Abstract:
A constant voltage circuit configured to convert an input voltage into an output voltage having a predetermined level is disclosed. The constant voltage circuit includes a differential amplifier circuit configured to produce an output signal having a voltage level in response to a reference voltage and the output voltage; and an output circuit configured to receive the output signal and produce a current in response to the voltage level of the output signal. The output voltage is proportional to the current. The output circuit includes plural output transistors and a transistor selecting unit configured to select one or more output transistors to be operated among the plural output transistors to produce the current depending on the level of the output voltage.

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates to a constant voltage circuit, and more particularly to a constant voltage circuit capable of making a quick response to a wide range of output currents such as a minute current and a large current, and capable of stable operation with high efficiency. 
     2. Description of the Related Art 
     In electronic devices such as portable phones, mobile PCs, and car navigation systems, a constant voltage power source having a constant voltage circuit and capable of supplying a stable voltage is used as a power source. When using such a constant voltage power source in a device with a large output current, the constant voltage power source is required to have a circuit configured to realize a high speed response by improving a ripple removing ratio and a load transient response. For example, when the constant voltage power source is used in a device with a wide range of output current, such as a portable phone having an operation mode and a standby mode, a circuit configuration capable of receiving a maximum output current is required. As a result, a current consumption is increased as a whole. In the standby mode of the portable phone, in which a high ripple removing ratio and a high load transient response are not required, an unnecessary current is consumed, which results in increasing the wasted current. In view of this, a constant voltage circuit for suppressing this wasted power consumption has been suggested. 
     Each of Patent Documents 1 and 2 discloses a constant voltage circuit configured to increase or decrease a bias current supplied to a differential amplifier in the constant voltage circuit depending on the amount of output current. 
       FIG. 8  shows the constant voltage circuit disclosed in Patent Document 1. In  FIG. 8 , a constant voltage circuit  101  includes a reference voltage circuit Vref, a differential amplifier circuit  102 , a bias current generating circuit  103 , and an output circuit  104 . 
     In this circuit, a PMOS transistor M 7  and an output transistor M 1  form a current mirror circuit. Therefore, a drain current in proportion to a drain current (output current) of the output transistor M 1  is generated in the PMOS transistor M 7 . This current is supplied as a drain current of an NMOS transistor M 8 . Since the NMOS transistor M 8  and an NMOS transistor M 9  form a current mirror circuit, a drain current of the NMOS transistor M 9  is in proportion to the drain current of the output transistor M 1 . The drain current of the NMOS transistor M 9  is a part of a bias current of the differential amplifier circuit  102 , therefore, the bias current of the differential amplifier circuit  102  increases and decreases in accordance with an increase and a decrease of the output current. 
     In this manner, the bias current of the differential amplifier circuit  102  is increased and decreased in accordance with the increase and decrease of the output current. Therefore, a response speed is increased when the output current is increased. In this manner, the current consumption and the response speed are set appropriately. 
     [Patent Document 1] Japanese Patent Application Publication No. 3-158912 
     [Patent Document 2] Japanese Patent Application Publication No. 2006-99526 
     In the constant voltage circuits configured to change the bias current of the differential amplifier circuit in accordance with the output current as disclosed in Patent Documents 1 and 2, an operation of the constant voltage circuit becomes unstable when the output current is small. That is, for example, a constant voltage power source having a large output transistor a capable of outputting an output current of 1 A or more can be stably operated when the output current is large. However, this constant voltage power source cannot be stably operated when the output current is small since a bias current of a differential amplifier circuit becomes small and a phase margin is decreased. Moreover, there is a problem in that a response speed is extremely low when the bias current is small. This is because a transistor having a large ratio of gate width to gate length and thus having large gate capacitance is used as an output transistor to realize an operation with a large current. When a bias current is small, it takes time to charge and discharge the gate capacitance. Therefore, the response speed is drastically decreased when the output current is small. 
     SUMMARY 
     The present invention is made in view of the aforementioned circumstances and it is an object of at least one embodiment of the present invention to provide In an aspect of this disclosure, there is provided a constant voltage circuit of which response speed is not decreased when an output current is small and which operates stably with a wide range of amounts of output current. 
     According to another aspect, a constant voltage circuit configured to convert an input voltage into an output voltage having a predetermined level includes a differential amplifier circuit configured to produce an output signal having a voltage level in response to a reference voltage and the output voltage; and an output circuit configured to receive the output signal and produce a current in response to the voltage level of the output signal. The output voltage is proportional to the current. The output circuit includes plural output transistors and a transistor selecting unit configured to select one or more output transistors to be operated among the plural output transistors to produce the current depending on the level of the output voltage. 
     According to another aspect, a constant voltage circuit configured to convert an input voltage into an output voltage having a predetermined level includes a differential amplifier circuit configured to produce an output signal having a voltage level determined in response to a reference voltage and the output voltage; an output circuit including at least a first transistor, a second transistor, and a first switching unit configured to supply a current supplied by the second transistor to be added to a current supplied by the first transistor to produce a combined current or block the current supplied by the second transistor depending on the voltage level of the output signal of the differential amplifier circuit. The output circuit is configured to produce an output current of the current supplied by the first transistor or the combined current in response to the voltage level of the output signal of the differential amplifier circuit. The output voltage is proportional to the output current. The constant voltage circuit further includes a bias current supply circuit including at least a first current source, a second current source, and a second switching unit configured to supply a current supplied by the second current source to be added to a current supplied by the first current source to produce a combined current source or block the current supplied by the second current source. The bias current supply circuit is configured to supply a bias current of the first current source or the combined current source to the differential amplifier circuit. The constant voltage circuit further includes a determination circuit configured to control switching of the second switching unit depending on the voltage level of the output signal of the differential amplifier circuit. 
