Abstract:
A DC-DC converting apparatus including a step-up and step-down circuit stepping up/down an input voltage to generate an output voltage and a PWM control circuit. The PWM control circuit generates an error signal, first to third voltages, a first triangular wave signal varying between the first and second voltages, and a second triangular wave signal varying between the third voltage and a fourth voltage determined based on the first to third voltages. The PWM control circuit compares the error signal with the first and second triangular wave signals and causes the step-up and step-down circuit to step up/down the input voltage based on the comparison. The first to fourth voltages V 1  to V 4  satisfy V 1&lt; V 4&lt; V 2&lt; V 3  and V 4= V 3− (V 2− V 1 ). At least one of the first to third voltages is variably set to make a time in which voltage ranges of the first and second triangular wave signals overlap longer than a delay time caused by the comparison.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This patent application is based on and claims priority to Japanese patent application No. 2004-178323 filed on Jun. 16, 2004 in the Japan Patent Office, the entire contents of which are incorporated by reference herein.  
         [0002]     The invention relates to a DC-DC (direct current to direct current) converting method and apparatus, and more particularly to a DC-DC converting method and apparatus which stably performs step-up and step-down conversions by suitably setting a voltage range where a voltage range of a triangular wave signal used for a step-up control overlaps a voltage range of a triangular wave signal used for a step-down control.  
       DISCUSSION OF THE BACKGROUND  
       [0003]     In recent years, small-size mobile equipment, such as a mobile phone has been widely used. Such small-size mobile equipment includes a small-size rechargeable battery as a power source. To downsize batteries and extend their operation time, attempts have been made to improve battery performance and to reduce electric power consumption in small-size mobile equipment. Further, it is desirable to widen a usable voltage range of batteries to reduce the number of batteries and make them usable for a longer time. Therefore, some power supply circuits are provided with a step-up and step-down DC-DC converter capable of supplying a load with a constant voltage even when a voltage provided by the battery exceeds or falls below a voltage level required by the load. The step-up and step-down DC-DC converter is not selective in power supply voltage and thus can adapt to a variety of power input such as a battery and an AC (alternating current) adapter.  
         [0004]      FIG. 1  illustrates a configuration of a background step-up and step-down DC-DC converter  100 . The step-up and step-down DC-DC converter  100  includes an input terminal IN for receiving an input voltage VB, an output terminal OUT for outputting a predetermined output voltage Vout, a PWM (pulse-width modulation) control circuit  101 , and a step-up and -down circuit  102 .  
         [0005]     The step-up and step-down circuit  102  includes an inductor La, a capacitor Ca, and transistors Ma to Md.  
         [0006]     The PWM control circuit  101  includes an error amplifier circuit  111 , a triangular wave generation circuit  112 , a step-down comparator circuit CMPa, a step-up comparator circuit CMPb, a control circuit  113 , and a predriver  114 .  
         [0007]     The error amplifier circuit  111  includes an operational amplifier circuit AMPa, a reference voltage generation circuit  117 , resistors R 110  and R 111 , and a feedback resistor R 112 . The reference voltage generation circuit  117  generates and outputs a predetermined reference voltage Vref. The resistors R 110  and R 111  divide the output voltage Vout and generate a feedback voltage VFB. The operational amplifier circuit AMPa compares the reference voltage Vref with the feedback voltage VFB, and generates and outputs an error signal Sa based on the result of the comparison.  
         [0008]     The triangular wave generation circuit  112  includes a first triangular wave generation circuit  121 , a second triangular wave generation circuit  122 , a constant current source  123 , a battery  124 , and resistors R 101  to R 103 . The first triangular wave generation circuit  121  generates a first triangular wave signal TWa used for performing a step-down control, and the second triangular wave generation circuit  122  generates a second triangular wave signal TWb used for performing a step-up control.  
         [0009]     The first triangular wave generation circuit  121  receives a first voltage Va used for setting a lower limit voltage of the first triangular wave signal TWa, a second voltage Vb used for setting an upper limit voltage of the first triangular wave signal TWa, and current output from the constant current source  123  and used for setting a gradient of a waveform of the first triangular wave signal TWa.  
         [0010]     The second triangular wave generation circuit  122  receives a third voltage Vc used for setting an upper limit voltage of the second triangular wave signal TWb, current output from the constant current source  123  and used for setting a gradient of the second triangular wave TWb, and a clock signal CLKa output from the first triangular wave generation circuit  121  to be used for synchronizing actions of the second triangular wave generation circuit  122 . The currents input from the constant current source  23  to the first and second triangular wave generation circuits  121  and  122  are equal in value.  
         [0011]     As illustrated in a timing diagram of  FIG. 2 , the first triangular wave signal TWa forms a triangular waveform which varies between the first voltage Va and the second voltage Vb, while the second triangular wave signal TWb forms a triangular waveform which varies between the third voltage Vc and the fourth voltage Vd.  
         [0012]     When the first triangular wave signal TWa reaches the first voltage Va (i.e., the lower limit voltage of the first triangular wave TWa), the first triangular wave generation circuit  121  outputs the clock signal CLKa to the second triangular wave generation circuit  122 . Upon input of the clock signal CLKa to the second triangular wave generation circuit  122 , the voltage of the second triangular wave signal TWb which has been decreasing starts to increase.  
