Abstract:
A power circuit for generating a standard voltage based on an input voltage, includes a step-down circuit, a step-up circuit, a voltage determiner, a voltage output unit, and a circuit actuator. The step-down circuit steps down the input voltage to the standard voltage, when the input voltage is larger than the standard voltage. The step-up circuit steps up the input voltage to the standard voltage, when the input voltage is smaller than the standard voltage. The voltage determiner determines whether the input voltage is larger than or smaller than the standard voltage. The voltage output unit receives the input voltage, and outputs the input voltage as a supply voltage when the voltage determiner determines that the input voltage is larger than the standard voltage, and outputs no voltage when the voltage determiner determines that the input voltage is smaller than the standard voltage. And the circuit actuator can detect the supply voltage, actuate the step-down circuit when the supply voltage is detected, and actuate the step-up circuit, when the supply voltage is not detected.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to voltage control in a power circuit, especially to voltage control in a power circuit for supplying electric power to an electronic machine by utilizing voltages from conventional power sources for generating low and high voltages.  
         [0003]     2. Description of the Related Art  
         [0004]     Some electronic machines can use a plurality of power sources. In such electric machines, it is necessary to change various voltages to a constant voltage, and to use the constant voltage. Therefore, in such electronic machines, step-down power circuits for stepping down the voltage from power sources, and step-up power circuits for stepping up the voltage from power sources, are used together.  
         [0005]     A conventional power circuit that applies a constant voltage, generally includes both a step-up circuit for stepping up a voltage from a power source, being lower than an output voltage, and a step-down circuit for stepping down a voltage from another power source, being higher than an output voltage. In such power circuits, energy loss tends to be large mainly because of heat loss caused by a series regulators used for stepping down voltage in a step-down circuit.  
         [0006]     On the other hand, when using only one of a step-up and step-down power circuit, some problems regarding power supply may occur. For example, in a case where only a step-down power circuit and a battery that generates an input voltage higher than an output voltage are used, when the input voltage becomes lower than the output voltage, as a result of the input voltage dropping due to long time usage of the battery, an electric machine having the step-down power circuit may stop suddenly. This is because the input voltage can not be recovered to an acceptable level, due to the absence of a step-up circuit.  
       SUMMARY OF THE INVENTION  
       [0007]     Therefore, an object of the present invention is to provide a power circuit that can effectively apply a constant voltage by selectively operating a step-up circuit and a step down circuit according to the amount of the power source voltage.  
         [0008]     A power circuit according to the present invention, is for generating a standard voltage based on an input voltage. The power circuit includes a step-down circuit, a step-up circuit, a voltage determiner, a voltage output unit, and a circuit actuator. The step-down circuit steps down the input voltage to the standard voltage, when the input voltage is larger than the standard voltage. The step-up circuit steps up the input voltage to the standard voltage, when the input voltage is smaller than the standard voltage. The voltage determiner determines whether the input voltage is larger than or smaller than the standard voltage. The voltage output unit receives the input voltage, and outputs the input voltage as a supply voltage when the voltage determiner determines that the input voltage is larger than the standard voltage, and outputs no voltage when the voltage determiner determines that the input voltage is smaller than the standard voltage. The circuit actuator can detect the supply voltage, and actuates the step-down circuit when the supply voltage is detected, and that actuates the step-up circuit, when the supply voltage is not detected.  
         [0009]     The circuit actuator may comprise a first voltage applier that applies no voltage to the step-up circuit when the voltage determiner determines the input voltage is larger than the standard voltage, and that applies the input voltage to the step-up circuit when the voltage determiner determines that the input voltage is smaller than the standard voltage. The-first voltage applier may comprise an N-channel FET.  
         [0010]     The power circuit can further comprise a voltage controller that applies no voltage to the first voltage applier when the supply voltage is provided to the voltage controller, and applies voltage to the first voltage applier when the supply voltage is not provided to the voltage controller, so that the voltage controller controls the voltage applied to the step-up circuit by the first voltage applier. The voltage controller may comprise first and second transistors connected to each other.  
         [0011]     The power circuit can further comprise a switch for starting the power circuit, and the standard voltage can be applied to the first transistor when the switch is turned on.  
