Abstract:
A power supply apparatus includes a DC-to-DC converter performing a voltage conversion converting a voltage of a source power supplied from a direct current power source to a first predetermined voltage lower than the voltage of the source power and a voltage regulator carrying out a voltage regulation for regulating the first predetermined voltage of the source power output from the DC-to-DC converter to at least a second predetermined voltage lower than the first predetermined voltage.

Description:
FIELD OF THE INVENTION 
     This patent specification relates to a power supply method and apparatus, and more particularly to a power supply method and apparatus that effectively reduces power consumption. 
     BACKGROUND OF THE INVENTION 
     Conventionally, a power supply apparatus that reduces a direct current supplied from a direct current power source (e.g., a battery) to a predetermined voltage is classified into two types; one type using a voltage regulator and the other type using a DC-to-DC converter. 
     FIG. 1 shows an exemplary circuit of a background power supply apparatus using a voltage regulator  100 . In the voltage regulator  100  of FIG. 1, a P-channel-type MOS (metal oxide semiconductor) transistor  102  (hereinafter referred to as a P-MOS transistor  102 ) and resisters  103  and  104  are connected in series between a power source terminal applied with a power source voltage VDD by a direct current  101  (e.g., a battery including a secondary battery) and ground. The resisters  103  and  104  divide a voltage Vout which is compared by a voltage comparator  106  with a predetermined reference voltage Vref generated by a reference voltage generator  105 . Based on a comparison result, an operation of the P-MOS transistor  102  is controlled so that the voltage Vout is held at a desired value. In FIG. 1, a CPU  107  is an exemplary system that requires power from the voltage regulator  100 . 
     However, the above-described voltage regulator has a drawback that the P-MOS transistor  101  consumes a great amount of electric power for a reduction of the power source voltage VDD to the voltage Vout. More specifically, when the CPU  107  consumes a current of 100 mA, for example, and a voltage regulator  100  reduces the power source voltage VDD from 3.6 volts, for example, to 2 volts, for example, the P-MOS transistor  101  consumes the power of 0.16 W. That is, the voltage regulator consumes a difference of the battery voltage and the CPU&#39;s operational voltage. Such voltage regulator is undesirable for a system aiming a low power consumption since the CPU&#39;s operational voltage has been lowered in the recent years. 
     Accordingly, as shown in FIG. 2, a DC-to-DC converter is used in place of the voltage regulator as a power supply in a system (e.g., the CPU  107 ) using a battery. In FIG. 2, a DC-to-DC converter  110  reduces the power source voltage VDD to a predetermined voltage Vout and supplies the voltage Vout to the CPU  107 . 
     In general, a system using a battery as a source of power is provided with a sleep function or temporarily stopping the operations of the system to reduce an electrical power consumption on an as needed basis. In the case of the power supply apparatus of FIG. 2, it is attempted to reduce the power consumption by changing the output terminal of the DC-to-DC converter  110  to the CPU  107  in the sleep mode from a ground level to a high impedance level. This is because the DC-to-DC converter  110  is used as an apparatus that directly controls the power source required by the system (e.g., the CPU  107 ). 
     On the other hands, the DC-to-DC converter  110  is required to be always in an active state in the case the system (e.g., the CPU  107 ) in the sleep mode is intermittently activated to control certain components on an as needed basis. In such a case, the power consumption by the DC-to-DC converter  110  shares a large part of the total system power consumption. 
     SUMMARY OF THE INVENTION 
     This patent specification describes a novel power supply apparatus. In one example, this novel power supply apparatus includes a DC-to-DC converted and a voltage regulator. The DC-to-DC converter is arranged and configured to perform a voltage conversion for converting a voltage of a power source supplied from a direct current power source to a first predetermined voltage. The first predetermined voltage is lower than the voltage of the power source. The voltage regulator is arranged and configured to carrying out a voltage regulation for regulating the first predetermined voltage of the power source to at least a second predetermined voltage. The second predetermined voltage is lower than the first predetermined voltage. 
     The DC-to-DC converter may be turned into a non-active state to stop the voltage conversion and straight passes the voltage of the power source when an operation mode is changed to a sleep mode. 
     The DC-to-DC converter may include a switching circuit, a smoothing circuit, and a controller. The switching circuit is arranged and configured to perform a switching operation for switching the power source and to output a pulsating current voltage. The smoothing circuit is arranged and configured to smooth the pulsating current voltage output by the switching circuit and to output a smoothed voltage to the voltage regulator. The controller is arranged and configured to detect the smoothed voltage output from the smoothing circuit and to control the switching circuit to change a performance of the switching operation in response to a detection result of the smoothed voltage so that the smoothed voltage output by the smoothing circuit is substantially equal to the first predetermined voltage. The controller is turned into a non-active state to cause the switching circuit to stop the switching operation so as to pass the voltage of the power source through the switching circuit and to output the voltage of the power source to the smoothing circuit when the operation mode is changed to the sleep mode. 