     According to another aspect, a constant voltage circuit configured to convert an input voltage into an output voltage having a predetermined level includes a voltage input terminal, a voltage output terminal, a constant voltage circuit unit, and a determination circuit unit. The constant voltage circuit unit includes an output circuit and a differential amplifier circuit. The output circuit includes a first transistor, a second transistor, a first switching unit, and a sixth switching unit. The first and second transistors have sources connected together to the voltage input terminal, drains connected together to the voltage output terminal, and gates connected to each other through the first switching unit. The gate of the second transistor is connected to the voltage input terminal through the sixth switching unit. The differential amplifier circuit has a non-inverting input terminal receiving a first reference voltage, an inverting input terminal receiving a divided voltage of the output voltage, an output terminal connected to the gate of the first transistor. A first current source and a second current source are connected in parallel to each other as bias current supply sources, and a second switching unit is connected between the first and second current sources. The determination circuit unit includes a current supply circuit and a comparator. The current supply circuit includes a third transistor and a fourth transistor having sources connected together to the voltage input terminal, drains connected to each other, and gates connected to the gates of the first and second transistors respectively. The determination circuit unit further includes a third current source, a third switching unit, and a fourth switching unit which are connected in series between the sources and drains of the third and fourth transistors in parallel to the third and fourth transistors. The comparator has a non-inverting input terminal connected to the drains of the third and fourth transistors which are connected together and an inverting input terminal to which one of a second reference voltage and a third reference voltage is selectively connected through a fifth switching unit, and an output terminal to output an output signal. The first, second, and sixth switching units in the constant voltage circuit unit and the third to fifth switching units in the determination circuit unit are controlled by the output signal of the comparator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a constant voltage circuit of an embodiment of the present invention; 
         FIG. 2  is a timing chart of an operation of a major part in the constant voltage circuit shown in  FIG. 1 ; 
         FIG. 3  is a constant voltage circuit of a second embodiment of the present invention; 
         FIG. 4  is a timing chart of an operation of a major part in the constant voltage circuit shown in  FIG. 3 ; 
         FIG. 5  is a diagram showing a detail of a comparator  12  shown in  FIG. 1 ; 
         FIG. 6  shows an integrated stabilizing power source circuit provided in a periphery of a first output transistor M 1  and a second output transistor M 2  in the constant voltage circuit shown in  FIG. 1 ; 
         FIG. 7  shows a small signal equivalent circuit of an area  20  shown in  FIG. 3 ; and 
         FIG. 8  shows a conventional constant voltage circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention are described with reference to the drawings. 
       FIG. 1  shows a constant voltage circuit of an embodiment of the present invention. This constant voltage circuit includes a constant voltage circuit unit  1  and a determination circuit unit  2 . The constant voltage circuit unit  1  outputs a predetermined constant voltage output Vout from a voltage output terminal in response to an input voltage Vdd inputted from a voltage input terminal. The determination circuit unit  2  monitors an output current of the constant voltage circuit unit  1  and compares the output current with a predetermined value. The determination circuit unit  2  then transmits a comparison result to the constant voltage circuit unit  1 , thereby switches S 1 , S 2 , and S 6  of the constant voltage circuit unit  1  are controlled. 
     The constant voltage circuit unit  1  includes a reference voltage source Vr, a differential amplifier  11 , bias current sources I 1  and I 2 , a first output transistor M 1 , a second output transistor M 2 , resistors R 1  to R 3 , the switches S 1 , S 2 , and S 6 , an input terminal Vdd, and an output terminal Vout. Further, the determination circuit unit  2  includes PMOS transistors M 3  and M 4 , a comparator  12 , a first reference voltage source Va 1 , a second reference voltage source Va 2 , inverters  13  to  19 , current sources I 3  to I 6 , capacitors C 1  to C 3 , a resistor R 4 , and switches S 3  to S 5 . 
     In this configuration, an outline of the constant voltage circuit is described. 
     In the constant voltage circuit unit  1 , a current of the current source I 1  is always applied as a bias current of the differential amplifier  11 . When a load current of the constant voltage circuit is increased, that is when an output current is increased, the switch S 2  is turned on, thereby a current of the current source I 2  is additionally supplied to the current of the current source I 1  as a bias current of the differential amplifier  11 . In this manner, when the output current is small, only the current source I 1  is used. When the output current is large, on the other hand, the currents of the current sources I 1  and I 2  are used as a bias current of the differential amplifier  11 . Similarly, as for the output transistors, the first output transistor M 1  is always used. On the other hand, the second output transistor M 2  is used only when the output current is large. That is, when the output current is small, only the first output transistor M 1  is used. When the output current becomes large, the switch S 1  is turned on while the switch S 6  is turned off. In this manner, the first output transistor M 1  and the second output transistor M 2  are both used. 
     Here, the current source I 2  and the second output transistor M 2  are larger in size than the current source I 1  and the first output transistor M 1  respectively. By using the second output transistor M 2 , the circuit may oscillate. In the configuration of the determination circuit unit  2  of this embodiment, oscillation of the circuit is prevented. This prevention of oscillation will be described in detail below. 
     Hereinafter, the embodiment of the present invention is described in detail. 
     In the constant voltage circuit unit  1  shown in  FIG. 1 , the reference voltage Vr is inputted to a non-inverting input terminal of the differential amplifier  11 . A detection voltage Vf obtained by dividing an output voltage Vout by the resistors R 1  and R 2  is inputted to an inverting input terminal of the differential amplifier  11 . The other terminal of the resistor R 2  is connected to ground potential Vss. An output of the differential amplifier  11  is connected to a gate of the first output transistor M 1  formed of a PMOS transistor. The first output transistor M 1  has a source connected to the input terminal Vdd and a drain connected to the output terminal Vout. A source and a drain of the second output transistor M 2  formed of a PMOS transistor are connected to the source and the drain of the first output transistor M 1  respectively. A gate of the second output transistor M 2  is connected to the output of the differential amplifier  11  through the switch S 1 . The gate of the second output transistor M 2  is pulled-up to the input terminal voltage Vdd through the switch S 6  and the resistor R 3 . The current source I 1  capable of always supplying a bias current is connected to the differential amplifier  11  between the input terminal Vdd and the differential amplifier  11 . Moreover, the current source I 2  and the switch S 2  which are connected in series are connected in parallel to the current source I 1 . 
     When the output current is small in the constant voltage circuit unit  1 , the switches S 1  and S 2  are turned off and the switch S 6  is turned on. In addition, only the first output transistor M 1  is operated as an output transistor and only the first current source I 1  is used as a current source. When the output current is large, on the other hand, the switches S 1  and S 2  are turned on and the switch S 6  is turned off. Then, both the first output transistor M 1  and the second output transistor M 2  are operated as output transistors, and the first and second current sources I 1  and I 2  are both operated as current sources. This will be described in detail below. 
     Next, the determination circuit unit  2  is described. 