         [0012]     A constant voltage can always be applied to the second transistor by an external power source, while the power circuit is working. And the voltage controller can apply a constant voltage to the first voltage applier when the input voltage is not applied to the voltage controller.  
         [0013]     The circuit actuator can further comprise a second voltage applier that applies no voltage to the step-up circuit when the supply voltage is applied to the second voltage applier, and that applies the standard voltage to the step-up circuit when the supply voltage is not applied to the second voltage applier. The second voltage applier can further comprise a third transistor connected between the voltage output unit and the step-up circuit.  
         [0014]     The power circuit can further comprise a switch connected to the third transistor and the step-up circuit for starting the power circuit. And the standard voltage can be applied to the third transistor and the step-up circuit when the switch is turned on. The third transistor may be connected to a ground.  
         [0015]     The power circuit can further comprise a third voltage applier that applies the supply voltage to the step-down circuit when the supply voltage is applied to the third voltage applier, and that applies no voltage to the step-down circuit when the supply voltage is not applied to the third voltage applier.  
         [0016]     The third voltage applier can comprise fourth and fifth transistors connected to each other.  
         [0017]     The power circuit can further comprise a switch for starting the power circuit, and the standard voltage is applied to the fourth transistor when the switch is turned on.  
         [0018]     The fifth transistor may apply the supply voltage to the step-down circuit when the supply voltage is applied to the fifth transistor, and apply no voltage to the step-down circuit when the supply voltage is not applied to the fifth transistor.  
         [0019]     The circuit actuator can comprise a first voltage applier, a voltage controller, a second voltage applier and a third voltage applier, and a switch for applying the standard voltage to the circuit actuator.  
         [0020]     The first voltage applier applies no voltage to the step-up circuit when the voltage determiner determines the input voltage is larger than the standard voltage, and applies the input voltage to the step-up circuit when the voltage determiner determines the input voltage is smaller than the standard voltage.  
         [0021]     The voltage controller applies no voltage to the first voltage applier when the supply voltage is provided to the voltage controller, and applies voltage to the first voltage applier when the supply voltage is not provided to the voltage controller, so that the voltage controller controls the voltage applied to the step-up circuit by the first voltage applier.  
         [0022]     The second voltage applier applies no voltage to the step-up circuit when the supply voltage is applied to the second voltage applier, and applies the standard voltage to the step-up circuit when the supply voltage is not applied to the second voltage applier.  
         [0023]     The third voltage applier applies the supply voltage to the step-down circuit when the supply voltage is applied to the third voltage applier, and applies no voltage to the step-down circuit when the supply voltage is not applied to the third voltage applier.  
         [0024]     And the voltage controller, the second voltage applier, and the third voltage applier are respectively connected to the voltage output unit and connected to an output terminal of the step-down circuit and an output terminal of the step-up circuit. When the switch is on, the standard voltage is applied to the circuit actuator so that the voltage controller, the second voltage applier, and the third voltage applier work in accordance with the supply voltage output by the voltage output unit, and when the switch is off, the standard voltage is not applied to the circuit actuator so that the voltage controller, the second voltage applier, and the third voltage applier do not work regardless of the supply voltage output by the voltage output unit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The present invention will be better understood from the description of the preferred embodiment of the invention set forth below together with the accompanying drawings, in which:  
         [0026]      FIG. 1  is a block diagram of the power circuit of the embodiment of the present invention;  
         [0027]      FIG. 2  is a timing chart representing change of voltages at each terminal when an enable switch in the power circuit is in the on state; and  
         [0028]      FIG. 3  is a timing chart representing change of voltages at each terminal when the enable switch is in the off state. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     Hereinafter, the preferred embodiment of the present invention is described with reference to the attached drawing.  
         [0030]      FIG. 1  is a block diagram of the power circuit of the embodiment of the present invention.  FIG. 2  and  FIG. 3  are timing charts representing change of voltages at each terminal when an enable switch in the power circuit is in the on state and off state, respectively.  