         [0013]     The gradients of the first and second triangular wave signals TWa and TWb are determined by the value of the current output from the constant current source  123 . Therefore, the first and second triangular wave signals TWa and TWb have equal amplitudes. A fourth voltage Vd, which is a lower limit voltage of the second triangular wave signal TWb, is a voltage obtained by subtracting a voltage difference between the second and first voltages Vb and Va from the third voltage Vc.  
         [0014]     The fourth voltage Vd should be lower than the second voltage Vb to smooth the switching between the step-up operation and the step-down operation performed in the step-up and step-down DC-DC converter  100 . In other words, a voltage range of the first triangular wave signal TWa used for the step-down control should partly overlap a voltage range of the second triangular wave signal TWb used for the step-up control.  
         [0015]     In a recent attempt to further reduce mobile equipment size and power consumption, a PWM control frequency of a step-up and step-down DC-DC converter is increased. If the PWM control frequency is increased, the inductor La and the capacitor Ca which occupy space of the step-up and step-down DC-DC converter may be downsized, and power efficiency may be improved. The increase of the PWM control frequency is, therefore, effective in reducing electric power consumption.  
         [0016]     If the PWM control frequency is increased, however, time periods Ta and Tb shown in  FIG. 2 , in which the voltage range of the first triangular wave signal TWa overlaps the voltage range of the second triangular wave signal TWb, are reduced.  
         [0017]     As illustrated in  FIG. 3 , both the step-down comparator circuit CMPa and the step-up comparator circuit CMPb have two types of delay time periods, i.e., first delay time periods TDa and TDb and second delay time periods Tr and Tf. In the first delay time periods TDa and TDb, two input voltages input into the comparator circuit, reverse in voltage levels, and affect the signal output from the comparator circuit. In the second delay time period Tr, which is a rise time of an output signal output from the comparator circuit, the output signal output from the comparator circuit increases from a relatively low level (LOW) and reaches a relatively high level (HIGH). Meanwhile, in the second delay time period Tf, which is a fall time of the output signal output from the comparator circuit, the output signal output from the comparator circuit decreases from the HIGH level and reaches the LOW level.  
         [0018]     If each of the time periods Ta and Tb (i.e., the time periods in which the voltage range of the first triangular wave signal TWa overlaps the voltage range of the second triangular wave signal TWb) is shorter than a sum of TDa, TDb, Tr, and Tf (i.e., TDa+TDb+Tr+Tf, which is hereinafter referred to as a total delay time period), an effective output pulse is not output during each of the time periods Ta and Tb from an output terminal of the comparator circuit, and thus the step-up operation and the step-down operation are prevented.  
         [0019]     There is a background method of increasing currents consumed by the comparator circuit to increase the operational speed and reduce the total delay time period of the comparator circuit. This method, however, contradicts the attempt to reduce electric power consumption by increasing the PWM control frequency. That is, if the electric power consumption by the comparator circuit is increased, reduction in electric power consumption may not be attained.  
       SUMMARY  
       [0020]     The invention provides a DC-DC converting apparatus. In one example, a DC-DC converting apparatus includes a step-up and step-down circuit and a pulse-width modulation control circuit. The step-up and step-down circuit is configured to step-up and step-down an input voltage to generate and output a predetermined output voltage. The pulse-width modulation control circuit is configured to generate an error signal based on the predetermined output voltage and a predetermined reference voltage, first to third voltages, a first triangular wave signal varying between the first and second voltages, and a second triangular wave signal varying between the third voltage and a fourth voltage determined based on the first to third voltages. The pulse-width modulation control circuit is further configured to perform a comparison of the error signal with the first and second triangular wave signals, and to cause the step-up and step-down circuit to step-up and step-down the input voltage based on a result of the comparison. The first to fourth voltages satisfy V 1 &lt;V 4 &lt;V 2 &lt;V 3  and V 4 =V 3 −(V 2 −V 1 ), wherein V 1  is the first voltage, V 2  is the second voltage, V 3  is the third voltage, and V 4  is the fourth voltage. Further, at least one of the first to third voltages is variably set such that a time period in which the voltage ranges of the first and second triangular wave signals overlap, is longer than a delay time period caused by the comparison.  