     A source and a gate of the PMOS transistor M 3  are connected to the source and the gate of the first output transistor M 1  respectively. That is, the source of the PMOS transistor M 3  is connected to the input terminal Vdd. In this manner, the PMOS transistor M 3  and the first output transistor M 1  form a current mirror circuit. The output current is monitored by the PMOS transistor M 3 . Similarly, a source and a gate of the PMOS transistor M 4  are connected to the source and gate of the second output transistor M 2  respectively. The PMOS transistor M 4  and the second output transistor M 2  form a current mirror circuit. Drains of the PMOS transistors M 3  and M 4  are connected together and grounded through the resistor R 4 . The resistor R 4  functions as a current voltage converter capable of converting a drain current of the PMOS transistors M 3  and M 4  into a voltage. As described above, the PMOS transistors M 3  and M 4  form current mirror circuits with the first output transistor M 1  and the second output transistor M 2  respectively. Therefore, the drain current of the PMOS transistors M 3  and M 4  is in proportion to the output current. Since the resistor R 4  converts this current into a voltage, a voltage drop Vb at the resistor R 4  is in proportion to the output current. The switches S 3  and S 4  are connected in series to the current source I 3 . These serially connected switches are connected between the sources and drains of the PMOS transistors M 3  and M 4 . The voltage Vb is inputted to a non-inverting input terminal of the comparator  12 . An inverting input terminal of the comparator  12  is connected to a common terminal c of the switch S 5 . The first reference voltage source Va 1  is connected between a terminal a of the switch S 5  and ground potential Vss. The second reference voltage source Va 2  is connected between a terminal b and ground potential Vss. Here, the second reference voltage source Va 2  is set lower than the first reference voltage Va 1 . An output CMPo of the comparator  12  is connected to inputs of the inverters  13  and  17 , and control terminals of the switches S 3  and S 5 . A capacitor C 1  is connected between an output of the inverter  13  and ground potential Vss and to an input of the inverter  14 . A current source I 4  is connected between a positive side power source terminal of the inverter  13  and the input terminal Vdd. An output A of the inverter  14  is connected to an input of the inverter  15  and a control terminal of the switch S 2  in the constant voltage circuit unit  1 . The capacitor C 2  is connected between an output B of the inverter  15  and ground potential Vss. Moreover, the output B of the inverter  15  is connected to an input of the inverter  16 . The output B of the inverter  15  is connected to a control terminal of the switch S 6  of the constant voltage circuit unit  1 . An output C of the inverter  16  is connected to a control terminal of the switch S 1  in the constant voltage circuit unit  1 . Further, a current source I 5  is connected between a negative side power source terminal of the inverter  15  and ground potential Vss. The capacitor C 3  is connected between an output of the inverter  17  and ground potential Vss. The output of the inverter  17  is connected to an input of the inverter  18 . A current source I 6  is connected between the negative side power source terminal of the inverter  17  and ground potential Vss. An output of the inverter  18  is connected to an input of the inverter  19 . An output D of the inverter  19  is connected to a control terminal of the switch S 4 . 
     Here, the inverters  17  to  19 , the current source I 6 , and the capacitor C 3  form a first delay circuit. By the first delay circuit, the output CMPo of the comparator  12  is delayed for a delay time of Td 3  and transmitted to the control terminal of the switch S 4 . The inverters  13  and  14 , the current source I 4 , the capacitor C 1 , the inverters  15  and  16 , the current source I 5 , and the capacitor C 2  form a second delay circuit. By the second delay circuit, the output CMPo of the comparator  12  is delayed for a delay time Td 1  or Td 2  and transmitted to the switches S 1 , S 2 , and S 6 . These delay times Td 1  to Td 3  are described in detail below. 
     The determination circuit unit  2  determines whether the output current is larger or smaller than a predetermined value. In response to this determination, the switches S 1 , S 2 , and S 6  of the constant voltage circuit unit  1  are controlled and the second output transistor M 2  and the second current source I 2  are turned on or off (these elements are used or not used). 
     The switches S 1  to S 4  and S 6  are turned on when a high level (H-level) signal is inputted to control terminals and turned off when a low level (L-level) signal is inputted to the control terminals. The common terminal c and the terminal a of the switch S 5  are connected when an L-level signal is inputted to a control terminal, and the common terminal c and the terminal b are connected when an H-level signal is inputted to the control terminal. 
       FIG. 2  is a timing chart of an operation of a major part in the constant voltage circuit shown in  FIG. 1 . Part (a) in  FIG. 2  shows changes of a gate voltage Vm 1   g  of the first output transistor M 1  and a gate voltage Vm 2   g  of the second output transistor M 2  with respect to a time t. Part (b) in  FIG. 2  shows changes of the voltage Va of the inverting input terminal and the voltage Vb of the non-inverting input terminal of the comparator  12  with respect to the time t. Part (c) in  FIG. 2  shows changes of the output signal CMPo of the comparator  12 , the output A of the inverter  14 , the output B of the inverter  15 , the output C of the inverter  16 , and the output D of the inverter  19  in  FIG. 1 . 
     Next, an operation of the constant voltage circuit shown in  FIG. 1  is described with reference to  FIG. 2 . 
     In a vertical axis of parts (a) and (b) of  FIG. 2 , Vdd denotes a voltage level of the input terminal voltage, Va 1  denotes a voltage level of the first reference voltage Va 1 , and Va 2  denotes a voltage level of the second reference voltage Va 2 . In a vertical axis of part (c) of  FIG. 2 , CMPo denotes an output signal level of the comparator  12 , A denotes an output signal level of the inverter  14 , B denotes an output signal level of the inverter  15 , C denotes an output signal level of the inverter  16 , and D denotes an output signal level of the inverter  19 . The signals A, B, C, and D correspond to control signals of the switches S 2 , S 6 , S 1 , and S 4 , respectively. 
     In parts (a) to (c) of  FIG. 2 , in the initial state when the output current is 0 A, a current is not supplied to the resistor R 4  since the first output transistor M 1  and the PMOS transistor M 3  form a current mirror circuit. Therefore, a voltage drop is not generated at the resistor R 4 . That is, the voltage Vb of the non-inverting input terminal of the comparator  12  is 0 V. On the other hand, the first reference voltage Va 1  or the second reference voltage Va 2  is applied to the inverting input terminal of the comparator  12 . Therefore, the output CMPo of the comparator  12  is at an L-level. Since the output CMPo of the comparator  12  is at an L-level, the output A of the inverter  14  and the output C of the inverter  16  become L-level. On the other hand, the output B of the inverter  15  and the output D of the inverter  19  become H-level. Therefore, the switches S 1  to S 3  are turned off and the switches S 4  and S 6  are turned on (see  FIG. 2(   c )). The common terminal c of the switch S 5  is connected to the terminal a at this time. Since the switch S 1  is off and the switch S 6  is on, the gate of the second output transistor M 2  is pulled up to the input terminal voltage Vdd by the resistor R 3 . Therefore, the second output transistor M 2  is off. Since the switch S 2  is off, the current source I 1  is supplied as a bias current of the differential amplifier  11 . Since the switch S 3  is off, the current of the current source I 3  is not supplied to the resistor R 4  even when the switch S 4  is on. Further, since the common terminal c of the switch S 5  is connected to the terminal a, the first reference voltage Va 1  is connected to the inverting input terminal of the comparator  12 . 