         [0031]     A power circuit  10  is provided in a digital camera (not shown), and has a battery  14 , first to third digital transistors  16  to  18 , an N-channel MOSFET  20 , a step-up circuit  40 , a step-down circuit  50 , and a voltage detecting device  60  (see  FIG. 1 ). Although the battery  14  usually inputs a voltage, when a USB power source  13  is connected to the power circuit  10  via the USB terminal  12 , the USB power source  13  inputs a voltage to the power circuit  10  instead of the battery  14 . The input voltages of the battery  14  and the USB power source  13  are 1.8(V) and 5(V) respectively.  
         [0032]     The input voltage from the USB power source  13  and the battery  14  is applied to the step-up circuit  40  or the step-down circuit  50 , via the first to third digital transistors  16  to  18 , the N-channel MOSFET  20 , and the voltage detecting device  60 , or it is applied directly. The power circuit  10  controls the voltages in both cases, that is in the case where the USB power source  13  inputs voltage and in the other case where the battery  14  does, so that a constant voltage (a standard voltage) of 3.3(V) is output from an output terminal  24 , regardless of the input voltages. Note that a motor driving power circuit  70  provided in the digital camera as well as the power circuit  10 , inputs a constant voltage of 5(V) to the power circuit  10 .  
         [0033]     In the power circuit  10 , an enable switch  26  is provided. When the enable switch  26  is turned on by a user, a pull-up voltage of the standard voltage output from the output terminal  24 , is applied to the first to third digital transistors  16  to  18  (see T 0  in  FIG. 2 ). However, when no voltage is output from the output terminal  24 , another pull-up voltage from the USB power source  13  or the battery  14 , is applied to the first to third digital transistors  16  to  18 .  
         [0034]     When the enable switch  26  is on, the USB power source  13  is connected to the power circuit  10  and a voltage of 5(V) being larger than the standard voltage is provided to the power circuit  10 , so that the power circuit  10  is in the “high-voltage controlling state (I)” (see  FIG. 2 ). In the “high-voltage controlling state (I)”, only the step-down circuit  50  functions. On the other hand, when the enable switch  26  is on and a voltage of 1.8(V) being smaller than the standard voltage is provided to the power circuit  10  by the battery  14 , the power circuit  10  changes to the “low-voltage controlling state (II)” (see  FIG. 2 ). And then, in this case, only the step-up circuit  40  works.  
         [0035]     When the enable switch  26  is off, the output voltage from the output terminal  24  is 0(V), in both a “high-voltage state (III)” where a 5(V) voltage is applied to the power circuit  10  by the USB power source  13 , and a “low-voltage state (IV)” where a 1.8(V) voltage is applied by the battery  14  (see  FIG. 3 ). Operations of the power circuit  10  in each of these states, are explained below.  
         [0036]     In the “high-voltage controlling state (I)”, the enable switch  26  is on, and the USB power circuit  13  applies a 5(V) voltage to the voltage detecting device  60 , the step-down circuit  50 , and the N-channel MOSFET  20  (T 1 ). In the voltage detecting device  60 , a voltage detecting unit  62  is provided. When the input voltage applied to an input terminal V in  of the voltage detecting unit  62  is determined as being larger than the standard voltage of 3.3(V), the voltage detecting unit  62  outputs the input voltage as a supply voltage from an output terminal V out . And when the input voltage applied to the input terminal V in  is determined as being smaller than the standard voltage of 3.3(V), the voltage detecting unit  62  outputs no voltage from the output terminal V out . Therefore, when a 5(V) voltage is applied to the voltage detecting device  60 , the supply voltage of 5(V) is output from the output terminal V out  (T 2 ).  
         [0037]     The supply voltage of 5(V) output from the output terminal V out , is applied to a terminal A 1  of the first digital transistor  16  (T 3 ). Here, a voltage of 3.3(V) is applied to a terminal A 2  of the first digital transistor  16  (T 4 ), because the enable switch  26  is on, as mentioned above. As a result of this, a collector current does not flow in a first transistor  27  included in the first digital transistor  16 , because a voltage in the reverse direction is applied between a base and an emitter of the first transistor  27 . Therefore, the second transistor  28  changes to the off state and no voltage is output from a terminal A 3 , because a base current does not flow to a second transistor  28  included in the first digital transistor  16  (T 5 ).  
