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 . referring now to the drawings , wherein like reference numerals designate identical or corresponding parts throughout the several views , and more particularly to fig3 thereof , a power supply apparatus 1 according to a preferred embodiment is described . as illustrated in fig3 the power supply apparatus 1 includes a dc - to - dc ( direct current to direct current ) converter 2 , and a voltage regulator 3 . the dc - to - dc converter 2 reduces a power source voltage vdd to a predetermined voltage va and has an output terminal outputting the voltage va . the power source voltage vdd is supplied from a direct current power source 10 that can be composed of various kinds of batteries including secondary batteries . the voltage regulator 3 reduces the predetermined voltage va output from the dc - to - dc converter 2 to a predetermined voltage vb and has an output terminal outputting the voltage vb . as shown in fig3 in the power supply apparatus 1 , the dc - to - dc converter 2 is connected between the power supply line from the current power source 10 and the grounding . the voltage regulator 3 is connected between the output terminal of the dc - to - dc converter 2 and the grounding . the output terminal of the voltage regulator 3 is connected to a power supply terminal of a cpu ( central processing unit ) 11 . the cpu 11 is shown as an exemplary device requiring a power supply . other devices such as a dsp ( digital signal processor ), memories , and so on which form , together with the cpu 11 , a system apparatus also require a power supply . the voltage regulator 3 includes a p - channel - type mos ( metal oxide semiconductor ) transistor 21 ( hereinafter referred to as a p - mos transistor 21 ), resisters 22 and 23 , a reference voltage generator 24 , and a voltage comparator 25 . the p - mos transistor 21 and the resisters 22 and 23 are connected in series between the output terminal of the dc - to - dc converter 2 and the grounding , and the voltage regulator 3 has an output terminal drawn from a line connecting the p - mos transistor 21 to the resister 22 . the voltage comparator 25 has an input terminal connected to a line placed between the resisters 22 and 23 and another input terminal to receive a reference voltage vref output from the reference voltage generator 24 . the voltage comparator 25 has an output terminal connected to a gate of the p - mos transistor 21 . the resisters 22 and 23 divide the voltage vb , and the voltage comparator 25 compares the voltage divided by the resisters 22 and 23 to the reference voltage vref output from the reference voltage generator 24 . when the divided voltage is equal to or greater than the reference voltage vref , the voltage comparator 25 controls the operation of the p - mos transistor 21 so that a current flowing the p - mos transistor 21 is reduced . on the other hands , when the divided voltage is smaller than the reference voltage vref , the voltage comparator 25 controls the p - mos transistor 21 to increase the flowing current . the cpu 11 has a sleep function for turning the connected system apparatus into a low power consuming state ( hereinafter referred to as a sleep mode ) by temporarily stopping operations of the associated components . to turn into the sleep mode , the cpu 11 sends a sleep signal slp to the dc - to - dc converter 2 . in a normal operation mode , that is , not in the sleep mode , no sleep signal slp is sent to the dc - to - dc converter 2 from the cpu 11 and the dc - to - dc converter 2 generates the voltage va by reducing the power source voltage vdd supplied by the direct current power source 10 and outputs the voltage va to the voltage regulator 3 . the voltage regulator 3 reduces the voltage va applied as a source power by the dc - to - dc converter 2 to obtain the voltage vb and supplies the voltage vb to the cpu 11 as a source power . in this way , the power supply apparatus 1 reduces the power source voltage vdd supplied by the direct current power source 10 to the voltage va with the dc - to - dc converter 2 , further reduces the voltage va to the voltage vb with the voltage regulator 3 , and supplies the voltage vb as a source power to the cpu 11 . with this configuration , it is possible to minimize a value of voltage that the voltage regulator 3 bears to reduce as a load . when the power source voltage vdd is 3 . 6 volts , for example , the voltage va output by the dc - to - dc converter 2 may be set to 2 . 0 volts , for example , and the voltage vb output by the voltage regulator 3 may be set to 1 . 