     In the aforementioned state, the output current is increased. When the output current is increased, the gate voltage Vm 1   g  of the first transistor M 1  is decreased ( FIG. 2(   a )). At the same time, the gate voltage of the PMOS transistor M 3  is decreased. Therefore, the voltage Vb of the non-inverting input terminal of the comparator  12  is increased ( FIG. 2(   b )). However, connection states of the switches are not changed until the output current reaches a value of a predetermined first current value. 
     When the output current reaches the predetermined current value at a time t 1 , the voltage Vb reaches the first reference voltage Va 1  ( FIG. 2(   b )). When the output current is further increased higher than the first current value, the voltage Vb becomes higher than the first reference voltage Va 1 . Therefore, the output CMPo of the comparator  12  is inverted to an H-level ( FIG. 2(   c )). Then, the switch S 3  is turned on, therefore, the current of the current source I 3  is supplied to the resistor R 4 . As a result, the voltage Vb rapidly rises ( FIG. 2(   b )). Note that the current value of the current source I 3  in this embodiment is substantially equal to or larger than the drain current of the PMOS transistor M 3 , which flows when the output current becomes equal to the first current value. As shown in  FIG. 2(   b ), the voltage Vb rises as high as (2×Va 1 ), which is about twice as high as the first reference voltage Va 1  at a timing of the time t 1 . Further, by the inversion of the output CMPo of the comparator  12 , the common terminal c of the switch S 5  is connected to the terminal b. Therefore, the second reference voltage Va 2  is connected to the inverting input terminal of the comparator  12 . Since the second reference voltage Va 2  is set a little lower than the first reference voltage Va 1 , the inverting input terminal voltage Va of the comparator  12  is a little decreased from the voltage level Va 1  to Va 2  as shown in  FIG. 2(   b ). Moreover, since the output of the comparator is at an H-level, an output of the inverter  13  becomes an L-level. Since an output circuit of the inverter  13  has low impedance on a low side, a charge of the capacitor C 1  is discharged instantly. Therefore, since the input of the inverter  14  becomes L-level with little delay, the output A of the inverter  14  changes to an H-level in a moment when the output CMPo of the comparator  12  becomes an H-level ( FIG. 2(   c )). Moreover, when the output A of the inverter  14  becomes an H-level, the switch S 2  is turned on. Therefore, a current value of the current source I 2  is additionally provided to a bias circuit of the differential amplifier  11 . As a result, the operation of the differential amplifier  11  becomes faster. Consequently, the voltage Vm 1   g  drops rapidly after the time t 1 . 
     When the output A of the inverter  14  becomes an H-level, the output B of the inverter  15  changes from an H-level into an L-level. However, since the current source I 5  is inserted between a power source on a negative side of the inverter  15  and ground potential Vss, the charge charged in the capacitor C 2  when the output B of the inverter  15  is at an H-level is discharged through the current source I 5 . Thus, it takes time until the output B of the inverter  15  changes from an H-level to an L-level. This delay time is shown as Td 1  in  FIG. 2(   c ). After the time Td 1 , the output B of the inverter  15  becomes an L-level. When the voltage of the capacitor C 2  becomes as low as or lower than an input threshold voltage of the inverter  16  at a time t 2 , the output C of the inverter  16  becomes an H-level almost at the same timing ( FIG. 2(   c )). Then, the switch S 6  is turned off and the switch S 1  is turned on. Then, the output of the differential amplifier  11  is inputted to the gate of the second output transistor M 2 . Before the switch S 1  is turned on, the gate of the second output transistor M 2  had been pulled-up to the input voltage Vdd by the resistor R 3 . Therefore, the gate voltage Vm 2   g  of the second output transistor M 2  had been the input voltage Vdd ( FIG. 2(   a )). Further, since there is gate capacitance of the second output transistor M 2  between the gate of the second output transistor M 2  and the input terminal Vdd, the output of the differential amplifier  11  momentarily rises as high as the input terminal voltage Vdd right after the switch S 1  is turned on. Therefore, there is a moment when both the first output transistor M 1  and the second output transistor M 2  are turned off. 
     When both the first output transistor M 1  and the second output transistor M 2  are turned off, the PMOS transistors M 3  and M 4  are also turned off. Therefore, only a current of the current source I 3  is supplied to the resistor R 4 . As described above, the output current of the current source I 3  is substantially equal to the drain current of the PMOS transistor M 3 , which flows when the output current becomes equal to the first current value. Therefore, the voltage Vb drops almost as low as the first reference voltage Va 1  ( FIG. 2(   b )). However, since the second reference voltage Va 2  lower than the first reference voltage Va 1  is inputted to the inverting input terminal of the comparator  12 , the output CMPo of the comparator  12  is not inverted. 
     At a time t 3 , the charge in the gate capacitance of the second output transistor M 2  is discharged by the output current of the differential amplifier  11 . Then, the constant voltage circuit unit  1  switches to a stable operation. In this case, since the current value of the current source I 3  is supplied to the resistor R 4  in addition to the drain currents of the PMOS transistors M 3  and M 4 , the voltage Vb becomes twice as high as the first reference voltage Va 1  or higher ( FIG. 2(   b )). As described above, since the bias current of the differential amplifier  11  is increased by turning on the switch S 2  before connecting the second output transistor M 2  to the output of the differential amplifier  11  by turning on the switch S 1 , the differential amplifier  11  has a larger output current and is capable of faster response before the second output transistor M 2  is connected. Therefore, less time is required to charge the gate capacitance of the second output transistor M 2  by the output current of the differential amplifier  11  as compared to the case of turning on the switches S 1  and S 2  at the same time. As a result, fluctuation of an output voltage caused when the second output transistor M 2  is connected can be suppressed. 
