voltage with the error voltage and generate a reset pulse to turn off the switching transistor when the slope voltage reaches the error voltage, and a slope voltage maintenance mechanism to keep the slope voltage at the ground voltage from when the reset pulse is generated to when a subsequent clock signal is generated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent specification claims priority from Japanese Patent Application No. 2008-229945, filed on Sep. 8, 2008 in the Japan Patent Office, which is hereby incorporated by reference herein in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a current-mode control type DC-DC (direct current to direct current) converter used for power supplies in electronic devices, and a control method for a current-mode control type DC-DC converter. 
         [0004]    2. Discussion of the Background 
         [0005]    Currently, as power supply circuits used in portable electronic devices, non-insulated DC-DC converters that include inductors capable of downsizing and obtaining higher efficiency are widely used. 
         [0006]    Classified by feedback method, there are two types of DC-DC converters, those employing a voltage-mode control method and those employing a current-mode control method. 
         [0007]    The current-mode control type DC-DC converters have a number of advantages. For example, a line regulation expressed as a percentage of change in output voltage relative to the change in input voltage is higher, compensating signal phases as well as controlling the current are easier, and they are adapted to have a large capacity of electric power by arranging multiple current mode DC-DC converters in parallel. Therefore, at present, current-mode control type DC-DC converters are widely used. 
         [0008]      FIG. 3  illustrates a configuration of a known DC-DC converter  100 . The DC-DC converter  100  includes a slope voltage generation circuit  115 , a fixed slope compensation voltage generation circuit  116 , an amended slope compensation voltage generation circuit  117 , a reference voltage generation unit  121 , an error amplifier circuit  110 , a PWM (pulse width modulation) control comparator  111 , a RS (Reset-Set) flip-flop circuit  112 , a driver circuit  113 , a switching transistor M 101 , a synchronous rectification transistor M 102 , PMOS (P-channel metal oxide semiconductor) transistors M 103  and M 104 , an inductor L 101 , a capacitor C 101 , and resistors R 101  through R 103 . The DC-DC converter  100  further includes a power input terminal Vin, ground terminals Vss, an output terminal Vout, and an external-control bias PABIAS. 
         [0009]    The slope voltage generation circuit  115  includes operational amplifier circuits  118  and  119 , PMOS transistors M 105 , M 106  and M 107 , NMOS (N-channel metal oxide semiconductor) transistor M 108 , and resistors R 104  through R 108 . The PMOS transistors M 106  and M 107  form a current mirror circuit. 
         [0010]    The fixed slope compensation voltage generation circuit  116  includes an electric current source L 101 , a PMOS transistor M 109 , a NMOS transistor M 110 , a capacitor C 102 , and a resistor R 109 . 
         [0011]    The amended slope compensation voltage generation circuit  117  includes an operational amplifier circuit  120 , PMOS transistors M 113  and M 114 , NMOS transistors M 111 , M 112 , and M 115 , and a resistor R 110 . The PMOS transistors M 113  and M 114  form one current mirror circuit, and the NMOS transistors M 111  and M 112  form another current mirror circuit. 
         [0012]    With reference to a timing chart shown in  FIG. 4 , operation of the known DC-DC converter  100  is described below. 
         [0013]    A clock signal is inputted to a set terminal S of the RS flip-flop circuit  112 , and the RS flip-flop circuit  112  is set up at every clock signal period. When the RS flip-flop circuit  112  is set up, an output signal of an output terminal Q thereof becomes high, and the signal is applied to an input terminal I of the driver circuit  113 . Then, the driver circuit  113  turns both a control signal PHS outputted from an output terminal P and a control signal NLS outputted from an output terminal N low. Therefore, the switching transistor M 101  is turned on, and the synchronous rectification transistor M 102  is turned off. At this time, the PMOS transistors M 103  and M 104  forming a series circuit  114  connected in parallel to the switching transistor M 101  are turned on. 