         [0021]     The invention further provides another DC-DC converting apparatus. In one example, this DC-DC converting apparatus includes a step-up and step-down circuit and a pulse-width modulation control circuit. The step-up and step-down circuit is configured to step-up and step-down an input voltage according to a control signal input to generate and output a predetermined output voltage. The pulse-width modulation control circuit is configured to generate an error signal indicating an error in the feedback voltage proportional to the predetermined output voltage and a predetermined reference voltage, first to third voltages, a first triangular wave signal used for stepping down the input voltage, and a second triangular wave signal used for stepping up the input voltage. The pulse-width modulation control circuit is further configured to compare the error signal with the first and second triangular wave signals, and to output the control signal to the step-up and step-down circuit based on the result of the comparison. Further, the pulse-width modulation control circuit includes a triangular wave generation circuit and a comparator circuit. The triangular wave generation circuit is configured to set the first to third voltages and a fourth voltage so as to satisfy V 1 &lt;V 4 &lt;V 2 &lt;V 3  and V 4 =V 3 −(V 2 −V 1 ), and to generate the first and second triangular wave signals, wherein V 1  is the first voltage setting a lower limit voltage of the first triangular wave signal, V 2  is the second voltage setting an upper limit voltage of the first triangular wave signal, V 3  is the third voltage setting an upper limit voltage of the second triangular wave signal, and V 4  is the fourth voltage setting a lower limit voltage of the second triangular wave signal. The comparator circuit is configured to compare the error signal with the first and second triangular wave signals. Further, at least one of the first to third voltages is variably set such that a time period in which the voltage ranges of the first and second triangular wave signals overlap is longer than a delay time period of the comparator circuit.  
         [0022]     In the DC-DC converting apparatus, the triangular wave generation circuit may include a constant voltage generation circuit, a constant current source, a first triangular wave generation circuit, and a second triangular wave generation circuit. The constant voltage generation circuit may be configured to generate and output the first to third voltages. The constant current source may be configured to generate and output a predetermined constant current which is variably set to the desired constant voltage and used for setting respective gradients of the first and second triangular wave signals. The first triangular wave generation circuit may be configured to receive the first and second voltages and the predetermined constant current and to generate and output the first triangular wave signal. The second triangular wave generation circuit may be configured to receive the third voltage and the predetermined constant current and to generate and output the second triangular wave signal. In the DC-DC converting apparatus, at least one of the first to third voltages may be variably set.  
         [0023]     In the DC-DC converting apparatus, the constant voltage generation circuit may include a first constant voltage source configured to generate and output the second voltage which is variably set to the desired constant voltage, a second constant voltage source configured to generate and output the third voltage, and a voltage dividing circuit configured to divide the second voltage in order to generate and output the first voltage.  
         [0024]     In the DC-DC converting apparatus, the constant voltage generation circuit may include a first constant voltage source configured to generate and output the second voltage which is variably set to the desired constant voltage, a second constant voltage source configured to generate and output the third voltage, and a voltage dividing circuit configured to divide the third voltage to generate and output the first voltage.  
         [0025]     In the DC-DC converting apparatus, the constant voltage generation circuit may include a first constant voltage source configured to generate and output the second voltage which is variably set to the desired constant voltage, a second constant voltage source configured to generate and output the third voltage, and a third constant voltage source configured to generate and output the first voltage.  
         [0026]     In the DC-DC converting apparatus, the predetermined constant current output from the constant current source may be variably set such that frequencies of the first and second triangular wave signals are kept constant at predetermined values.  
         [0027]     The invention further provides a DC-DC converting method for stepping up and stepping down an input voltage to generate and output a predetermined output voltage. In one example, a DC-DC converting method for stepping up and stepping down an input voltage to generate and output a predetermined output voltage includes: generating an error signal based on the predetermined output voltage and a predetermined reference voltage; generating first to third voltages; generating a first triangular wave signal varying between the first and second voltages and a second triangular wave signal varying between the third voltage and a fourth voltage based on the first to third voltages; setting the first to fourth voltages so as to satisfy V 1 &lt;V 4 &lt;V 2 &lt;V 3  and V 4 =V 3 −(V 2 −V 1 ), wherein V 1  is the first voltage, V 2  is the second voltage, V 3  is the third voltage, and V 4  is the fourth voltage; comparing the error signal with the first and second triangular wave signals; and stepping up and stepping down the input voltage based on a result of the comparison. In this method, at least one of the first to third voltages V 1  to V 3  is variably set such that a time period in which voltage ranges of the first and second triangular wave signals overlap with each other is longer than a delay time period caused by the comparison.  
         [0028]     The invention also provides another DC-DC converting method for stepping up and stepping down an input voltage to generate and output a predetermined output voltage. In one example, this DC-DC converting method for stepping up and stepping down an input voltage to generate and output a predetermined output voltage includes: providing a step-up and step-down circuit and a pulse-width modulation control circuit; providing a triangular wave generation circuit and a comparator circuit in the pulse-width modulation control circuit; causing the pulse-width modulation control circuit to generate an error signal indicating an error in the feedback voltage which is proportional to the predetermined output voltage and a predetermined reference voltage; causing the triangular wave generation circuit to set first to fourth voltages so as to satisfy V 1 &lt;V 4 &lt;V 2 &lt;V 3  and V 4 =V 3 −(V 2 −V 1 ), and to generate a first triangular wave signal used for stepping down the input voltage and a second triangular wave signal used for stepping up the input voltage, wherein V 1  is the first voltage setting a lower limit voltage of the first triangular wave signal, V 2  is the second voltage setting an upper limit voltage of the first triangular wave signal, V 3  is the third voltage setting an upper limit voltage of the second triangular wave signal, and V 4  is the fourth voltage setting a lower limit voltage of the second triangular wave signal; causing the comparator circuit to compare the error signal with the first and second triangular wave signals; and causing the step-up and step-down circuit to step-up and step-down the input voltage based on a result of the comparison. In this method, at least one of the first to third voltages is variably set such that a time period in which the voltage ranges of the first and second triangular wave signals overlap is longer than a delay time period of the comparator circuit.  