         [0038]     The N-channel MOSFET  20  has a terminal B 1  as a gate, a terminal B 2  as a source, and a terminal B 3  as a drain. The gate terminal B 1  is connected to the terminal A 3  of the first digital transistor  16 , in this case, no voltage is applied to the terminal B 1  (T 6 ). Therefore, the N-channel MOSFET  20  is in the off state, and no voltage is output from the terminal B 2  of a source (T 8 ), although the 5(V) voltage from the USB power source  13  is applied to the terminal B 3  of a drain (T 7 ).  
         [0039]     On the other hand, 3.3(V) is applied to a terminal C 2  of the second digital transistor  17 , because the enable switch  26  is on (T 9 ). In addition to this, a voltage of 5(V) is applied to a terminal C 4  of the second digital transistor  17  from the output terminal V out  of the voltage detecting unit  62  (T 10 ). As a result, collector current flows to a third transistor  29  of the second digital transistor  17  via a terminal C 6 , and base current flows to a fourth transistor  30  of the second digital transistor  17  via a terminal C 5 . And then, since voltage is applied between the base and the emitter of the fourth transistor  30  in a forward direction, the fourth transistor  30  becomes on and 5(V) is applied to the step down circuit  50  via a terminal C 3  (T 11 ).  
         [0040]     The step step-down circuit  50  has a step-down DC converter  52  and a dual MOSFET  54  including two P-channel MOSFETs. A first P-channel MOSFET included in the dual MOSFET  54 , has a first gate terminal G 1 , a first source terminal S 1 , and a first drain terminal D 1 , and a second P-channel MOSFET has a second gate terminal G 2 , a second source terminal S 2 , and a second drain terminal D 2 . When 5(V) from the terminal C 3  of the second digital transistor  17  is applied to a terminal CE of the step-down DC converter  52 , and a voltage from the USB power source  13  is applied to a terminal VDD, a control pulse having an amplitude of 5(V) is provided to the first and second gate terminals G 1  and G 2  from a terminal EXT.  
         [0041]     In the dual MOSFET  54 , 5(V) is applied to the first source terminal S 1  from the USB power source  13 , and then, drain current flows in the first P-channel MOSFET. Current flows to a first capacitor  36  in the step-up circuit  40  from the second source terminal S 2 , and the first capacitor  36  is charged, because the drain current flows to the second drain terminal D 2  of the second P-channel MOSFET. Note that current flowing to the first capacitor  36  from the second source terminal S 2 , is smoothed by a first schottky diode  51  and a first coil  34 .  
         [0042]     Voltage generated by an electric charge at the first capacitor  36 , is output from the output terminal  24  after being smoothed by a second coil  53  and a second capacitor  41 . A feedback terminal FB of the step-down converter  52  detects a divided voltage of the first capacitor  36  divided by a first and second resistors  47  and  48 . A duty ratio of the voltage applied to the first and second gate terminals G 1  and G 2  of the dual MOSFET  54  from the terminal EXT of the step-down DC converter  52  is modulated, so that the output voltage (standard voltage) output by the output terminal  24  becomes a constant voltage of 3.3(V). Note that the reason the dual MOSFET  54  has the first and second P-channel MOSFETs, is to prevent the reverse flow of current from the step-up circuit  40 .  
         [0043]     A voltage of 5(V) is applied to a terminal D 1  of the third digital transistor  18  from the output terminal V out  of the voltage detecting unit  62 , as well as the terminal C 4  of the second digital transistor  17  (T 12 ). Therefore, base current flows to a fifth transistor  31  of the third digital transistor  18 , collector current also flows because the emitter is connected to the ground  32 , and a terminal D 3  of the third digital transistor  18  is shorted to the ground  32  (T 13 ). That is, no voltage is applied to the step-up circuit  40  because the third digital transistor  18  is in the on state, so that the step-up circuit  40  does not work in the “high-voltage controlling state (I)”.  
         [0044]     Summarizing the above description, in the “high-voltage controlling state (I)”, a step-up DC converter  42  and a MOSFET  44  in the step-up circuit  40  do not work, and the voltage of 5(V) input to the power circuit  10  is stepped down by the step-down circuit  50 , so that the constant voltage of 3.3(V) is output by the output terminal  24  (T 14 ).  