         [0014]    Subsequently, when the switching transistor M 101  is turned on, an electric current IL is supplied from a power input terminal Vin to the inductor L 101 . At this time, a voltage drop that is proportional to the inductor current IL is generated across a source and a drain of the switching transistor M 101 . The voltage drop is divided by the PMOS transistors M 103  and M 104 , and the divided voltage is picked up as a voltage between a source and a drain of the PMOS transistor M 103 . This voltage is a voltage Vsense. 
         [0015]    The voltage Vsense is supplied to a non-inverting input terminal of the operational amplifier circuit  118 . An inverting output terminal of the operational amplifier circuit  118  is connected to a source of the PMOS transistor M 105 , and an output terminal thereof is connected to a gate of the PMOS transistor M 105 . The resistor R 106  is connected between the source of the transistor M 105  and the power input terminal Vin. The resistor R 108  is connected between a drain of the transistor M 105  and the ground terminal Vss. 
         [0016]    Therefore, a drain voltage VA of the PMOS transistor M 105  is proportional to the voltage Vsense and is a voltage changed to a ground standard voltage. Since the inductor current IL is increased over time, the voltage VA is increased over time as shown in  FIG. 4 . 
         [0017]    It is to be noted that the voltage VA starts from a voltage in excess of 0 V (Volt) because the DC-DC converter operates in a continuous mode, in which the inductor current IL flows through the synchronous rectification transistor M 102  while the switching transistor M 101  is off, and the inductor current IL does not decrease to 0 A (Ampere) until the switching transistor M 101  is turned on next time. 
         [0018]    Next, ignoring operation of the amended slope compensation voltage generation circuit  117 , operation of the fixed slope compensation voltage generation circuit  116  is described below. 
         [0019]    When the control signal PHS is high, the NMOS transistor M 110  is on, and a capacitor C 102  discharges. At this time, the PMOS transistor M 109  is off, and an electric current supply from the electric current source I 101  to the capacitor C 102  is stopped. 
         [0020]    As described above, when the control signal PHS becomes low by inputting the clock signal to the RS flip-flop circuit  112 , the NMOS transistor M 110  is tuned off, and the PMOS transistor M 109  is turned on. Then, the capacitor C 102  is recharged by the electric current source I 101 , and a voltage VB at a junction node between a drain of the PMOS transistor M 109  and the capacitor C 102  is linearly increased as shown in  FIG. 4 . 
         [0021]    The voltage VB is added to the voltage VA via the resistors R 107  and R 109 , thus generating a voltage VC shown in  FIG. 4 . The voltage VC is applied to an operating amplifier circuit  119  and to a non-inverting input terminal of the PWM comparator  111  via the current mirror circuit constituted by the PMOS transistors M 107  and M 106 . 
         [0022]    By contrast, an output voltage Vout of the known DC-DC converter  100  is divided by the resistors R 101  and R 102  and inputted to an inverting input terminal of the error amplifier circuit  110 . The reference voltage Vref is applied to a non-inverting input terminal of the error amplifier circuit  110 . 
         [0023]    The error amplifier circuit  110  outputs an error voltage Verr that is an amplified difference voltage between the divided output voltage Vout and the reference voltage Vref. The error voltage Verr is applied to an inverting input terminal of the PWM comparator  111 . 
         [0024]    As the voltage VC and the error voltage Verr in shown in  FIG. 4  indicate, when the voltage VC is increased over time and reaches the error voltage Verr, an output signal of the PWM comparator  111  becomes high, and the RS flip-flop circuit  112  is reset. 
         [0025]    Then, the output signal outputted from the output terminal Q becomes low, and the driver circuit  113  receives the signal thus outputted and switches the control signals PHS and NLS to high level. 
         [0026]    Subsequently, the switching transistor M 101  is turned off, and the synchronous rectification transistor M 102  is turned on. At this time, because the NMOS transistor M 110  is turned on, the capacitor C 102  discharges, and the voltage VB is decreased to the ground voltage. 