8 volts , for example . thus , the power consumption of the voltage regulator 3 can be reduced . in the sleep mode , that is , during the time the dc - to - dc converter 2 receives the sleep signal slp from the cpu 11 , the dc - to - dc converter 2 is fell into an inactive status to stop its operation . when stopping the operation , the dc - to - dc converter 2 outputs the power source voltage vdd supplied by the direct current power source 10 straight as the voltage va without performing the voltage reduction . accordingly , the power source voltage vdd is applied as a source power to the voltage regulator 3 . at this time , however , the cpu 11 operates in the sleep mode and consumes almost no electric power . therefore , the voltage regulator 3 consumes almost no electric power . on the other hands , the cpu 11 may perform its operation at intervals of a relatively short time period ( e . g ., 1 second ) during the sleep mode . in such an operation mode at intervals , the voltage regulator 3 reduces the power source voltage vdd applied thereto through the dc - to - dc converter 2 to the voltage vb , thereby obtaining a source power required for the cpu 11 to operate . at this time , the electric power consumed by the cpu 11 is relatively small and therefore the p - mos transistor 21 of the voltage regulator 3 consumes a relatively small amount of electric power . [ 0061 ] fig4 illustrates an exemplary internal structure of the dc - to - dc converter 2 . as illustrated in fig4 the dc - to - dc converter 2 includes a switching circuit 31 , a smoothing circuit 32 , and a controller 33 . the switching circuit 31 switches the power source voltage vdd supplied by the direct current power source 10 and outputs a resultant pulsating current voltage . the smoothing circuit 32 smoothes the pulsating current voltage output by the switching circuit 31 . the controller 33 controls the switching operation of the switching circuit 31 . the switching circuit 31 includes a p - mos transistor 41 and a parasite diode connected between a drain and a source of the p - mos transistor 41 . in the p - mos transistor 41 , the source is applied with the power source voltage vdd from the direct current power source 10 , a gate is connected to the controller 33 , and the drain is connected to the smoothing circuit 32 . a substrate gate of the pmos transistor 41 is connected to the source thereof . the smoothing circuit 32 includes a smoothing choke coil 45 , a smoothing capacitor 46 , and a flywheel diode 47 . the smoothing choke coil 45 and the smoothing capacitor 46 form a choke input type smoothing circuit that smoothes the pulsating current voltage input from the p - mos transistor 41 and outputs a resultant voltage . the flywheel diode 47 has a cathode connected to an input terminal of the smoothing choke coil 45 and an anode connected to the grounding . the direct current smoothed through the smoothing circuit 32 is output to the voltage regulator 3 as the voltage va , as well as to the controller 33 . the controller 33 outputs a pulse signal having a predetermined frequency ( e . g ., in a range from several hundreds khz to one mhz ) to the gate of the p - mos transistor 41 when receiving no input of the predetermined sleep signal slp from the cpu 11 ). the controller 33 observes the voltage output from the smoothing circuit 32 and controls a duty cycle of the pulse signal output to the gate of the pmos transistor 41 so that the voltage output from the smoothing circuit 32 is equal to the predetermined voltage va ( e . g ., 2 . 0 volts ). more specifically , the controller 33 reduces the duty cycle so that the p - mos transistor 41 turns on for a relatively longer time period when the voltage output from the smoothing circuit 32 is smaller than the predetermined voltage va . also , the controller 33 increases the duty cycle so that the p - mos transistor 41 turns on for a relatively shorter time period when the voltage output from the smoothing circuit 32 is greater than the predetermined voltage va . further , the controller 33 maintains the duty cycle when the voltage output from the smoothing circuit 32 is equal to the predetermined voltage va . on the other hands , the controller 33 is turned into a non - active state and stops its operation when receiving the predetermined sleep signal slp from the cpu 11 , and an input to the gate of the p - mos transistor 41 is at a low level . thereby , the p - mos transistor 41 is turned into an on state , and the voltage output from the smoothing circuit 32 is equal to the power source voltage vdd supplied by the direct current power source 10 . the above - described power supply apparatus 1 has the voltage regulator 3 configured to output a single voltage vb . alternatively , the above - described power supply apparatus 1 may have the voltage regulator 3 outputting a plurality of different voltages . also , the switching circuit 31 and the controller 33 of the dc - to - dc converter 2 and