         [0045]     When the power source is switched from the USB power source  13  to the battery  14 , the state is changed from the “high-voltage controlling state (I)” to the “low-voltage controlling state (II)”. In the “low-voltage controlling state (II)”, a voltage of 1.8 (V) (T 15 ) is applied to the voltage detecting device  60 , the step-down circuit  50 , and the N-channel MOSFET  20 . When 1.8 (V) (being smaller than 3.3 (V)) is applied to the input terminal V in  of the voltage detecting unit  62 , no voltage is output from the output terminal V out  (T 16 ) as mentioned above, and no voltage is applied to the terminal A 1  of the first digital transistor  16  (T 17 ).  
         [0046]     The enable switch  26  is on at this time, and then 3.3 (V) is applied to the terminal A 2  of the first digital transistor  16  (T 4 ). As a result of this, in the first transistor  27 , a voltage is applied between the base and the emitter in the reverse direction, and the collector current flows. Base current flows at the second transistor  28  because of the collector current at the first transistor  27 , and 5(V) is applied to a terminal A 4  of the first transistor  16  from the motor driving power circuit  70 , so that a voltage of 5(V) is output from the terminal A 3  of the third transistor  28  (T 18 ).  
         [0047]     Therefore, 5(V) is applied to the gate terminal B 1  of the N-channel MOSFET  20  (T 19 ), and 1.8(V) from the battery  14  is also applied to the drain terminal B 3  (T 20 ), so that 1.8 (V) is applied to a third capacitor  43  of the step-up circuit  40  (T 21 ) from the source terminal B 2 . That is, the N-channel MOSFET  20  turns on. The N-channel MOSFET  20  can efficiently apply the power source voltage from the battery  14  of the primary power source, to the step-up circuit  40 , because the resistance of the N-channel MOSFET  20  is quite low, and the gate voltage is also low, 1.8 (V).  
         [0048]     On the other hand, although 3.3 (V) is applied to the terminal C 2  of the second digital transistor  17  (T 9 ), no voltage is applied to the terminal C 4  from the output terminal V out  of the voltage detecting unit  62  (T 22 ). As a result of this, collector current does not flow to the fourth transistor  30 , and no voltage is applied to the step-down circuit  50  from the terminal C 3  (T 23 ). That is, the second digital transistor  17  turns off, and the step-down circuit  50  does not work in the “low-voltage controlling state (II)”.  
         [0049]     Voltage at the terminal D 1  of the third digital transistor  18  is 0 (V) at this time (T 24 ), because no voltage is applied from the output terminal V out  of the voltage detecting unit  62  (T 16 ). As a result of this, the third digital transistor  18  turns off, and voltage at the terminal D 3  becomes 3.3 (V), because of the on state of the enable switch  26  and a pull-up by a third resistor  49  (T 25 ). And then, 3.3 (V) is applied to the step-up DC converter  42  of the step-up circuit  40 .  
         [0050]     When the 3.3 (V) is applied to a terminal CE of the step-up DC converter  42 , and voltage from the motor driving power circuit  70  is applied to a terminal VDD, the step-up DC converter  42  outputs a control pulse having an amplitude of 5(V) to a gate terminal G of the MOSFET  44  from the terminal EXT. The MOSFET  44  is an N-channel MOSFET having four drain terminals, these are a first to a fourth terminals D 1  to D 4 , for efficiently releasing heat, and when voltage is applied to the gate terminal G, current flows from the first to the fourth terminals D 1  to D 4 . When voltage is applied to the gate terminal G of the MOSFET  44  from the terminal EXT of the step-down DC converter  52 , current flows to a ground GND from a second coil  45 , via the drain and the source. On the other hand when voltage is not applied to the MOSFET  44  from the terminal EXT of the step-down DC converter  52 , electric power stored in the second coil  45  is provided to the first capacitor  36 , via a second schottky diode  55 . As a result of this, the first capacitor  36  is charged.  