         [0029]     The DC-DC converting method may further include, in the triangular wave generation circuit, a constant voltage generation circuit configured to generate and output the first to third voltages, a constant current source configured to generate and output a predetermined constant current which is variably set and used for setting the respective gradients of the first and second triangular wave signals, a first triangular wave generation circuit configured to receive the first and second voltages and the predetermined constant current and to generate and output the first triangular wave signal, and a second triangular wave generation circuit configured to receive the third voltage and the predetermined constant current and to generate and output the second triangular wave signal, and to variably set at least one of the first to third voltages.  
         [0030]     The DC-DC converting method may further include, in the constant voltage generation circuit, a first constant voltage source configured to generate and output the second voltage which is variably set to the desired constant voltage, a second constant voltage source configured to generate and output the third voltage, and a voltage dividing circuit configured to divide the second voltage to generate and output the first voltage.  
         [0031]     The DC-DC converting method may further include, in the constant voltage generation circuit, a first constant voltage source configured to generate and output the second voltage which is variably set to the desired constant voltage, a second constant voltage source configured to generate and output the third voltage, and a voltage dividing circuit configured to divide the third voltage to generate and output the first voltage.  
         [0032]     The DC-DC converting method may further include, in the constant voltage generation circuit, a first constant voltage source configured to generate and output the second voltage which is variably set, a second constant voltage source configured to generate and output the third voltage, and a third constant voltage source configured to generate and output the first voltage.  
         [0033]     The DC-DC converting method may further include variably setting the predetermined constant current output from the constant current source such that frequencies of the first and second triangular wave signals are kept constant at predetermined values.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]     A more complete appreciation of the disclosure and many of the advantages thereof are readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0035]      FIG. 1  is a circuit diagram illustrating a configuration of a background step-up and step-down DC-DC converter;  
         [0036]      FIG. 2  is a timing diagram illustrating waveforms of first and second triangular wave signals generated in the step-up and step-down DC-DC converter of  FIG. 1 ;  
         [0037]      FIG. 3  is a timing diagram illustrating delay time periods of a step-down comparator circuit and a step-up comparator circuit used in the step-up and step-down DC-DC converter of  FIG. 1 ;  
         [0038]      FIG. 4  is a circuit diagram illustrating a configuration of a step-up and step-down DC-DC converter according to an exemplary embodiment of the invention;  
         [0039]      FIG. 5  is a timing diagram illustrating relationships between first and second triangular wave signals and first to fourth voltages generated in the step-up and step-down DC-DC converter of  FIG. 4  according to an exemplary embodiment of the invention;  
         [0040]      FIG. 6  provides timing diagrams illustrating changes of the first and second triangular wave signals generated in the step-up and step-down DC-DC converter of  FIG. 4  according to an exemplary embodiment of the invention;  
         [0041]      FIG. 7  is a circuit diagram illustrating a configuration of a step-up and step-down DC-DC converter according to an exemplary embodiment of the invention;  
         [0042]      FIG. 8  is a circuit diagram illustrating a configuration of a step-up and step-down DC-DC converter according to another exemplary embodiment of the invention;  
         [0043]      FIG. 9  provides timing diagrams illustrating changes of the first and second triangular wave signals generated in the step-up and step-down DC-DC converter of  FIG. 8  according to another exemplary embodiment of the invention; and  
         [0044]      FIG. 10  is a circuit diagram illustrating a configuration of a step-up and step-down DC-DC converter according to an exemplary embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0045]     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the purpose of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so used and it is to be understood that substitutions for each specific element can include any technical equivalents that operate in a similar manner.  
         [0046]     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, more particularly to  FIG. 4 , which illustrates a configuration of a step-up and step-down DC-DC converter  200  according to an embodiment of the invention.  
         [0047]     The step-up and step-down DC-DC converter  200  of  FIG. 4  includes an input terminal IN, an output terminal OUT, a PWM control circuit  2 , and a step-up and step-down circuit  3 . The step-up and step-down DC-DC converter  200  receives an input voltage VB input from the input terminal IN, converts the input voltage VB to a predetermined constant voltage, and outputs the predetermined constant voltage from the output terminal OUT as an output voltage Vout.  
         [0048]     The PWM control circuit  2  includes a triangular wave generation circuit  11 , an error amplifier circuit  12 , a step-down comparator circuit CMP 1 , a step-up comparator circuit CMP 2 , a control circuit  13 , and a predriver  14 .  