     The DC-to-DC converter may output the voltage of the power source without performing the voltage conversion when the operation mode is changed to the sleep mode. 
     The converter may include a switching circuit, a smoothing circuit, and a controller. The switching circuit is arranged and configured to perform a switching operation for switching the power source and outputting a pulsating current voltage. The smoothing circuit is arranged and configured to smooth the pulsating current voltage output from the circuit and to output a smoothed voltage to the voltage regulator. The controller is arranged and configured to detect the smoothed voltage output from the smoothing circuit and to control the switching circuit to change a performance of the switching operation in response to a detection result of the smoothed voltage so that the smoothed voltage output from the smoothing circuit is substantially equal to the first predetermined voltage. The controller causes the switching circuit to stop the switching operation so as to pass the voltage of the power source through the switching circuit and to output the voltage of the power source to the smoothing circuit when the operation mode is changed to the sleep mode. 
     The controller may connect a load to an output terminal of the smoothing circuit and controls a current flowing the load so as to reduce the voltage output from the smoothing circuit to the first predetermined voltage when the voltage output from the smoothing circuit is lower than the first predetermined voltage and when the operation mode is changed to a normal operation mode. 
     The controller may include a transistor, a comparator, and a current control circuit. The transistor operates as the load. The comparator performs a first comparison for comparing the voltage output from the smoothing circuit with the first predetermined voltage when the operation mode is changed to the normal operation mode and outputs a first comparison result. The current control circuit is arranged and configured to control the transistor to produce a current flowing therethrough in response to the first comparison result of the comparator when the operation mode is changed to the normal operation mode. 
     The current control circuit may control the transistor to increase the current at a first predetermined pace when the voltage output from the smoothing circuit is determined as greater than the first predetermined voltage based on the first comparison result performed by the comparator. 
     The current control circuit may control the transistor to continue to increase the current at the first predetermined pace for a first predetermined time period when the voltage output from the smoothing circuit is determined as substantially equal to the first predetermined voltage based on the first comparison result performed by the comparator. The current control circuit may further control the transistor to produce a saturated current flowing therethrough for a second predetermined time period immediately after the first predetermined time period. 
     The current control circuit may control the transistor to decrease the current at a second predetermined pace for a third predetermined time period immediately after the second predetermined time period. 
     The controller may detect a current output from the switching circuit and controls the switching circuit to vary the current in response to the detected current when the operation mode is changed to the sleep mode. 
     The controller may control the switching circuit to straight output the voltage of the power source to the smoothing circuit when the current detected is smaller than a predetermined value and to reduce the current output therefrom to a value smaller than the predetermined value in a predetermined manner when the current is greater than the predetermined value. 
     The controller may perform a second comparison between a reference voltage dropping at a substantially constant pace and the voltage output from the smoothing circuit in response to the detected voltage when the operation mode is changed to the normal operation mode, and controls a duty cycle of the switching operation performed by the switching circuit according to a result of the second comparison during a time the voltage output from the smoothing circuit is reduced to the first predetermined voltage. 
     The controller may perform a third comparison between another predetermined reference voltage and the voltage output from the smoothing circuit in response to the detected voltage, and controls a duty cycle of the switching operation performed by the switching circuit according to a result of the third comparison when the voltage output from the smoothing circuit is reduced to the first predetermined voltage. 
     This patent specification further describes a novel method of power supply. In one example, this novel method includes the steps of performing and regulating. The performing step performs a DC-to-DC conversion with a DC-to-DC converter to achieve a voltage conversion for converting a voltage of a power source supplied from a direct current power source to a first predetermined voltage. The first predetermined voltage is lower than the voltage of the power source. The regulating step regulates the first predetermined voltage of the power source to at least a second predetermined voltage. The second predetermined voltage is lower than the first predetermined voltage. 
     The performing step may turn the DC-to-DC converter into a non-active state to stop the DC-to-DC conversion and straight passes the voltage of the power source through the DC-to-DC converter to the voltage regulator when an operation mode is changed to a sleep mode. 
     The performing step may include the steps of executing, smoothing, detecting, changing, and stopping. The executing step executes a switching operation for switching the power source to output a pulsating current voltage. The smoothing step smoothes the pulsating current voltage output by the switching circuit to output a smoothed voltage to the voltage regulator. The detecting step detects the smoothed voltage output in the smoothing step. The changing step changes a performance of the switching operation in response to a detection result of the smoothed voltage so that the smoothed voltage output in the smoothing step is substantially equal to the first predetermined voltage. The stopping step stops the switching operation when the operation mode is changed to the sleep mode so as to apply the voltage of the power source to the smoothing circuit. 