the voltage regulator 3 can be integrated into a single ic chip . [ 0068 ] fig5 shows a dc - to - dc converter 202 which can be used as an alternative to the dc - to - dc converter 2 . the dc - to - dc converter 202 of fig5 is similar to the dc - to - dc converter 2 of fig4 except for a smoothing circuit 232 and a controller 233 . the smoothing circuit 232 includes a high active n - channel - type mos ( metal oxide semiconductor ) transistor 51 ( hereinafter referred to as a n - mos transistor 51 ) in place of the flywheel diode 47 of the smoothing circuit 32 . the controller 233 of fig5 is similar to the controller 33 of fig4 except for generation of control signals s 1 and s 2 . in the dc - to - dc converter 202 , the n - mos transistor 51 is connected between the drain of the p - mos transistor 41 and the grounding , as shown in fig5 so that the p - mos transistor 41 and the n - mos transistor 51 are controlled by the controller 233 with the control signals s 1 and s 2 . a time chart of fig6 shows a relationship between the control signals s 2 and s 2 . a shown in fig6 the sleep signal slp output by the cpu 11 is held at a low level during the normal operation mode and at a high level during the sleep mode . during the normal operation mode , the controller 233 generates the control signals s 1 and s 2 which differently rise and fall from each other and sends them to the p - mos transistor 41 and the n - mos transistor 51 , respectively . thereby , the p - mos transistor 41 and the n - mos transistor 51 are controlled so as not to be turned on at the same time . this n - mos transistor 51 can be integrated with the switching circuit 31 , the controller 233 , and the voltage regulator 3 into a single ic chip . in this way , the power supply apparatus 1 generates and supplies the stable predetermined voltage vb to the cpu 11 during the time the cpu 11 operates in the normal operation mode by efficiently reducing the power source voltage vdd to the voltage va with the dc - to - dc converter 202 and finally regulating the voltage va with the voltage regulator 3 to obtain the voltage vb . thereby , the power supply apparatus 1 can achieve a relatively low power consumption of the voltage regulator 3 in the normal operation mode . also , during the sleep mode , the power supply apparatus 1 causes the dc - to - dc converter 202 to turn into an inactive state to reduce the power consumption , and generates the predetermined stable vb by reducing the power source voltage vdd to the voltage vb directly with the voltage regulator 3 . that is , since the devices including the cpu , the dsp , the memories , etc . are turned into the sleep mode and do not need the power source , the voltage vb is not used by the devices and no power is consumed . when the cpu 11 , for example , operates at intervals of a predetermined time period ( e . g ., one second ) in the sleep mode , the cpu 11 can operate with the stable voltage vb supplied . [ 0071 ] fig7 shows a dc - to - dc converter 302 according to another preferred embodiment . the dc - to - dc converter 302 can be used as an alternative to the dc - to - dc converter 2 of fig4 and is similar to it , except for a control circuit 333 . this control circuit 333 can be used in place of the control circuit 233 of the dc - to - dc converter 202 , as a further alternative . as shown in fig7 the control circuit 333 includes a duty control circuit 61 , an undershoot preventive circuit 62 , and an overshoot preventive circuit 63 . the duty control circuit 61 controls a duty cycle of a pulse signal output to the gate of the p - mos transistor 41 so that a voltage vo output from the smoothing circuit 32 becomes the predetermined voltage va . the undershoot preventive circuit 62 and the overshoot preventive circuit 63 operate to protect occurrences of an undershoot and an overshoot , respectively , of the voltage vo . connections of the sleep signal slp to the duty control circuit 61 , the undershoot preventive circuit 62 , and the overshoot preventive circuit 63 are not shown in fig7 for the sake of simplicity . referring to fig8 mechanisms causing undershooting and overshooting waveforms are explained . in the sleep mode , the input to the gate of the p - mos transistor 41 is at a low level and the power source voltage vdd passes through the switching circuit 31 and the smoothing circuit 32 so that the voltage vo has the same voltage level as the power source voltage vdd , as described above . when the sleep mode ( referred to as m 1 in fig8 ) is changed to the normal operation mode ( referred to as m 2 in fig8 ), the voltage regulator 3 needs a certain time period as a transition time ( referred to as m 3 in fig8 ) to respond to the mode change . accordingly , during the transition time the voltage vo is