         [0051]     Note that a feedback terminal FB of the step-up converter  42  detects a divided voltage of the voltage at the first capacitor  36  divided by a first and second resistor  47  and  48 , and modulates a duty ratio of the voltage applied to the gate terminal G of the MOSFET  44  from the terminal EXT of the step-up DC converter  42 , so that the voltage smoothed by the second coil  53  and the second capacitor  41 , becomes a constant 3.3 (V).  
         [0052]     Summarizing the above description, when a voltage of 1.8(V) is input to the power circuit  10 , the step-down circuit  50  does not work, and the voltage of 1.8(V) input to the power circuit  10  is stepped up by the step-up circuit  40  in the “low-voltage controlling state (II)”, so that the voltage of 3.3 (V) is output from the output terminal  24  (T 14 ).  
         [0053]     In the “high-voltage state (III)” where a voltage is applied to the power circuit  10  by the USB power source  13 , and the enable switch  26  is off (see  FIG. 3 ), no voltage is applied to the first and second digital transistors  16  and  17  (T 26 ) because there is no pull-up. A voltage of 5 (V) from the USB power source  13  (T 27 ) is applied to the voltage detecting device  60 , the step-down circuit  50 , and the N-channel MOSFET  20 . And then, as the voltage applied to the input terminal V in  of the voltage detecting unit  62  is 5 (V), being larger than the standard voltage of 3.3(V), a voltage of 5 (V) is output from the output terminal V out  (T 28 ).  
         [0054]     Although the voltage of 5 (V) output from the voltage detecting device  60  is applied to the terminal A 1  of the first digital transistor  16  (T 29 ), no voltage is applied to the terminal A 2  of the first digital transistor  16  (T 30 ), because the enable switch  26  is off. As a result, in the first transistor  27  included in the first digital transistor  16 , a voltage is applied between the base and the emitter in the reverse direction, and the collector current does not flow. Therefore, in the second transistor  28  of the first digital transistor  16 , base current does not flow, the first digital transistor  16  turns off, and no voltage is output from the terminal A 3  (T 31 ).  
         [0055]     Accordingly, in the N-channel MOSFET  20 , no voltage is applied to the gate terminal B 1  (T 32 ), and 5(V) is applied to the drain terminal B 3  (T 33 ), so that no voltage is output from the source terminal B 2  (T 34 ).  
         [0056]     On the other hand, no voltage is applied to the terminal C 2  of the second digital transistor  17  because the enable switch  26  is off (T 35 ), and collector current does not flow to the third transistor  29 . Although 5 (V) is applied to the terminal C 4  from the output terminal V out  (T 36 ), collector current does not flow to the third transistor  29 , so that base current does not flow to the fourth transistor  30 , and then the fourth transistor  30  turns off. As a result of this, no voltage is applied to the step-down circuit  50  from the terminal C 3  (T 37 ). Therefore, the step-down circuit  50  does not function in the “high-voltage state (III)”.  
         [0057]     A voltage of 5 (V) is applied to the terminal D 1  of the third digital transistor  18  from the output terminal V out  (T 38 ). And then, because the emitter of the third digital transistor  18  is connected to the ground  32 , the terminal D 3  is shorted to the GND, the same as in the “high-voltage controlling state (I)”. Further, because the enable switch  26  is off, pull up does not occur and voltage at the terminal D 3  becomes GND level (T 39 ), so that the step-up circuit  40  does not work.  
         [0058]     As mentioned above, although the voltage of 5 (V) is applied to the power circuit  10 , no voltage is output from the output terminal  24  (T 40 ) in the “high-voltage state (III)”. This is because voltage is not applied to the step-up circuit  40  and to the step-down circuit  50 .  
         [0059]     On the other hand, when the state has changed to the “low-voltage state (IV)” where the battery  14  applies voltage to the power circuit  10  and the enable switch  26  is off, no voltage is applied to the first and second digital transistors  16  and  17  (T 26 ) because the enable switch  26  is off. At this time, because the voltage from the battery  14  is 1.8 (V) (T 41 ), being smaller than the standard voltage of 3.3 (V), no voltage is applied by the voltage detecting device  60  (T 42 ). Therefore, no voltage is applied to the terminal B 1  of the N-channel MOSFET  20  from the terminal A 3  (T 32 ), and no voltage is output from the terminal B 2  of the N-channel MOSFET  20  (T 34 ).  