         [0049]     The triangular wave generation circuit  11  includes a first triangular wave generation circuit  21 , a second triangular wave generation circuit  22 , a first constant voltage source  23 , a second constant voltage source  24 , a constant current source  25 , and resistors R 1  and R 2 . The first triangular wave generation circuit  21  generates a first triangular wave signal TW 1  used for performing a step-down control, and the second triangular wave generation circuit  22  generates a second triangular wave signal TW 2  used for performing a step-up control. The first constant voltage source  23  generates and outputs a second voltage V 2  which is variably set to the desired constant voltage. The second constant voltage source  24  generates and outputs a predetermined third voltage V 3 . The constant current source  25  generates and outputs constant current which is variably set to the desired constant voltage. The resistors R 1  and R 2  divide the second voltage V 2  to generate a first voltage V 1 . The step-down comparator circuit CMP 1  and a step-up comparator circuit CMP 2  form a comparator circuit. The first constant voltage source  23 , the second constant voltage source  24 , and the resistors R 1  and R 2  form a constant voltage generation circuit. The resistors R 1  and R 2  form a voltage dividing circuit.  
         [0050]     The resistors R 1  and R 2  are connected in series between the ground (GND) and the first constant voltage source  23  which outputs the second voltage V 2 . The first triangular wave generation circuit  21  receives the second voltage V 2  used for setting an upper limit voltage of the first triangular wave signal TW 1 , and the first voltage V 1  used for setting a lower limit voltage of the first triangular wave signal TW 1 . The second triangular wave generation circuit  22  receives the third voltage V 3  used for setting an upper limit voltage of the second triangular wave signal TW 2 , and a clock signal CLK 1  output from the first triangular wave generation circuit  21  and used for synchronizing actions of the second triangular wave generation circuit  22 . The first triangular wave generation circuit  21  and the second triangular wave generation circuit  22  receive constant current output from the constant current source  25  which are used for setting respective gradients of the first and second triangular wave signals TW 1  and TW 2 . The first triangular wave signal TW 1  output from the first triangular wave generation circuit  21  is input in a non-inverting input terminal of the step-down comparator circuit CMP 1 , while the second triangular wave signal TW 2  output from the second triangular wave generation circuit  22  is input in a non-inverting input terminal of the step-up comparator circuit CMP 2 .  
         [0051]     The error amplifier circuit  12  includes an operational amplifier circuit AMP 1 , a reference voltage generation circuit  31 , resistors R 10  and R 11 , and a feedback resistor R 12 . The reference voltage generation circuit  31  generates and outputs a predetermined reference voltage Vref. The resistors R 10  and R 11  divide the output voltage Vout and generate a feedback voltage VFB. The resistors R 10  and R 11  are connected in series between the output terminal OUT and the ground GND. An inverting input terminal of the operational amplifier circuit AMP 1  is connected to a connection point between the resistors R 10  and R 11 , while a non-inverting input terminal of the operational amplifier circuit AMP 1  receives input of the reference voltage Vref. The feedback resistor R 12  is connected between an output terminal of the operational amplifier circuit AMP 1  and the inverting input terminal of the operational amplifier circuit AMP 1 . The output terminal of the operational amplifier circuit AMP 1  is connected to the inverting input terminal of the step-down comparator circuit CMP 1  and the inverting input terminal of the step-up comparator circuit CMP 2 . The operational amplifier circuit AMP 1  compares the reference voltage Vref with the feedback voltage VFB, and generates and outputs an error signal S 1  based on a result of the comparison.  
         [0052]     The step-down comparator circuit CMP 1  compares a voltage of the first triangular wave signal TW 1  with a voltage of the error signal S 1  and outputs a step-down mode switching signal S 2 , which is a binary signal indicating a result of the comparison, to the control circuit  13 .  
         [0053]     The step-up comparator circuit CMP 2  compares a voltage of the second triangular wave signal TW 2  with the voltage of the error signal S 1  and outputs a step-up mode switching signal S 3 , which is a binary signal indicating a result of the comparison, to the control circuit  13 .  
         [0054]     The control circuit  13  outputs a step-up and step-down control signal S 4  to the predriver  14  according to the step-down mode switching signal S 2  and the step-up mode switching signal S 3  input therein.  
         [0055]     The predriver  14  drives switching elements M 1  to M 4  of the step-up and step-down circuit  3  according to the step-up and step-down control signal S 4  input in the predriver  14  from the control circuit  13 .  
         [0056]     The step-up and step-down circuit  3  includes the switching elements M 1  to M 4 , an inductor L 1 , and a capacitor C 1 . The switching elements M 1  and M 2  are NMOS (N-channel metal oxide semiconductor) transistors which perform a step-down control to the output voltage Vout. Meanwhile, the switching elements M 3  and M 4  are NMOS transistors which perform a step-up control to the output voltage Vout. The step-up and step-down circuit  3  performs a step-up operation and a step-down operation of the output voltage Vout according to switching signals S 11  to S 14  output from the predriver  14  of the PWM control circuit  2 .  
         [0057]     The switching element M 1  and M 2  are connected in series between the input terminal IN and the ground GND, while the switching elements M 3  and M 4  are connected in series between the output terminal OUT and the ground GND. The inductor L 1  is connected between a connection point of the switching elements M 1  and M 2  and a connection point of the switching elements M 3  and M 4 . The capacitor C 1  is connected between the output terminal OUT and the ground GND. The switching signals S 11  to S 14  output from the predriver  14  are input in corresponding gates of the switching elements M 1  to M 4 .  