     When the output CMPo of the comparator  12  becomes an H-level at the time t 1 , the output of the inverter  17  changes from an H-level to an L-level. However, the current source I 6  is inserted between the negative side power source of the inverter  17  and ground potential Vss. Therefore, the charge charged in the capacitor C 3  when the inverter  17  outputs an H-level signal is slowly discharged through the current source I 6 . As a result, it takes time until the output of the inverter  17  changes from an H-level to an L-level. This delay time is shown as Td 3  in  FIG. 2(   c ). The delay time Td 3  is longer than the delay time Td 1 . Moreover, the delay time Td 3  is set as long as or longer than a time that it takes until the gate capacitance of the second output transistor M 2  is discharged by the output of the differential amplifier  11 . In this manner, the second output transistor M 2  can be securely connected. 
     When the voltage of the capacitor C 3  becomes as low as or lower than a threshold voltage of the inverter  18  at a time t 4 , the output of the inverter  18  becomes an H-level. Thus, the output D of the inverter  19  of a subsequent stage is an L-level ( FIG. 2(   c )). Then, the switch S 4  is turned off to block the current of the current source I 3  from being supplied to the resistor R 4 . Therefore, the voltage Vb drops by a voltage substantially equal to the first reference voltage Va 1  ( FIG. 2(   b )). 
     In this manner, the switch S 3  is turned on at the time t 1  in accordance with the increase of the output current, thereby the current of the current source I 3  is inputted to the comparator  12 . In addition, the switch S 2  is turned on almost at the same time, thereby the current source I 2  is operated. The switch S 1  is turned on at a time t 2  with a delay of the time Td 1  after the switch S 3  is turned on. Then, since the second output transistor M 2  can be operated, the circuit can receive a large load. In this manner, the current source I 2  and the second output transistor M 2  which are additionally provided are operated in the periods t 1  to t 4  including the periods t 2  and t 3  as transient periods. After the time t 4 , the current is in a large current mode (high speed mode). As described above, the second reference voltage Va 2  has a voltage level lower than that of the first reference voltage Va 1 . When the voltage Vb becomes higher than the first reference voltage Va 1  to invert the output CMPo of the comparator  12 , the second reference voltage Va 2  is inputted to the inverting input terminal of the comparator instead of the first reference voltage Va 1 . Therefore, the second output transistor M 2  can be securely connected. 
     In this circuit configuration, the second output transistor M 2  is larger in size than the first output transistor M 1 . Therefore, when the output current is increased, the switch S 2  is turned on, the second output transistor M 2  is turned on, and the switch S 6  is turned off. Then, there is the moment when both the first and second output transistors M 1  and M 2  are turned off as described above. Then, a current flowing through the PMOS transistor M 3  which monitors the first output transistor M 1  is decreased. Then, the determination circuit unit  2  determines that the output current has decreased and ends up oscillating. To solve this problem, the determination circuit unit  2  has an oscillation preventive function. When the output current is small, the current flowing through the PMOS transistor M 3  is small, therefore, the comparator  12  outputs an L-level signal while the output D of the inverter  19  becomes an H-level. Therefore, the switch S 4  is on. When the output current gradually increases to be higher than the first current value 1, the output of the comparator  12  is inverted to an H-level, which turns on the switch S 3  (the time t 1  in  FIG. 2(   c )). At this time, since a signal inputted to the inverter  17  is delayed by the capacitor C 3 , the switch S 4  remains on. In this manner, there is a time when both the switches S 3  and S 4  are on (Td 3  in  FIG. 2(   c )). Therefore, the current of the current source I 3  is supplied to the non-inverting input terminal of the comparator  12  as described above. Here, the switch S 4  is on only for the delay time caused by the capacitor C 3 . After this delay time, the switch S 4  is turned off, therefore, the current supply of the current source I 3  is stopped at a time t 4  ( FIG. 2(   c )). In the period of Td 3 , the first output transistor M 1  and the second output transistor M 2  are both operated, thereby a proper current is supplied to the PMOS transistors M 3  and M 4  which form a current mirror circuit. Therefore, when the output D of the inverter  19  becomes an L-level and the switch S 4  is turned off, the voltage Vb becomes stable. 
     Next, the output current which has been increasing starts decreasing at a time t 5 . When the output current becomes as small as or smaller than the predetermined second current value at a time t 6 , the voltage Vb becomes lower than the second reference voltage Va 2 . As a result, the output CMPo of the comparator  12  is inverted from an H-level into an L-level ( FIG. 2(   c )). Then, the switch S 3  is turned off. Further, since the output CMPo is at an L-level, the common terminal c of the switch S 5  is connected to the terminal a, the first reference voltage Va 1  is connected to the inverting input terminal of the comparator  12 , and the input voltage Va of the inverting input terminal of the comparator  12  becomes Va 1  ( FIG. 2(   b )). Moreover, since the output CMPo is at an L-level, the inverter  17  outputs an H-level signal. Here, since the output circuit of the inverter  17  has low impedance on a high side, the capacitor C 3  can be instantly charged. As a result, an input signal to the inverter  18  becomes an H-level with little delay. Therefore, an output of the inverter  18  changes to an L-level soon after the output CMPo of the comparator  12  changes to an L-level. Therefore, the output D of the inverter  19  which receives the output of the inverter  18  changes to an H-level with little delay ( FIG. 2(   c )). When the output D of the inverter  19  becomes an H-level, the switch S 4  is turned on. However, the current of the current source I 3  is not supplied to the resistor R 4  since the switch S 3  is off at this time. 
     When the output CMPo of the comparator  12  becomes an L-level, the inverter  13  outputs an H-level signal. Since the current source I 4  is connected between a power source terminal on a positive side of the inverter  13  and the input terminal Vdd, it takes time to charge the capacitor C 1 , causing a delay time of Td 2  ( FIG. 2(   c )). 