maintained at a voltage level around the power source voltage vdd , which is greater than the predetermined voltage va , for the above - mentioned certain time after the sleep mode is terminated . this causes the controller 333 to rise the voltage to a high level input to the gate of the p - mos transistor 41 so that the p - mos transistor 41 is turned off and shuts off the power source voltage vdd . that is , at the end of the transition time , the voltage regulator 3 starts its operation under the condition that the voltage vo is maintained at a voltage level around the power source voltage vdd . this causes the dc - to - dc converter 302 to fall to a state of being loaded by the voltage regulator 3 . in this case , when a load current io ( e . g ., 200 ma ) flows from the smoothing circuit 32 , the voltage vo may be dropped so rapidly as to produce an undershooting waveform w 1 , as shown in fig8 . as a result , the voltage vo is momentarily reduced to a value considerably smaller than the predetermined voltage va . on the other hands , the voltage vo may be risen so rapidly as to produce an overshooting waveform w 2 , as shown in fig8 when the p - mos transistor 41 is turned on immediately after the mode is changed from the normal operation mode to the sleep mode in order to cause the power source voltage vdd to pass through the p - mos transistor 41 . in this case , the voltage vo may produce an overshooting waveform w 2 , as shown in fig8 and is momentarily risen over a value considerably greater than the power source voltage vdd . the undershoot preventive circuit 62 prevents an occurrence of the above - described undershooting waveform w 1 and the overshoot preventive circuit 63 prevents an occurrence of the above - described overshooting waveform w 2 . the duty control circuit 61 includes a voltage detection circuit 71 and a duty controller 72 . the voltage detection circuit 71 detects the voltage vo , and the duty controller 72 controls a duty cycle of a pulse signal input to the gate of the p - mos transistor 41 in response to the voltage vo detected by the voltage detection circuit 71 . the voltage detection circuit 71 includes an operational amplifier 73 , a voltage dividing circuit 74 , a vrl generator 75 . the voltage dividing circuit 74 divides the voltage vo , and includes resisters 76 and 77 and an n - channel - type mos ( metal oxide semiconductor ) transistor 78 ( hereinafter referred to as an n - mos transistor 78 ). the vrl generator 75 generates a reference voltage vrl . the resisters 76 and 77 are connected in series between the line of the voltage vo and the grounding . the n - mos transistor 78 has a gate that receives an inverse sleep signal slpb ( not shown ) generated by the inverse of the sleep signal slp . in the voltage detection circuit 71 , the sleep signal slp is in a low state in the sleep mode and therefore the inverse sleep signal slpb input to the gate of the n - mos transistor 78 is in a high state . thereby , the n - mos transistor 78 is turned on and is brought into conduction . the voltage vo is then divided by the resisters 76 and 77 and a divided voltage vd is generated between the resisters 76 and 77 . the operational amplifier 73 has an inverse input terminal receiving the divided voltage vd and a non - inverse input terminal receiving the reference voltage vrl output from the vrl generator 75 . the operational amplifier 73 compares the divided voltage vd to the reference voltage vrl and outputs a voltage to the duty controller 72 in response to the comparison result . the duty controller 72 generates a pulse signal having a duty cycle in response to the voltage received from the operational amplifier 73 and outputs the pulse signal to the gate of the p - mos transistor 41 . on the other hands , when the mode is changed from the normal operation mode to the sleep mode , the sleep signal slp in a high state is output from the cpu 11 . accordingly , the operational amplifier 73 , the vrl generator 75 , and the duty controller 72 are caused to stop the respective operations . at the same time , in the voltage dividing circuit 74 , the gate of the n - mos transistor 78 is turned off and is out of conduction . as a result , the voltage vo is divided and the divided voltage vd is generated . when the duty controller 72 stops its operation , the output terminal thereof is in an open state and in a high impedance state . the undershoot preventive circuit 62 includes an n - channel - type mos ( metal oxide semiconductor ) transistor 81 ( hereinafter referred to as an n - mos transistor 81 ), an operations amplifier 83 , and a current control circuit 83 . the n - mos transistor 81 operates as a load to consume a current ia flowing from the output terminal of the smoothing circuit 32 to the