         [0058]     Operations of the step-up and step-down DC-DC converter  200  shown in  FIG. 4  is described with reference to  FIG. 5 , which is a timing diagram illustrating the relationships between the first triangular wave signal TW 1 , the second triangular wave signal TW 2 , and the first to fourth voltages V 1  to V 4 .  
         [0059]     As illustrated in  FIG. 5 , the first triangular wave signal TW 1  forms a waveform which varies between the first voltage V 1  and the second voltage V 2 , while the second triangular wave signal TW 2  forms a waveform which varies between the third voltage V 3  and the fourth voltage V 4 .  
         [0060]     The fourth voltage V 4  shown in  FIG. 5  is a lower limit voltage of the second triangular wave TW 2 . When the first triangular wave signal TW 1  reaches the first voltage V 1  (i.e., the lower limit voltage of the first triangular wave signal TW 1 ), the first triangular wave generation circuit  21  outputs the clock signal CLK 1  to the second triangular wave generation circuit  22 . Upon input of the clock signal CLK 1  to the second triangular wave generation circuit  22 , the voltage of the second triangular wave signal TW 2 , which has been decreasing, starts to increase.  
         [0061]     The gradients of the first and second triangular wave signals TW 1  and TW 2  are determined by a value of the constant current output from the constant current source  25 . Therefore, the first and second triangular wave signals TW 1  and TW 2  have equal amplitudes. The fourth voltage V 4  (i.e., the lower limit voltage of the second triangular wave signal TW 2 ) is a voltage obtained by subtracting a voltage difference between the second and first voltages V 2  and V 1  from the third voltage V 3 . That is, the fourth voltage V 4  is expressed as V 4 =V 3 −(V 2 −V 1 ). The fourth voltage V 4  should be lower than the second voltage V 2  to smooth the switching between the step-up operation and the step-down operation performed in the step-up and step-down DC-DC converter  200 . In other words, a voltage range of the first triangular wave signal TW 1  used for the step-down control should partly overlap a voltage range of the second triangular wave signal TW 2  used for the step-up control.  
         [0062]     When the input voltage VB is lower than the output voltage Vout, a voltage of the error signal S 1  output from the operational amplifier AMP 1  falls between the second voltage V 2  and the third voltage V 3 . Accordingly, the step-up comparator circuit CMP 2  outputs the step-up mode switching signal S 3  to the control circuit  13 , and the step-up operation is performed to control the output voltage Vout to a predetermined level. When the output voltage Vout decreases, the voltage of the error signal S 1  increases, and the step-up operation is performed to control the output voltage Vout to increase up to a predetermined level.  
         [0063]     When the input voltage VB is higher than the output voltage Vout, on the other hand, the voltage of the error signal S 1  falls between the first voltage V 1  and the fourth voltage V 4 . Accordingly, the step-down comparator circuit CMP 1  outputs the step-down mode switching signal S 2  to the control circuit  13 , and the step-down operation is performed to control the output voltage Vout to a predetermined level. When the output voltage Vout decreases, the voltage of the error signal S 1  decreases, and the step-down operation is performed to control the output voltage Vout to increase up to a predetermined level.  
         [0064]     When the input voltage VB and the output voltage Vout are at an approximately equal level, the voltage of the error signal S 1  falls between the fourth voltage V 4  and the second voltage V 2 . Accordingly, the step-down comparator circuit CMP 1  outputs the step-down mode switching signal S 2  to the control circuit  13 , and the step-up comparator circuit CMP 2  outputs the step-up mode switching signal S 3  to the control circuit  13 . As a result, the step-up operation and the step-down operation are performed, respectively, to control the output voltage Vout to be at a predetermined level.  
         [0065]     FIGS.  6  ( a ) and  6  ( b ) illustrate changes of the first triangular wave signal TW 1  and the second triangular wave signal TW 2  generated in the step-up and step-down DC-DC converter  200  of  FIG. 4 .  FIG. 6  ( a ) illustrates an example of an initial state in which the second voltage V 2  (i.e., the output voltage output from the first constant voltage source  23 ) is set at 0.8 volts, and the first voltage V 1  is set at 0.2 volts, for example. Further, the third voltage V 3  (i.e., the output voltage output from the second constant voltage source  24 ) is set at 1.2 volts, for example. In this case, the fourth voltage V 4  becomes 0.6 volts according to the above equation V 4 =V 3 −(V 2 −V 1 ), and a voltage range in which the voltage range of the first triangular wave signal TW 1  overlaps the voltage range of the second triangular wave signal TW 2  is 0.2 volts (i.e., 0.8−0.6=0.2).  
         [0066]      FIG. 6  ( b ) illustrates a state in which the second voltage V 2  is increased from 0.8 volts to 0.85 volts. In this case, the first voltage V 1  increases to 0.21 volts, and the third voltage V 3  stays unchanged at 1.2 volts. The fourth voltage V 4  becomes 0.56 volts according to the above equation V 4 =V 3 −(V 2 −V 1 ). Accordingly, the voltage range in which the voltage range of the first triangular wave signal TW 1  overlaps the voltage range of the second triangular wave signal TW 2  is 0.29 volts (i.e., 0.85−0.56=0.29), which is 0.09 volts larger than 0.2 volts of the initial state.  