     Therefore, the switch S 2  is turned off at a time t 7  after the delay time Td 2  has passed after the output CMPo of the comparator  12  becomes an L-level. As a result, the current supply of the current source I 2  as a bias current of the differential amplifier  11  is blocked, thereby only the current of the current source I 1  is supplied as the bias current of the differential amplifier  11 . Further, the output B of the inverter  15  which receives the L-level output A from the inverter  14  becomes an H-level. Then, since the high side of the inverter  15  has low impedance, the capacitor C 2  is instantly charged. Therefore, when the output A of the inverter  14  becomes an L-level, the output C of the inverter  16  becomes an L-level immediately. As a result, the switch S 1  is turned off ( FIG. 2(   c )), blocking a connection between the output of the differential amplifier  11  and a gate of the second output transistor M 2 . On the other hand, since the switch S 6  is turned on, the gate voltage Vm 2   g  of the second output transistor M 2  is pulled-up to the input terminal Vdd by the resistor R 3  to be as high as the input voltage Vdd ( FIG. 2(   a )). Moreover, since the gate of the first output transistor M 1  is connected to the output of the differential amplifier  11 , the gate voltage Vm 1   g  drastically drops as shown in  FIG. 2(   a ). When the connection between the differential amplifier  11  and the gate of the second output transistor M 2  is blocked, the differential amplifier  11  charges only the gate capacitance of the first output transistor M 1 . Since the gate capacitance of the first output transistor M 1  is small, the output voltage Vout is not changed even when the bias current is changed to only the current source I 1  at the same time as blocking the connection between the differential amplifier  11  and the gate of the second output voltage Vout. 
     As described above, in the constant voltage circuit of this embodiment, the bias current of the differential amplifier  11  is changed in accordance with the output current. Therefore, a driving efficiency of the constant voltage circuit is improved when the output current is small. At the same time, a driving property of the constant voltage circuit is switched by connecting or blocking the second output transistor M 2  in accordance with the output current. As a result, the constant voltage circuit is capable of high speed response when the output current is small and also receiving a large output current. 
     The bias current of the differential amplifier is changed in the constant voltage circuit disclosed in Patent Documents 1 and 2, however, a driving state of an output transistor is not changed in accordance with the output current in these conventional techniques. When switching the driving state in the present invention, a small output current mode (only the first output transistor M 1  is operated) and a large output current mode (the first and second output transistors M 1  and M 2  are operated) are switched by comparing an output current with a predetermined output current value as a reference. At this time, there is an unstable period (for example, a period when the mode should originally be in the large output current mode but the modes are switched plural times) when switching the modes. This problem is solved as follows in the circuit configuration of this embodiment. Specifically, a predetermined voltage corresponding to the current source I 3  is added to the voltage Vb at a timing of the time t 1  shown in  FIG. 2(   b ). Therefore, even when the voltage level of the voltage Vb becomes unstable in the period until the time t 4 , the voltage level of the voltage Vb does not become lower than the reference voltage Va 2 . As a result, a mode of the constant voltage circuit can be fixed to a required mode. In  FIG. 2(   b ), after the voltage Vb becomes stable at the time t 4 , the constant voltage circuit operates in the large output current mode. 
     Further, since a ratio of a gate width to a gate length of the second output transistor M 2  is set as high as or higher than a ratio of a gate width to a gate length of the first output transistor M 1 , a bias current value as large as or larger than the original bias current value is supplied to the differential amplifier  11 . In this manner, a wide range of output voltage can be obtained. 
     Next, a second embodiment of the present invention is described with reference to  FIGS. 3 and 4 . 
       FIG. 3  shows a constant voltage circuit showing the second embodiment of the present invention. 
       FIG. 3  is different from  FIG. 1  in that a circuit  21  shown by a broken line is provided instead of a circuit  20  shown by a broken line in  FIG. 1 . That is, a constant current inverter  23  including resistors R 21  and R 22 , a switch S 21 , an inverter  22 , a power source voltage Vdd, a current source I 21 , and a PMOS transistor M 21  is provided in  FIG. 3  instead of the circuit including the resistor R 4 , the first reference voltage Va 1 , the second reference voltage Va 2 , the switch S 5 , and the comparator  12  shown in  FIG. 1 . Other than this difference,  FIG. 3  has a configuration similar to that of  FIG. 1 , therefore, a description of  FIG. 3  will be made on only the aforementioned difference. 
     In  FIG. 3 , drains of the PMOS transistors M 3  and M 4  are commonly connected and grounded through the resistors R 21  and R 22 . The switch S 21  is connected to both ends of the resistor R 22  in parallel to the resistor R 22 . By turning on and off the switch S 21 , combined resistance of the resistors R 21  and R 22  is variably switched. The resistors R 21  and R 22  function as a current-voltage converter capable of converting a drain current of the PMOS transistors M 3  and M 4  into a voltage. As described above, the PMOS transistors M 3  and M 4  form a current mirror circuit with the first output transistor M 1  and the second output transistor M 2  respectively. Therefore, the drain current of the PMOS transistors M 3  and M 4  is in proportion to the output current. Since the resistors R 21  and R 22  convert this current into a voltage, a voltage drop Vb at the resistors R 21  and R 22  is in proportion to the output current. The current source I 21  and the PMOS transistor M 21  are connected in series between the power source voltage terminal Vdd and ground potential Vss. The current source I 21  and the PMOS transistor M 21  form a constant current inverter  23 . A voltage Vb is inputted to a gate of the PMOS transistor M 21 . An output of the constant current inverter  23  is inputted to the inverter  22 . An output CMPo of the inverter  22  is connected to a control terminal of the switch S 21  to turn on and off the switch S 21 . 
       FIG. 4  is a timing chart showing a major part of the constant voltage circuit shown in  FIG. 3 . Part (a) in  FIG. 4  shows changes of the gate voltage Vm 1   g  of the first output transistor M 1  with respect to a time t and of the gate voltage Vm 2   g  of the second output transistor M 2  with respect to a time t. Part (b) in  FIG. 4  shows changes of the voltage Vb inputted to the gate of the PMOS transistor M 21  with respect to the time t. Part (c) in  FIG. 4  shows changes of a level of the output signal CMPo of the inverter  22 , the output A of the inverter  14 , the output B of the inverter  15 , the output C of the inverter  16 , and the output D of the inverter  19  in  FIG. 3 . 
     Here, parts (a) and (c) in  FIG. 4  are the same as parts (a) and (c) in  FIG. 2 , however, the description made with reference to  FIG. 2  will be repeated below to describe part (b) in  FIG. 4 . 
     In a vertical axis of part (a) in  FIG. 4 , Vdd denotes a voltage level of a voltage inputted to an input terminal. In a vertical axis of part (c) in  FIG. 4 , CMPo denotes an output signal level of the inverter  22 , A denotes an output signal level of the inverter  14 , B denotes an output signal level of the inverter  15 , C denotes an output signal level of the inverter  16 , and D denotes an output signal level of the inverter  19 . The signals A, B, C, and D correspond to control signals of the switches S 2 , S 6 , S 1 , and S 4  respectively. Moreover, in the vertical axis of part (b) in  FIG. 4 , Vt denotes a level of a threshold voltage of the constant current inverter  23 . 