         [0027]    Further, because the PMOS transistor M 109  is turned off, the electric current from the electric current source I 101  is interrupted. Additionally, because the PMOS transistors M 103  and M 104  are turned off, the voltage Vsense becomes substantially equal to the input voltage Vin, and the voltage VA is decreased to the ground voltage. 
         [0028]    Then, when the clock signal becomes high next time and the control signal PHS becomes low, the DC-DC converter  100  repeats the above-described operation. 
         [0029]    However, in the above-described known slope voltage generation circuit, after the transistor M 101  is turned off, the voltage VC is not immediately decreased to the ground voltage but is decreased slowly as indicated by solid curved lines shown in  FIG. 4 . 
         [0030]    This situation occurs because it takes time to discharge the charge contained in a stray capacitance generated in an area from the junction node C to the non-inverting input terminal of the PWM comparator  111 . If the above-described time lengthens and the voltage VC is not decreased to the ground voltage until a next clock signal is inputted, the residual voltage is added to the voltage VC in the next cycle and an accurate switching period cannot be obtained. As a result, the output voltage fluctuates, which is a problem. 
         [0031]    In view of the foregoing, there is market demand for DC-DC converters capable of reducing fluctuations in the output voltage. 
       SUMMARY OF THE INVENTION 
       [0032]    In view of the foregoing, one illustrative embodiment of the present invention provides a current-mode control type DC-DC converter to control an output voltage that includes a switching transistor e turned on every time a clock signal is input thereto in predetermined cycles, an inductor supplied with electric current when the switching transistor is turned on, an error amplifier circuit to output an error voltage that is generated by amplifying a difference between a predetermined reference voltage and a divided voltage by dividing the output voltage of the DC-DC converter, a slope voltage generation circuit to generate a slope voltage that is generated by performing slope compensation on an inductor current, a PWN comparator to compare the slope voltage with the error voltage and generate a reset pulse when the slope voltage reaches the error voltage and the reset pulse turns the switching transistor off, and a slope voltage maintenance mechanism to keep the slope voltage at ground voltage during a time period from when the reset pulse is generated to when a subsequent clock signal is generated. 
         [0033]    Another illustrative embodiment of the present invention provides a current-mode control type DC-DC converter to control an output voltage that includes switching means turned on every time a clock signal is input thereto in predetermined cycles, inductor means supplied with electric current when the switching means is turned on, error amplifier means for outputting an error voltage that is generated by amplifying a difference between a predetermined reference voltage and a divided voltage that is generated by dividing the output voltage of the DC-DC converter, slope voltage generation means for generating a slope voltage by performing slope compensation on an inductor current, PWN comparing means for comparing the slope voltage with the error voltage and generate a reset pulse to turn off the switching means when the slope voltage reaches the error voltage, and slope voltage maintenance means for keeping the slope voltage at the ground voltage during a time period from when the reset pulse is generated to when a subsequent clock signal is generated. 
         [0034]    Another illustrative embodiment of the present invention provides a method for controlling a current-mode control type DC-DC converter that includes turning on a switching transistor every time a clock signal is input thereto in predetermined cycles, supplying electric current to an inductor (L 1 ) when the switching transistor is turned on, outputting an error voltage that is generated by amplifying a difference between a predetermined reference voltage and a divided voltage that is generated by dividing the output voltage of the DC-DC converter, generating a slope voltage by performing slope compensation on an inductor current, comparing the slope voltage with the error voltage, generating a reset pulse when the slope voltage reaches the error voltage, turning the switching transistor off in accordance with the reset pulse, and keeping the slope voltage at the ground voltage during a time period from when the reset pulse is generated to when a subsequent clock signal is generated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
           [0036]      FIG. 1  illustrates circuitry of a current-mode control type DC-DC converter  1  according to an illustrative embodiment of the present invention; 
           [0037]      FIG. 2  is a timing chart of operation of the DC-DC converter  1  shown in  FIG. 1 ; 
           [0038]      FIG. 3  illustrates related-art circuitry of a current-mode control type DC-DC converter  100 ; and 
           [0039]      FIG. 4  is a timing chart of operation of the DC-DC converter  100  shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0040]    In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
         [0041]    Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views thereof, particularly to  FIGS. 1 and 2 , a DC-DC converter according to an example embodiment of the present invention is described below. 