         [0067]     As observed from FIGS.  6  ( a ) and  6  ( b ), time periods T 1  and T 2 , in which the voltage range of the first triangular wave signal TW 1  overlaps with the voltage range of the second triangular wave signal TW 2  are respectively longer in  FIG. 6  ( b ) than in  FIG. 6  ( a ). If an output voltage output from the first constant voltage source  23  is increased, the PWM control frequency slightly decreases but can be restored by adjusting the value of the constant current output from the constant current source  25 .  
         [0068]      FIG. 7  illustrates a configuration of a step-up and step-down DC-DC converter  300  according to another exemplary embodiment of the invention. A detailed description is omitted for the components shown in  FIG. 7 , which were described with reference to  FIG. 4 . However, the differences between the step-up and step-down DC-DC converter  200  of  FIG. 4  and the step-up and step-down DC-DC converter  300  of  FIG. 7  are described. The step-up and step-down DC-DC converter  300  is different from the step-up and step-down DC-DC converter  200  in that the resistors R 1  and R 2  are replaced by a third constant voltage source  27  in the step-up and step-down DC-DC converter  300 . In this case, the third constant voltage source  27  generates and outputs the first voltage V 1  which is variably set to the desired voltage. The first constant voltage source  23 , the second constant voltage source  24 , and the third constant voltage source  27  form a constant voltage generation circuit. With this configuration, the step-up and step-down DC-DC converter  300  can provide similar operation of the step-up and step-down DC-DC converter  200 .  
         [0069]     As described above, in the step-up and step-down DC-DC converters  200  and  300 , the value of the second voltage V 2  output from the first constant voltage source  23  is set such that the time periods T 1  and T 2 , in which the voltage range of the first triangular wave signal TW 1  overlaps the voltage range of the second triangular wave signal TW 2 , are longer than the delay time periods of the step-down comparator circuit CMP 1  and the step-up comparator circuit CMP 2 . Accordingly, the step-up operation and the step-down operation of the output voltage Vout can be performed even during the time periods T 1  and T 2  in which the voltage range of the first triangular wave signal TW 1  overlaps the voltage range of the second triangular wave signal TW 2 . As a result, the output voltage Vout can be stabilized.  
         [0070]     A constant voltage source outputting a constant voltage which is variably set to the desired constant voltage may be used as the second constant voltage source  24  in the step-up and step-down DC-DC converters  200  and  300 . With this configuration, the third voltage V 3  may be decreased to extend the time periods T 1  and T 2 , in which the voltage range of the first triangular wave signal TW 1  overlaps the voltage range of the second triangular wave signal TW 2 . In this case, however, the highest voltage within a voltage range of the error signal S 1  is decreased, and thus this method of decreasing the third voltage V 3  to increase the time periods T 1  and T 2  is limited to when the error signal S 1  has a relatively sufficient voltage range.  
         [0071]     The first voltage V 1  is generated by dividing the second voltage V 2  in the step-up and step-down DC-DC converters  200 . Alternatively, the first voltage V 1  may be generated by dividing the third voltage V 3 , as in a step-up and step-down DC-DC converters  400  according to another embodiment.  
         [0072]      FIG. 8  illustrates a configuration of the step-up and step-down DC-DC converters  400 . A detailed description is omitted for the components shown in  FIG. 8 , which were described with reference to  FIG. 4 . However, the differences between the step-up and step-down DC-DC converters  200  of  FIG. 4  and the step-up and step-down DC-DC converter  400  of  FIG. 8  are described. The step-up and step-down DC-DC converter  400  is different from the step-up and step-down DC-DC converters  200  in that the resistors R 1  and R 2  of the step-up and step-down DC-DC converter  200  are replaced by resistors R 3  and R 4  which divide the third voltage V 3  to generate the first voltage V 1 .  
         [0073]     The step-up and step-down DC-DC converter  400  includes the input terminal IN, the output terminal OUT, a PWM control circuit  2   a , and the step-up and the step-down circuit  3 .  
         [0074]     The PWM control circuit  2   a  includes a triangular wave generation circuit  11   a , the error amplifier circuit  12 , the step-down comparator circuit CMP 1 , the step-up comparator circuit CMP 2 , the control circuit  13 , and the predriver  14 .  
         [0075]     The triangular wave generation circuit  11   a  includes the first triangular wave generation circuit  21 , the second triangular wave generation circuit  22 , the first constant voltage source  23 , the second constant voltage source  24 , the constant current source  25 , and the resistors R 3  and R 4 . The first constant voltage source  23 , the second constant voltage source  24 , and the resistors R 3  and R 4  form a constant voltage generation circuit. The resistors R 3  and R 4  form a voltage dividing circuit.  
         [0076]     The resistors R 3  and R 4  are connected in series between the ground GND and the second constant voltage source  24  which outputs the third voltage V 3 . The first triangular wave generation circuit  21  receives the second voltage V 2  used for setting the upper limit voltage of the first triangular wave signal TW 1 , and the first voltage V 1  which is generated by dividing the third voltage V 3  with the resistors R 3  and R 3  and which is used for setting the lower limit voltage of the first triangular wave signal TW 1 .  