     In parts (a) to (c) of  FIG. 4 , the first output transistor M 1  and the PMOS transistor M 3  form a current mirror circuit. Therefore, since a current is not supplied to the resistor R 21  in the initial state, that is when the output current is 0 A, a voltage drop is not caused by the resistor R 21 . That is, since the voltage Vb (input voltage Vb to the gate of the PMOS transistor M 21 ) of the input terminal of the constant current inverter  23  formed of the current source I 21  and the PMOS transistor M 21  is 0 V, the output CMPo of the inverter  22  is at an L-level. Since the output CMPo of the inverter  22  is at an L-level, the output A of the inverter  14  and the output C of the inverter  16  are at an L-level. On the other hand, the output B of the inverter  15  and the output D of the inverter  19  become an H-level. Therefore, the switches S 1  to S 3  are turned off and the switches S 4  and S 6  are turned on (see  FIG. 4(   c )). The switches S 1  to S 4  and S 6  are turned off when an L-level signal is inputted to their control terminals and turned on when an H-level signal is inputted to their control terminals. A switch which is turned on when an L-level signal is inputted to its control terminal and turned off when an H-level signal is inputted to its control terminal is used as the switch S 21 . Therefore, the switch S 21  is turned on at this time (when the output CMPo is at an L-level). Note that a switch which is turned on when an H-level signal is inputted to its control terminal may be used as the switch S 21  similarly to the other switches. In that case, the output CMPo may be inputted to the switch S 21  through an inverter and the like. 
     Since the switch S 1  is off and the switch S 6  is on, the gate of the second output transistor M 2  is pulled-up to the input terminal voltage Vdd by the resistor R 3 . Therefore, the second output transistor M 2  is off. Since the switch S 2  is off, the current of the current source I 1  is supplied as a bias current of the differential amplifier  11 . Further, since the switch S 3  is off, the current of the current source I 3  is not supplied to the resistor R 21  even when the switch S 4  is on. Since the switch S 21  is on, a connection between the resistors R 21  and R 22  is grounded. 
     In the aforementioned state, the output current is increased. When the output current is increased, the gate voltage Vm 1   g  of the first output transistor M 1  is decreased ( FIG. 4(   a )). At the same time, the gate voltage of the PMOS transistor M 3  is decreased. Therefore, the voltage Vb inputted to the input terminal of the constant current inverter  23  is increased ( FIG. 4(   b )). However, connection states of the switches are not changed until the output current reaches a level of a predetermined first current value. 
     When the output current reaches the predetermined first current value at a time t 1 , the voltage Vb becomes a threshold voltage Vt of the constant current inverter  23  ( FIG. 4(   b )). When the output current is further increased to be higher than the first current value, the voltage Vb becomes higher than the threshold voltage Vt of the constant current inverter  23 . Therefore, the output CMPo of the inverter  22  is inverted to an H-level ( FIG. 4(   c )). Then, the switch S 3  is turned on, therefore, the current of the current source I 3  is supplied to the resistor R 21 . As a result, the voltage Vb rapidly rises ( FIG. 4(   b )). Note that the current value of the current source I 3  in this embodiment is substantially equal to or higher than the drain current of the PMOS transistor M 3 , which flows when the output current becomes equal to the first current value. As shown in  FIG. 4(   b ), the voltage Vb rises as high as (2×Vt), which is twice as high as the threshold voltage Vt of the constant current inverter  23  at a timing of the time t 1 . Further, by the inversion of the output CMPo of the inverter  22 , the switch S 21  is turned off. Therefore, the drain current of the PMOS transistor M 21  and the current of the current source I 3  are supplied to the resistors R 21  and R 22 , which further increases the voltage Vb (a period from the time t 1  to t 2  in  FIG. 4(   b )). The output CMPo of the inverter  22  is at an H-level, therefore, the inverter  13  outputs an L-level signal. Since an output circuit of the inverter  13  has low impedance on a low side, a charge of the capacitor C 1  is discharged instantly. Therefore, since the input of the inverter  14  becomes an L-level with little delay, the output A of the inverter  14  changes to an H-level in a moment when the output CMPo of the inverter  22  becomes an H-level ( FIG. 4(   c )). Moreover, when the output A of the inverter  14  becomes an H-level, the switch S 2  is turned on. Therefore, a current value of the current source I 2  is additionally provided to a bias circuit of the differential amplifier  11 . As a result, the operation of the differential amplifier  11  becomes faster. Consequently, the voltage Vm 1   g  drops rapidly after the time t 1  as shown in  FIG. 4(   a ). 
     When the output A of the inverter  14  becomes an H-level, the output B of the inverter  15  changes from an H-level into an L-level. However, since the current source I 5  is inserted between a power source on a negative side of the inverter  15  and ground potential Vss, the charge charged in the capacitor C 2  when the output B of the inverter  15  is at an H-level is discharged through the current source I 5 . Thus, it takes time until the output B of the inverter  15  changes from an H-level to an L-level. This delay time is shown as Td 1  in  FIG. 4(   c ). After the time Td 1 , the output B of the inverter  15  becomes an L-level. When the voltage of the capacitor C 2  becomes as low as or lower than an input threshold voltage of the inverter  16  at a time t 2 , the output C of the inverter  16  becomes an H-level almost at the same timing ( FIG. 4(   c )). Then, the switch S 6  is turned off and the switch S 1  is turned on. Then, the output of the differential amplifier  11  is inputted to the gate of the second output transistor M 2 . Before the switch S 1  is turned on, the gate of the second output transistor M 2  had been pulled-up to the input voltage Vdd by the resistor R 3 . Therefore, the gate voltage Vm 2   g  of the second output transistor M 2  was the input voltage Vdd ( FIG. 4(   a )). Further, since there is gate capacitance of the second output transistor M 2  between the gate of the second output transistor M 2  and the input terminal Vdd, the output of the differential amplifier  11  momentarily rises as high as the input terminal voltage Vdd right after the switch S 1  is turned on. Therefore, there is a moment when both the first output transistor M 1  and the second output transistor M 2  are turned off. 