         [0042]      FIG. 1  illustrates circuitry of a current-mode control type DC-DC converter  1  according to the present embodiment. The current-mode control type DC-DC converter  1  includes a slope voltage generation circuit  20 , a reference voltage generation unit  21 , an error amplifier circuit  10 , a PWM (pulse width modulation) control comparator  11 , a RS (Reset-Set) flip-flop circuit  12 , and a driver circuit  13 . The current-mode control type DC-DC converter  1  also includes a switching transistor M 1 , a synchronous rectification transistor M 2 , PMOS (P-channel metal oxide semiconductor) transistors M 3  and M 4 , an inductor L 1 , a capacitor C 1 , and resistors R 1  and R 2 . 
         [0043]    The DC-DC converter  1  further includes a power input terminal Vin, ground terminals Vss, and an output terminal Vout. In the DC-DC converter  1 , a reference voltage Vref generated by the reference voltage generation unit  21  is applied to a non-inverting input terminal of the PWM comparator  11 , an input voltage Vi is applied to a predetermined portion between the power input terminal Vin and the ground terminal Vss, and an output voltage Vo is outputted from the output terminal Vout. 
         [0044]    The slope voltage generation circuit  20  includes operational amplifier circuits  14  and  15 , an electric current source I 1 , a PMOS transistor M 5 , and NMOS (N-channel metal oxide semiconductor) transistor M 6 , M 7 , and M 8 , a capacitor Cs, and resistors R 3  through R 6 . 
         [0045]    The output voltage Vo is divided by the resistors R 1  and R 2 , and the divided voltage Vfb is applied to an inverting input terminal of the error amplifier circuit  10 . The reference voltage Vref is applied to the non-inverting input terminal thereof. Then, the error amplifier circuit  10  outputs an error voltage Verr that is the amplified difference between the divided voltage Vfb and the reference voltage Vref from an output terminal thereof. The error voltage Verr is inputted to an inverting input terminal of the PWM comparator  11 . 
         [0046]    An output terminal of the PWM comparator  11  is connected to a reset terminal R of the RS flip-flop circuit  12 . A set terminal S of the RS flip-flop circuit  12  receives a clock signal CLK that is outputted from an oscillator, not shown. An output terminal Q of the RS flip flop circuit  12  is connected to an input terminal I of the driver circuit  13 . An output terminal P of the driver circuit  13  is connected to gates of the switching transistor M 1 , the PMOS transistor M 4 , and the NMOS transistors M 7  and M 8 . An output terminal N of the driver circuit  13  is connected to a gate of the synchronous rectification transistor M 2 . 
         [0047]    The switching transistor M 1  consists of a PMOS transistor, its source connected to the power input terminal Vin and its drain connected to an end of the inductor L 1  and a drain of the synchronous rectification transistor M 2 . 
         [0048]    The synchronous rectification transistor M 2  consists of a NMOS transistor whose source is connected to the ground terminal Vss. The other end of the inductor L 1  is connected to the output terminal Vout. The capacitor C 1  is connected between the output terminal Vout and the ground terminal Vss. 
         [0049]    A source of the PMOS transistor M 3  is connected to the power input terminal Vin, and its gate and drain are connected respectively to the ground terminal Vss and a source of the PMOS transistor M 4 . A drain of the PMOS transistor M 4  is connected to the drain of the switching transistor M 1 . 