         [0077]     FIGS.  9  ( a ) and  9  ( b ) illustrate changes of the first triangular wave signal TW 1  and the second triangular wave signal TW 2  generated in the step-up and step-down DC-DC converter  400  of  FIG. 8 .  FIG. 9  ( a ) illustrates an example of an initial state in which the third voltage V 3  (i.e., the output voltage output from the second constant voltage source  24 ) is set at 1.2 volts, and the first voltage V 1  is set at 0.2 volts, for example. Further, the second voltage V 2  (i.e., the output voltage output from the first constant voltage source  23 ) is set at 0.8 volts, for example. In this case, the fourth voltage V 4  becomes 0.6 volts according to the equation V 4 =V 3 −(V 2 −V 1 ), and a voltage range in which the voltage range of the first triangular wave signal TW 1  overlaps the voltage range of the second triangular wave signal TW 2  is 0.2 volts (i.e., 0.8−0.6=0.2).  
         [0078]      FIG. 9  ( b ) illustrates a state in which the second voltage V 2  is increased from 0.8 volts to 0.85 volts. In this case, the first voltage V 1  and the third voltage V 3  stay unchanged at 0.2 volts and 1.2 volts, respectively. The fourth voltage V 4  becomes 0.55 volts according to the above equation V 4 =V 3 −(V 2 −V 1 ). Accordingly, the voltage range in which the voltage range of the first triangular wave signal TW 1  overlaps the voltage range of the second triangular wave signal TW 2  is 0.3 volts (i.e., 0.85−0.55=0.3), which is 0.1 volts larger than the 0.2 volts of the initial state.  
         [0079]     As observed from FIGS.  9  ( a ) and  9  ( b ), the time periods T 1  and T 2 , in which the voltage range of the first triangular wave signal TW 1  overlaps the voltage range of the second triangular wave signal TW 2  are respectively longer in  FIG. 9  ( b ) than in  FIG. 9  ( a ). If the output voltage output from the first constant voltage source  23  is increased, the PWM control frequency decreases, as in the case of the step-up and step-down DC-DC converter  200  shown in  FIG. 4 . Therefore, variation in the PWM control frequency is cancelled out by increasing the value of the constant current output from the constant current source  25 .  
         [0080]      FIG. 10  illustrates a configuration of a step-up and step-down DC-DC converter  500  according to another embodiment of the invention. A detailed description is omitted for the components shown in  FIG. 10  which were described with reference to  FIGS. 4 and 8 . However, the differences between the step-up and step-down DC-DC converter  400  of  FIG. 8  and the step-up and step-down DC-DC converter  500  of  FIG. 10  are described. The step-up and step-down DC-DC converter  500  is different from the a step-up and step-down DC-DC converter  400  in that the resistors R 3  and R 4  are replaced by a third constant voltage source  28  in the step-up and step-down DC-DC converter  500 . The third constant voltage source  28  generates and outputs the first voltage V 1 . The first constant voltage source  23 , the second constant voltage source  24 , and the third constant voltage source  28  form a constant voltage generation circuit. With this configuration, the step-up and step-down DC-DC converter  500  can provide similar operations of the step-up and step-down DC-DC converter  400 .  
         [0081]     As described above, in the step-up and step-down DC-DC converter  400 , the first voltage V 1  is generated by dividing the third voltage V 3 . Accordingly, the step-up and step-down DC-DC converter  400  can provide a similar effect to that of the step-up and step-down DC-DC converter  200 .  
         [0082]     Further, a constant voltage source outputting a constant voltage which is variably set to the desired constant voltage may be used as the second constant voltage source  24  in the step-up and step-down DC-DC converters  400  and  500 . With this configuration, the third voltage V 3  may be decreased to increase the time periods T 1  and T 2 , in which the voltage range of the first triangular wave signal TW 1  overlaps the voltage range of the second triangular wave signal TW 2 . In this case, however, the highest voltage within a voltage range of the error signal S 1  is decreased, and thus this method of decreasing the third voltage V 3  to increase the time periods T 1  and T 2  is limited to when the error signal S 1  has a relatively sufficient voltage range.  
         [0083]     Two constant voltage sources are used in each of the step-up and step-down DC-DC converters  200  and  400 . Alternatively, the third voltage V 3  may be generated from the first voltage V 1  by using a constant voltage source outputting a constant voltage which is variably set to the desired constant voltage and at least three resistors that divide the output voltage output from the constant voltage source. Further, the step-up and step-down DC-DC converter may be provided with a constant voltage source which generates the first voltage V 1 , constant voltage sources which respectively generate the second voltage V 2  and the third voltage V 3  which are variably set to the desired constant voltage, and series-connected resistors. If more than one constant voltage sources are provided in the step-up and step-down DC-DC converter, the first to third voltages V 1  to V 3  change in values, depending on which constant voltage source is configured to output a constant voltage which is variably set to the desired constant voltage. Circuits used in the step-up and step-down DC-DC converter should be appropriately chosen according to purposes.  
         [0084]     The above-described embodiments are illustrative, and numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative and exemplary embodiments herein may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 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.