grounding . the operational amplifier 82 operates as a voltage comparator for comparing the divided voltage vd output from the voltage dividing circuit 74 to the reference voltage vrl output from the vrl generator 75 , and outputs a binary signal in response to the comparison result . the undershoot preventive circuit 62 further includes a current control circuit 83 . the current control circuit 83 controls the operation of the n - mos transistor 81 in accordance with the signal output from the operational amplifier 82 so as to control the current ia flowing from the output terminal of the smoothing circuit 32 . the operational amplifier 82 , the voltage dividing circuit 74 , and the vrl generator 75 together form a voltage determination circuit . in the undershoot preventive circuit 62 , when the mode is changed from the normal operation mode to the sleep mode , the sleep signal slp in a high state is output from the cpu 11 . accordingly , the operational amplifier 82 and the current control circuit 83 are caused to stop the respective operations and , at the same time , the gate of the n - mos transistor 81 is turned off and is out of conduction . since the p - mos transistor 41 is in an on state and is of conduction , the voltage vo is held at a level around the power source voltage vdd . when the mode is changed from the sleep mode to the normal operation mode , the operational amplifier 82 and the current control circuit 83 are turned into an active state and start the respective operations . at this time , the voltage vo has a voltage close to the power source voltage vdd which is greater than the predetermined voltage va and therefore the controller 333 outputs the voltage in a high state which turns off the p - mos transistor 41 . therefore , the divided voltage vd is greater than the reference voltage vrl and the operational amplifier 82 outputs a signal in a low state . when the low signal is input from the operational amplifier 82 to the current control circuit 83 , the current control circuit 83 raises a gate voltage vg of the n - mos transistor 81 . as a result , the n - mos transistor 81 generate the current ia in response to the gate voltage vg input , as shown in fig9 . the voltage vd is gradually reduced from the level of the power source voltage vdd to the predetermined voltage va . during this reduction of the voltage vd , the operational amplifier 82 changes the output from the low voltage to a high level voltage when the divided voltage vd is reduced to a level smaller than the reference voltage vrl . when the operational amplifier 82 outputs a high signal to the current control circuit 83 , the current control circuit 83 controls the gate voltage vg of the n - mos transistor 81 in a way as shown in fig1 . that is , the gate voltage vg is linearly raised during a predetermined time t 1 and is continuously raised during a predetermined time t 2 . further , the gate voltage vg is held at a level of the power source voltage vdd during a predetermined time t 3 and is reduced from the level of the voltage vg to the grounding level during a predetermined time t 4 . during these operations , the current ia flowing through the n - mos transistor 81 is changed in a way as shown in fig9 . the current during the predetermined time t 3 is a saturated current . also , during these operations , the voltage level of the gate voltage vg is changed in a way as shown in fig1 . the gate voltage vg is continuously raised in the predetermined time t 2 at the same voltage raising pace as in a predetermined time t 1 after the predetermined time t 1 , as shown in fig1 . this is because the duty control circuit 72 takes a certain delay time before starting the control of the operation of the p - mos transistor 41 after the voltage level of the voltage vo is changed to the predetermined voltage va . it should be noted that fig1 shows a case in which the current control circuit 83 receives a high signal from the operational amplifier 82 before the gate voltage vg is raised to a level of the power source voltage vdd after the predetermined time t 1 following the application of the gate voltage vg to the n - mos transistor 81 . on the other hands , when the current control circuit 83 raises the gate voltage vg to the level of the power source voltage vdd upon receiving a high signal from the operational amplifier 82 , the gate voltage vg changes in a way as shown in fig1 . in fig1 , a predetermined time t 1 &# 39 ; is equivalent to the predetermined time t 1 of fig1 but is relatively longer than the predetermined time t 1 , and the current control circuit 83 attempts to raise the gate voltage vg during the predetermined time t 2 . at this time , however , the gate voltage vg is raised to the level of the power source voltage vdd and , as a