         [0050]    A non-inverting input terminal of the operational amplifier circuit  14  is connected to the drain of the PMOS transistor M 3 , and an inverting input terminal thereof is connected to a source of the PMOS transistor M 5 . Its output terminal is connected to a gate of the PMOS transistor M 5 . The resistor R 3  is connected between the source of the PMOS transistor M 5  and the power input terminal Vin, and the resistor R 4  is connected between a drain of the FMOS transistor M 5  and the ground terminal Vss. The drain of the PMOS transistor M 5  is also connected to an end of the resistor R 5 . 
         [0051]    The electric current source I 1  is connected between a non-inverting input terminal of the operational amplifier circuit  15  and the power input terminal Vin. The inverting input terminal of the operational amplifier circuit  15  is also connected to one terminal of the capacitor Cs and a drain of the NMOS transistor M 8 . The other terminal of the capacitor Cs and a source of the NMOS transistor M 8  are respectively connected to the ground terminals Vss. An inverting input terminal of the operational amplified circuit  15  is connected to a source of the NMOS transistor M 6  and one end of the resistor R 6 , and an output terminal of the operational amplified circuit  15  is connected to a gate of the NMOS transistor M 6 . 
         [0052]    A drain of the NMOS transistor M 6  is connected to the power input terminal Vi. The other end of the resistor R 6  is connected to the other end of the resistor R 5 . A junction node between the resistor R 5  and R 6  is connected to the non-inverting input terminal of the PWM comparator  11 . The NMOS transistor M 7  is connected between the ground terminal Vss and a junction node between the resistors R 5  and R 6 . 
         [0053]    Next, operation of the circuitry shown in  FIG. 1  is described below. 
         [0054]      FIG. 2  is a timing chart of operation of the DC-DC converter  1  shown in  FIG. 1  and illustrates main signal operations in the present embodiment. In  FIG. 2 , reference characters CLK represent the clock signal inputted to the set terminal S of the RS flip-flop circuit  12  and PHS represent a control signal outputted from the output terminal P of the driver circuit  13 . PWMout represents an output signal that is a reset pulse outputted from the PWM comparator  11 , and IL is an electric current flowing through the inductor L 1 . 
         [0055]    A voltage VA is a voltage at the drain of the PMOS transistor M 5 , and a voltage VB is a voltage at the source of the NMOS transistor M 6 . A voltage Vslope is a voltage at the junction node between the resistor R 5  and resistor R 6  and is applied to the non-inverting input terminal of the PWM comparator  11 . 
         [0056]    When the clock signal CLK becomes high, the RS flip-flop circuit  12  is set up, and a high level signal is outputted from its output terminal Q. The signal thus outputted is transmitted to the input terminal I of the driver circuit  13 . Then, in the driver circuit  13 , the control signal PHS outputted from the output terminal P and a control signal NLS outputted from the output terminal N become low. 
         [0057]    When the control signal PHS becomes low, the switching transistor M 1  is turned on, and the electric current IL is supplied to the inductor L 1  from the power input terminal Vin (hereinafter “inductor current IL”). 
         [0058]    The inductor current IL right after the switching transistor M 1  is turned on is identical to the inductor current L 1  just before the switching transistor M 1  is turned on. Therefore, in continuous mode, the inductor current IL is a positive value, as shown in  FIG. 2 . 
         [0059]    In current-mode control, in order to provide a feedback loop corresponding to the inductor current IL in the DC-DC converter  1 , the inductor current IL is converted into a voltage, and the voltage is inputted into the PWM comparator  11 . Then, the PWM comparator  11  compares the voltage thus inputted with the output voltage Vo. Therefore, the voltage is proportional to the inductor current IL. The slope voltage generation circuit  20  is included for generating the voltage. 
         [0060]    The inductor current IL increases over time. An on-resistance value of the switching transistor M 1  is kept almost constant, and therefore, a voltage between the source and the drain of the switching transistor M 1  is proportional to the inductor current IL. 
         [0061]    Because the gate of the PMOS transistor M 3  is connected to the ground terminal Vss, the PMOS transistor M 3  is always on. The PMOS transistor M 4  is controlled to switch on/off in synchronization with the switching transistor M 1 . 