         [0060]     No voltage is applied to the terminal C 2  because the enable switch  26  is off (T 35 ), and voltage is not applied to the terminal C 4  by the output terminal V out  (T 43 ). Therefore, no voltage is applied to the step-down circuit  50  from the terminal C 3  of the second digital transistor  17  (T 37 ), so that the step-down circuit  50  does not work in the “low-voltage state (IV)”.  
         [0061]     And then, because no voltage is applied to the terminal D 1  of the third digital transistor  18  from the output terminal V out  (T 44 ) and the enable switch is off, the third digital transistor  18  is off. Further, because the enable switch  26  is off, pull up does not occur and voltage at the terminal D 3  becomes GND level (T 39 ), so that the step-up circuit  40  does not work.  
         [0062]     As mentioned above, although the voltage of 1.8 (V) is applied to the power circuit  10 , no voltage is output from the output terminal  24  (T 40 ) in the “low-voltage state (IV)”. This is because voltage is not applied to the step-up circuit  40 , nor to the step-down circuit  50 .  
         [0063]     Note that the power circuit  10  can be configured by conventional elements. For example, the first and second digital transistors  16  and  17  can be “EMD6” produced by ROHM CO., LTD., the third digital transistor  18  can be “DTG124EM” produced by ROHM CO., LTD., and the N-channel MOSFET  20  can be “Si2312DS” produced by VISHAY SILICONIX, and so on. Further, the voltage detecting unit  62  can be “XC61CC3302” produced by TOREX SEMICONDUCTOR LTD., the step-down DC converter  52  can be “XC6366D105MR” produced by TOREX SEMICONDUCTOR LTD., the dual MOSFET  54  can be “Si1903DL” produced by VISHAY SILICONIX, the step-up DC converter  42  can be “XC6368D105MR” produced by TOREX SEMICONDUCTOR LTD., and the MOSFET  44  can be “Si1406DH” produced by VISHAY SILICONIX.  
         [0064]     As mentioned above, in this embodiment, the power circuit  10  that can effectively provide a constant voltage, is provided by making one of the step-up circuit  40  and the step-down circuit  50  selectively operate and making the other not operate according to the input voltage from the power sources, and by using the first to third digital transistor  16   s  to  18 , the N-channel MOSFET  20 , and the voltage detecting device  60 . And the power circuit  10  does not output voltage regardless of the input voltage from the power sources, when the enable switch  26  is off, because the first and second digital transistors  16  and  17  are off, and both the step-up circuit  40  and step-down circuit  50  are non-operational.  
         [0065]     The amount of voltage applied to the power circuit  10  from the power sources such as the USB power source  13  and the battery  14 , are not limited to those of this embodiment, as long as one is larger than the output voltage from the power circuit  10 , and the other is smaller than that. That is, as long as the USB power source  13  inputs voltage larger than 3.3 (V), and the battery  14  outputs voltage smaller than 3.3 (V), any voltage amount can be input. Further, by changing the design of the step-up circuit  40  and the step-down circuit  50 , the amount of the output voltage from the output terminal V out  can be adjusted.  
         [0066]     The power sources are not limited to the USB power source  13  and the battery  14 . For example, another battery that can apply voltage larger than the standard voltage, can be used instead of the USB power source  13 . In this case, when the voltage input by the battery becomes lower than the output voltage, as a result of a gradual drop of the input voltage caused by long time usage of the battery, outputting the constant voltage is still possible. This is because the step-up circuit  40  automatically operates instead of the step-down circuit  50 .  
         [0067]     Both of the USB power source  13  and the battery  14  can be jointly used. In this case, the amount of the input voltage becomes that between both power sources, and one of the step-up circuit  40  and the step-down circuit  50  is automatically selected to change the input voltage, so that a constant voltage can be output.  
         [0068]     Finally, it will be understood by those skilled in the art that the foregoing description is of a preferred embodiment of the apparatus, and that various changes and modifications may be made to the present invention without departing from the scope thereof.  
         [0069]     The present disclosure relates to subject matter contained in Japanese Patent Application No. 2004-125829 (filed on Apr. 21, 2004) which is expressly incorporated herein, by reference, in its entirety.