result , the gate voltage vg is held at the level of the power source voltage vdd during the predetermined times t 2 and t 3 . the current control circuit 83 is previously provided with various kinds of settings associated with the gate voltage of the n - mos transistor 81 so that the voltage regulator 3 starts its operation and the load current io flows from the smoothing circuit 32 through the voltage regulator 3 during the time the current control circuit 83 reduces the gate voltage of the n - mos transistor 81 to the grounding level . more specifically , the above - mentioned various kinds of settings includes the voltage raising pace of the gate voltage vg of the n - mos transistor 81 , the predetermined times t 2 and t 3 in which the gate voltage vg is held at the level of the power source voltage vdd , and the pace of reducing the gate voltage vg from the level of the power source voltage vdd to the grounding level . the overshoot preventive circuit 63 is in an inactive state and maintains the output terminal at an open state in the normal operation mode . accordingly , the overshoot preventive circuit 63 stops applying a gate voltage to the p - mos transistor 41 . in the sleep mode , the overshoot preventive circuit 63 is turned into an active state and detects a current output from the p - mos transistor 41 . therefore , the overshoot preventive circuit 63 controls the gate voltage of the p - mos transistor 41 in accordance with the result of the current detection . during the sleep mode , the overshoot preventive circuit 63 raises the voltage vo to the level of the power source voltage vdd by making the gate voltage of the p - mos transistor 41 low to turn on the p - mos transistor 41 when the detected current is smaller than a predetermined value □ ( e . g ., 1 a ). when the detected current is greater than the predetermined value □ ( e . g ., 1 a ), the overshoot preventive circuit 63 continuously raises the gate voltage of the p - mos transistor 41 in response to the detected current so that the current supplied from the p - mos transistor 41 is successively reduced to the level smaller than the predetermined value □ ( e . g ., 1 a ). when the above - described operations are performed , the voltage vo is changed in a way as shown in fig1 . as a result , the voltage vo can be prevented from the undershooting during the time the mode is changed from the sleep mode to the normal operation mode and from the overshooting during the time the mode is changed from the normal operation mode to the sleep mode . in addition , the overshoot preventive circuit 63 also has a function for prevent an excessive current flowing from the p - mos transistor 41 in the sleep mode when a short circuit occurs in a load connected to the smoothing circuit 32 . with this , the power supply apparatus 1 can prevent an excessive current output from the dc - to - dc converter 2 in the sleep mode . as described above , in the dc - to - dc converter 302 of fig7 the reference voltage vrl and the divided voltage vd are compared by the operational amplifier 73 of the voltage detection circuit 71 and the duty controller 72 generates a pulse signal that has a duty cycle in response to the comparison result and applies the pulse signal to the gate of the p - mos transistor 41 . in this case , relationships among the voltage vo , the divided voltage vd , and the reference voltage vrl are as shown in fig1 , in which a portion enclosed with a chain line is shown in an enlarged form . fig1 indicates that the dc - to - dc converter 302 prevents the voltage vo from undershooting though the voltage vo may still be dropped to a level slightly lower than the predetermined voltage va since the dc - to - dc converter 302 is fell into an inactive - like state during the time the mode is changed from the sleep mode to the normal operation mode . [ 0091 ] fig1 shows a dc - to - dc converter 402 according to another preferred embodiment . the dc - to - dc converter 402 can be used as an alternative to the dc - to - dc converter 302 of fig7 and is similar to it , except for a control circuit 433 . this control circuit 433 can be used in place of the control circuit 233 of the dc - to - dc converter 202 , as a further alternative . in fig1 , connections of the sleep signal slp to each component inside the control circuit 433 are not shown for the sake of simplicity . the controller 433 of fig1 is similar to the controller 333 of fig7 except for additional circuits of a vr 2 generator 91 , a selection circuit 92 , and an operational amplifier 93 to the voltage detection circuit 71 of fig7 . the vr 2 generator 91 generates and outputs a reference voltage vr 2 . the selection circuit 92 exclusively selects one of the reference voltages vrl and vr 2 in accordance with a control