         [0062]    Further, the PMOS transistor M 3  is serially connected to the PMOS transistor M 4 , and the PMOS transistors M 3  and M 4  are connected in parallel to the switching transistor M 1 . Therefore, a voltage Vsense between the source and the drain of the PMOS transistor MS is identical to a voltage that is generated by dividing the voltage on both sides of the switching transistor M 1  by the on-resistances of the PMOS transistors MS and M 4 . In other words, the voltage Vsense is proportional to the inductor current IL. 
         [0063]    The voltage Vsense is applied to the non-inverting input terminal of the operational amplifier circuit  14 . The operational amplifier circuit  14  controls the voltage at the gate of the transistor MS so that the voltage at the source of the transistor MS is identical or similar to the voltage Vsense. 
         [0064]    As a result, when reference character Id 5  represents the current at the drain of the PMOS transistor MS, the drain current Id 5  is proportional to the voltage Vsense and is expressed by a formula Id 5 =Vsense/R 3 . 
         [0065]    When the resistance of the resistor R 4  is considerably smaller than that of the resistor RS, expressed as R 4 &lt;&lt;R 5 , almost the entire drain current Id 5  flows to the resistor R 4 , and the voltage VA at the drain of the PMOS transistor MS is expressed as VA=R 4 ×(Vsense/R 3 ). When the resistance of the resistor R 4  is equal to that of the resistor R 3 , expressed as R 4 =R 3 , the voltage VA is identical to the Vsense, expressed as VA=Vsense. Additionally, the voltage VA works as a standard of the ground voltage. 
         [0066]    Next, a slope compensation circuit is described. In the current-mode control, when a proportion of a period during which the switching transistor M 1  is on in entire operation period exceeds 50%, a sub-harmonic oscillation that makes the operation unstable is generated. Therefore, slope compensation to add another slope voltage to the slope voltage Vslope that is proportional to the inductor current IL is required. 
         [0067]    When the control signal PHS is low, the NMOS transistor M 8  is turned off. Then, the capacitor Cs is charged with a constant current by the electric current source I 1 , and the voltage at the terminal of the capacitor Cs is linearly increased. The operational amplifier circuit  15  controls the voltage at the gate of the NMOS transistor M 6  so that a voltage VB at the source of the NMOS transistor M 6  is identical to the voltage at the terminal of the capacitor Cs. Therefore, the voltage VB is increased from the ground voltage over time, as shown in  FIG. 2 . 
         [0068]    The voltage Vslope is a voltage value between the voltage VA and the voltage VB, and is expressed by formula 1 shown below. 
         [0000]        V slope= VB +( R 6×( VA−VB ))/( R 5+ R 6)   (1) 
         [0069]    When the resistor R 5  and the resistor R 6  have the same value, the relation can be expressed by formula 2 shown below. 
         [0000]        V slope= VB +( VA−VB )/2=( VA+VB )/2   (2) 
         [0070]    In other words, the voltage Vslope is half the sum of the voltage VB and the voltage VA. The voltage Vslope is applied to the non-inverting input terminal of the PWM comparator  11 . 
         [0071]    By contrast, the output voltage Vo of the DC-DC converter  1  is divided by the resistors R 1  and R 2 , and the divided voltage Vfb is applied to the inverting input terminal of the error amplifier circuit  10 . Additionally, the reference voltage Vref is applied to the non-inverting input terminal of the error amplifier circuit  10 , and the error amplifier circuit  10  outputs the error voltage Verr that is the amplified difference between the reference voltage Vref and the divided voltage Vfb. The voltage Verr is applied to the inverting input terminal of the PWM comparator  11 . 
         [0072]    When the slope voltage Vslope is increased and reaches the error voltage Verr, the PWM comparator  11  outputs the high level signal. The signal thus outputted is transmitted to the reset terminal R of the RS flip-flop circuit  12 . Therefore, the RS flip-flop circuit  12  is reset, and the output signal from the output terminal Q becomes low. Then, both the output signals from the output terminals P and N of the driver circuit  13  become high, that is, the control signals PHS and NLS become high. 