     The DC-to-DC converted may output the voltage of the power source without performing the voltage conversion when the operation mode is changed to the sleep mode. 
     The performing step may include the steps of executing, smoothing, detecting, changing, and stopping. The executing step executes a switching operation for switching the power source to output a pulsating current voltage. The smoothing step smoothes the pulsating current voltage output in the switching step to output a smoothed voltage to the voltage regulator. The detecting step detects the smoothed voltage output in the smoothing step. The changing step changes a performance of the switching operation in response to a detection result of the smoothed voltage so that the smoothed voltage output in the smoothing step is substantially equal to the first predetermined voltage. The stopping step stops the switching operation when the operation mode is changed to the sleep mode so as to apply the voltage of the power source to the smoothing circuit. 
     The above-mentioned novel method may further includes steps of providing, applying, and adjusting. The providing step provides a transistor as a load. The applying step applies the voltage output in the smoothing step to the transistor so that a current flows through the transistor when the voltage output in the smoothing step is lower than the first predetermined voltage and when the operation mode is changed to a normal operation mode. The adjusting step adjusts the current flowing the load so as to reduce the voltage output in the smoothing step to the first predetermined voltage. 
     The adjusting step may include the steps of performing and causing. The performing step performs a first comparison for comparing the voltage output in the smoothing step with the first predetermined voltage when the operation mode is changed to the normal operation mode to output a first comparison result. The causing step causes the transistor to produce a current flowing therethrough in response to the first comparison result of the comparing step when the operation mode is changed to the normal operation mode. 
     The causing step may cause the transistor to increase the current at a first predetermined pace when the voltage output in the smoothing step is determined as greater than the first predetermined voltage based on the first comparison result performed in the comparing step. 
     The causing step may cause the transistor to continue to increase the current at the first predetermined pace for a first predetermined time period when the voltage output in the smoothing step is determined as substantially equal to the first predetermined voltage based on the first comparison result performed in the comparing step, and may cause the transistor to produce a saturated current flowing therethrough for a second predetermined time period immediately after the first predetermined time period. 
     The causing step may cause the transistor to decrease the current at a second predetermined pace for a third predetermined time period immediately after the second predetermined time period. 
     The above-mentioned method may further include the steps of detecting and instructing. The detecting step detect a current output in the switching step when the operation mode is changed to the sleep mode. The instructing step instruct the switching step to change the current in response to the detected current. 
     The instructing step may instruct the switching step to straight output the voltage of the power source to the smoothing step when the current detected is smaller than a predetermined value and to reduce the current output in the switching step to a value smaller than the predetermined value in a predetermined manner when the current is greater than the predetermined value. 
     The novel method may further include the steps of performing and determining. The performing step performs a second comparison between a reference voltage dropping at a substantially constant pace and the voltage output in the smoothing step in response to the detected voltage during a time the voltage output in the smoothing step is reduced to the first predetermined voltage. The determining step determines a duty cycle of the switching operation performed in the switching step according to a result of the second comparison. 
     The above-mentioned novel method may further include the steps of performing and controlling. The performing step performs a third comparison between another predetermined reference voltage and the voltage output in the smoothing circuit in response to the detected voltage. The controlling step controls the duty cycle of the switching operation performed in the switching step according to a result of the third comparison when the voltage output in the smoothing step is reduced to the first predetermined voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a block diagram of a background power supply apparatus; 
     FIG. 2 is a block diagram of another background power supply apparatus; 
     FIG. 3 is a block diagram of a power supply apparatus including a DC-to-DC converter according to a preferred embodiment; 
     FIG. 4 is a circuit diagram of the DC-to-DC converter; 
     FIG. 5 is a circuit diagram of another DC-to-DC converter according to a preferred embodiment; 
     FIG. 6 is a time chart for explaining control signals of a controller included in the DC-to-DC converter of FIG. 5; 
     FIG. 7 is a circuit diagram of another DC-to-DC converter according to a preferred embodiment; 
     FIG. 8 is a time chart for explaining an undershoot and an overshoot of an output voltage Vo; 
     FIG. 9 is a time chart for showing an example of a current Ia flowing through an N-MOS transistor of an undershooting preventive circuit included in the DC-to-DC converter of FIG. 7; 
     FIG. 10 is a time chart for showing an example of a gate voltage Vg relative to the N-MOS transistor of the undershooting preventive circuit; 
     FIG. 11 is a time chart for showing another example of the gate voltage Vg relative to the N-MOS transistor of the undershooting preventive circuit; 
     FIG. 12 is a time chart for showing an example of the output voltage Vo output by the DC-to-DC converter of FIG. 7; 
     FIG. 13 is a time chart for explaining a relationship among the voltage Vo, a divided voltage Vd, and a reference voltage Vr 1  generated in the DC-to-DC converter of FIG. 7; 
     FIG. 14 is a circuit diagram of another DC-to-DC converter according to a preferred embodiment; and 
     FIG. 15 is a time chart for explaining a relationship among a voltage Vo, a divided voltage Vd, and a reference voltage Vr 1  generated in the DC-to-DC converter of FIG.  14 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     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 FIG. 3 thereof, a power supply apparatus  1  according to a preferred embodiment is described. As illustrated in FIG. 3, 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 FIG. 3, 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 ground. The voltage regulator  3  is connected between the output terminal of the DC-to-DC converter  2  and ground. 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 ground, 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 the current flowing through 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 power source by the DC-to-DC converter  2  to obtain the voltage Vb and supplies the voltage Vb to the CPU  11  as a power source. 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 power source 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 from the CPU  11 , the DC-to-DC converter  2  is put 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 power source 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 power source 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. 