signal and inputs the selected reference voltage to the operational amplifier 73 . the operational amplifier 93 controls the operation of the selection circuit 92 in accordance with the divided voltage vd . accordingly , in fig1 , a duty control circuit 61 a replaces the duty control circuit 61 of fig7 and a voltage detection circuit 71 a replaces the voltage detection circuit 71 of fig7 . as shown in fig1 , the controller 433 includes the duty control circuit 61 a , the undershoot preventive circuit 62 , and the overshoot preventive circuit 63 . the duty control circuit 61 a includes the voltage detection circuit 71 a and the duty controller 72 . the voltage detection circuit 71 a detects the voltage vo , and the duty controller 72 controls the duty cycle of a pulse signal input to the gate of the p - mos transistor 41 in response to the voltage vo detected by the voltage detection circuit 71 a . the voltage detection circuit 71 a includes the operational amplifier 73 , the voltage dividing circuit 74 , the vrl generator 75 , the vr 2 generator 91 , the selection circuit 92 , and the operational amplifier 93 . in the sleep mode , as in the case of the voltage detection circuit 71 of fig7 the operational amplifier 73 and the vrl generator 75 are caused to stop the respective operations , and the voltage dividing circuit 74 dividing the voltage vo to a voltage vd outputs the divided voltage vd . also , the vr 2 generator 91 , the selection circuit 92 , and the operational amplifier 93 are caused to stop the respective operations . each part of the voltage detection circuit 71 a starts to operate when the mode is changed from the sleep mode to the normal operation mode . the vr 2 generator 91 generates the reference voltage vr 2 and varies it at a predetermined pace such that the reference voltage vr 2 is reduced from a predetermined voltage vx lower than the divided voltage vd to the reference voltage vrl in a predetermined time period when the mode is changed from the sleep mode to the normal operation mode . the operational amplifier 93 performs a comparison between the divided voltage vd and the reference voltage vrl , and outputs a low level control signal to the selection circuit 92 when the divided voltage vd is determined as greater than the reference voltage vrl . the selection circuit 92 inputs the reference voltage vr 2 to a non - inverse input terminal of the operational amplifier 73 upon receiving the low level control signal from the operational amplifier 93 . on the other hands , the operational amplifier 93 outputs a high level control signal to the selection circuit 92 when the divided voltage vd is determined as smaller than the reference voltage vrl . the selection circuit 92 inputs the reference voltage vrl to the non - inverse input terminal of the operational amplifier 73 upon receiving the high level control signal from the operational amplifier 93 . with the above - described operations , the voltage vo , the divided voltage vd , and the reference voltage vr 2 are varied in a way as shown in fig1 , in which a portion enclosed with a chain line is shown in an enlarged form . fig1 indicates that the dc - to - dc converter 402 prevents the voltage vo from undershooting , which is a problematic phenomenon that the voltage vo is dropped to a level lower than the predetermined voltage va at a time the dc - to - dc converter 402 bears a sudden load , since the dc - to - dc converter 402 is in an active state during the time the mode is changed from the sleep mode to the normal operation mode . the reference voltage vr 2 may be controlled to be declined so that the voltage vo is reduced at a pace slower than the case shown in fig1 . in this way , the dc - to - dc converter 402 can prevent the voltage vo from undershooting and overshooting with the undershoot preventive circuit 62 and the overshoot preventive circuit 63 , respectively . the dc - to - dc converter 402 further prevents the voltage vo from undershooting when the mode is changed from the sleep mode to the normal operation mode by the arrangement that the duty control circuit 61 a uses the reference voltage vr 2 which is generated and varied by the vr 2 generator 91 at the predetermined pace such that the reference voltage vr 2 is reduced from a predetermined voltage vx lower than the divided voltage vd to the reference voltage vrl in the predetermined time period when the mode is changed from the sleep mode to the normal operation mode . 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 . this paten specification is based on japanese patent applications , no . jpap2001 - 038394 filed on feb . 15 , 2001 and no . jpap2001 - 189792 filed on jun . 22 , 2001 in the japanese patent office , the entire contents of which are incorporated by reference herein .