         [0073]    If for some reason the output voltage Vo exceeds a predetermined voltage, the error voltage Verr that is outputted from the error amplifier circuit  10  is decreased to the ground voltage. Under this condition, when the clock signal CLK becomes high and the switching transistor M 1  is turned on, causing the slope voltage Vslope to be outputted, the slope voltage Vslope starts from the ground voltage, and therefore, the output signal of the comparator  11  becomes unstable. Thus, the reset signal might be outputted to the RS flip-flop circuit  12  accidently. 
         [0074]    In order to solve this problem, an offset voltage is applied to an input portion of the PWM comparator  11 . In this configuration, when the error voltage Verr and the slope voltage Vslope are ground voltage, the output signal of the PWM comparator  11  becomes high. At this time, even when the clock signal CLK is inputted to the RS flip-flop circuit  12 , the configuration can prevent the S flip-flop circuit  12  from outputting the high level signal from the output terminal Q thereof. 
         [0075]    Herein, when the error voltage Verr exceeds the offset voltage, the output signal of the PWM comparator  11  becomes low, and thus the RS flip-flop circuit  12  can be set up by the clock signal CLK. As a result, even when the output signal of the error amplifier circuit  10  declines to the ground voltage, the DC-DC converter can maintain stable operation. 
         [0076]    When the control signals PHS and NLS become high, the switching transistor M 1  is turned off and the synchronous rectification transistor M 2  is turned on. Then, the inductor current IL that is a current flowing through the inductor L 1  is supplied from the ground terminal Vss via the synchronous rectification transistor M 2  and is decreased over time. 
         [0077]    Further, when the control signal PHS becomes high, the PMOS transistor M 4  is turned off. Then, the electric current does not flow at the drain of the PMOS transistor M 3 , and the voltage Vsense corresponding to the amount of the voltage drop of the PMOS transistor M 3  is 0 V. As a result, the electrical potential at the source of the PMOS transistor M 5  is identical to the input voltage Vi, and the drain current Id 5  of the PMOS transistor M 5  is 0 A. Then, the voltage VA is decreased to the ground voltage. 
         [0078]    When the control signal PHS becomes high, the NMOS transistors M 7  and M 8  are turned on. When the NMOS transistor M 7  is turned on, the non-inverting input terminal of the PWM comparator  11  is short-circuited to the ground terminal Vss. Therefore, the voltage Vslope is rapidly decreased to the ground voltage. As a result, voltage Vslope falling edge lag can be prevented. 
         [0079]    When the NMOS transistor M 8  is turned on, the capacitor Cs is discharged, and accordingly the voltage VB is rapidly decreased to the ground voltage. It is to be noted that, in the present embodiment, the electric current from the electric current source I 1  flows to the NMOS transistor M 8  while the NMOS transistor M 8  is on. However, by including a transistor corresponding to the PMOS transistor M 9  shown in  FIG. 3 , the electric current from the electric current source I 1  can be shut down while the NMOS transistor M 8  is on. 
         [0080]    The above-described operation is repeated when the clock signal CLK becomes high next time. 
         [0081]    As described above, in the present embodiment, after the reset pulse is outputted, the slope voltage Vslope is forcibly decreased to the ground voltage, and fluctuation of the output voltage Vo can be prevented. 
         [0082]    More particularly, when the switching transistor M 1  is turned off, the NMOS transistor M 7  is turned on so that the slope voltage Vslope is forcibly decreased to the ground voltage. Therefore, when the transistor M 1  is turned on next time, the output voltage Vo does not fluctuate because no previously generated slope voltage remains. 
         [0083]    Moreover, the PWM comparator  11  has the input offset voltage, and the DC-DC converter can executes stable operation, even when an overshoot is caused by rapid fluctuations in the load and the output voltage exceeds the predetermined voltage. 
         [0084]    Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.