     FIG. 4 illustrates an exemplary internal structure of the DC-to-DC converter  2 . As illustrated in FIG. 4, 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 P-MOS 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 ground. 
     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 P-MOS 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. 
     FIG. 5 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 FIG. 5 is similar to the DC-to-DC converter  2  of FIG. 4, 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 th smoothing circuit  32 . The controller  233  of FIG. 5 is similar to the controller  33  of FIG. 4, 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 ground, as shown in FIG. 5, 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 FIG. 6 shows a relationship between the control signals S 2  and S 2 . A shown in FIG. 6, 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 rise and fall differently 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 devices including the CPU, the DSP, 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. 
     FIG. 7 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 FIG.  4  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 FIG. 7, 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 FIG. 7, for the sake of simplicity. 
     Referring to FIG. 8, 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 FIG. 8) is changed to the normal operation mode (referred to as M 2  in FIG.  8 ), the voltage regulator  3  needs a certain time period as a transition time (referred to as M 3  in FIG. 8) 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 lo (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 FIG.  8 . 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 rise to rapidly so to produce an overshooting waveform W 2 , as shown in FIG. 8, 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 FIG.  8  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 Vr 1  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 Vr 1  generator  75  generates a reference voltage Vr 1 . The resisters  76  and  77  are connected in series between the line of the voltage Vo and ground. 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 Vr 1  output from the Vr 1  generator  75 . The operational amplifier  73  compares the divided voltage Vd to the reference voltage Vr 1  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 Vr 1  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 ground. 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 Vr 1  output from the Vr 1  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 Vr 1  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 conducting, 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 Vr 1  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 FIG.  9 . 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 Vr 1 . 
     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 FIG.  10 . 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 ground 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 FIG.  9 . 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 FIG.  10 . 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 FIG.  10 . 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 FIG. 10 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 FIG.  11 . In FIG. 11, a predetermined time t 1 ′ is equivalent to the predetermined time t 1  of FIG. 10 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 IOU 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 ground 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 ground 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 α (e.g.,  1 A). When the detected current is greater than the predetermined value α (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 α (e.g.,  1 A). 
     When the above-described operations are performed, the voltage Vo is changed in a way as shown in FIG.  12 . 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 prevents 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 FIG. 7, the reference voltage Vr 1  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 Vr 1  are as shown in FIG. 13, in which a portion enclosed with a chain line is shown in an enlarged form. FIG. 13 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. 
     FIG. 14 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 FIG.  7  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 FIG. 14, 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 FIG. 14 is similar to the controller  333  of FIG. 7, 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 FIG.  7 . The Vr 2  generator  91  generates and outputs a reference voltage Vr 2 . The selection circuit  92  exclusively selects one of the reference voltages Vr 1  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 FIG. 14, a duty control circuit  61   a  replaces the duty control circuit  61  of FIG. 7 and a voltage detection circuit  71   a  replaces the voltage detection circuit  71  of FIG.  7 . 
     As shown in FIG. 14, 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 Vr 1  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 FIG. 7, the operational amplifier  73  and the Vr 1  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 Vr 1  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 Vr 1 , 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 Vr 1 . 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 Vr 1 . The selection circuit  92  inputs the reference voltage Vr 1  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 FIG. 15, in which a portion enclosed with a chain line is shown in an enlarged form. FIG. 15 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 FIG.  15 . 
     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 Vr 1  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 patent 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.