     When both the first output transistor M 1  and the second output transistor M 2  are turned off, the PMOS transistors M 3  and M 4  are also turned off. Therefore, only a current of the current source I 3  is supplied to the resistor R 21 . As described above, the output current of the current source I 3  is set substantially equal to or larger than the drain current of the PMOS transistor M 3  which flows when the output current becomes equal to the first current value. Therefore, the voltage Vb drops almost as low as the threshold voltage Vt of the constant current inverter  23  ( FIG. 4(   b )). However, since a voltage generated at the resistors R 21  and R 22  is inputted to the constant current inverter  23  at this time, an output of the constant current inverter  23  is not inverted. 
     At a time t 3 , when the gate capacitance of the second output transistor M 2  is discharged by the output current of the differential amplifier  11 , the constant voltage circuit unit  1  operates stably. In this case, since the current value of the current source I 3  is supplied to the resistors R 21  and R 22  in addition to the drain current of the PMOS transistors M 3  and M 4 , the voltage Vb becomes twice as high as the threshold voltage Vt of the constant current inverter  23  or higher ( FIG. 4(   b )). As described above, since the bias current of the differential amplifier  11  is increased by turning on the switch S 2  before connecting the second output transistor M 2  to the output of the differential amplifier  11  by turning on the switch S 1 , the output current of the differential amplifier  11  becomes larger and a response speed becomes faster before the second output transistor M 2  is connected. As a result, a response speed becomes higher. Therefore, less time is required to charge the gate capacitance of the second output transistor M 2  by the output current of the differential amplifier  11  as compared to the case of turning on the switches S 1  and S 2  at the same time. As a result, fluctuation of an output voltage caused when the second output transistor M 2  is connected can be suppressed. 
     When the output CMPo of the inverter  22  becomes an H-level at the time t 1 , the output of the inverter  17  changes from an H-level to an L-level. However, the current source I 6  is inserted between the negative side power source of the inverter  17  and ground potential Vss. Therefore, the charge charged in the capacitor C 3  when the inverter  17  outputs an H-level signal is slowly discharged through the current source I 6 . As a result, it takes time until the output of the inverter  17  changes from an H-level to an L-level. This delay time is shown as Td 3  in  FIG. 4(   c ). The delay time Td 3  is longer than the delay time Td 1 . Moreover, the delay time Td 3  is set as long as or longer than a time that it takes until the gate capacitance of the second output transistor M 2  is discharged by the output of the differential amplifier  11 . In this manner, the second output transistor M 2  can be securely connected. 
     When the voltage of the capacitor C 3  becomes as low as or lower than a threshold voltage of the inverter  18  at a time t 4 , the output of the inverter  18  becomes an H-level. Thus, the output D of the inverter  19  of a subsequent stage is an L-level ( FIG. 4(   c )). Then, the switch S 4  is turned off to block the current of the current source I 3  from being supplied to the resistors R 21  and R 22 . Therefore, the voltage Vb drops by a voltage substantially equal to the threshold voltage Vt of the constant current inverter  23  ( FIG. 4(   b )). 
     In this manner, the switch S 3  is turned on at the time t 1  in accordance with the increase of the output current, thereby the current of the current source I 3  is inputted to the constant current inverter  23 . In addition, the switch S 2  is turned on almost at the same time, thereby the current source I 2  is operated. The switch S 1  is turned on at a time t 2  with a delay of the time Td 1  after the switch S 3  is turned on. Then, since the second output transistor M 2  can be operated, the circuit can receive a large load. In this manner, the current source I 2  and the second output transistor M 2  which are additionally provided are operated in the periods t 1  to t 4  including the periods t 2  and t 3  as transient periods. After the time t 4 , the circuit is in a large current mode (high speed mode). As described above, when the voltage Vb becomes higher than the threshold voltage Vt of the constant current inverter  23  and the output CMPo of the inverter  22  is inverted, the switch S 21  is turned off so that the voltage generated at the resistors R 21  and R 22  is inputted to the constant current inverter  23 . Therefore, the second output transistor M 2  can be securely connected. 
     In this circuit configuration, the second output transistor M 2  is larger in size than the first output transistor M 1 . Therefore, when the output current is increased, the switch S 2  is turned on, the second output transistor M 2  is turned on, and the switch S 6  is turned off. Then, there is the moment when both the first and second output transistors M 1  and M 2  are turned off as described above. Then, a current flowing through the PMOS transistor M 3  which monitors the first output transistor M 1  is decreased. The determination circuit unit  2  determines that the output current has decreased and ends up oscillating. To solve this problem, the determination circuit unit  2  has an oscillation preventive function. When the output current is small, the current flowing through the PMOS transistor M 3  is small, therefore, the constant current inverter  23  outputs an H-level signal while the output D of the inverter  19  becomes an L-level. Therefore, the switch S 4  is on. When the output current gradually increases to be higher than the first current value, the output of the constant current inverter  23  is inverted to an L-level, which turns on the switch S 3  (the time t 1  in  FIG. 4(   c )). At this time, since a signal inputted to the inverter  17  is delayed by the capacitor C 3 , the switch S 4  remains on. In this manner, there is a time when both the switches S 3  and S 4  are on (Td 3  in  FIG. 4(   c )). Therefore, the current of the current source I 3  is supplied to the constant current inverter  23  as described above. Here, the switch S 4  is on only for the delay time caused by the capacitor C 3 . After this delay time, the switch S 4  is turned off, therefore, the current supply of the current source I 3  is stopped at the time t 4  ( FIG. 4(   c )). In the period of Td 3 , the first output transistor M 1  and the second output transistor M 2  are both operated, thereby a proper current is supplied to the PMOS transistors M 3  and M 4  which form a current mirror circuit. Therefore, when the output D of the inverter  19  becomes an L-level and the switch S 4  is turned off, the voltage Vb becomes stable. 
     Next, the output current which has been increasing starts decreasing at a time t 5 . When the output current becomes as small as or smaller than the predetermined second current value at a time t 6 , the voltage Vb becomes lower than the threshold voltage Vt of the constant current inverter  23 . As a result, the output CMPo of the inverter  22  is inverted from an H-level into an L-level ( FIG. 4(   c )). Then, the switch S 3  is turned off. Further, since the output CMPo is at an L-level, the switch S 21  is turned on and the connection between the resistors R 21  and R 22  is grounded through the switch S 21 . Moreover, since the output CMPo is at an L-level, the inverter  17  outputs an H-level signal. Here, since the output circuit of the inverter  17  has low impedance on a high side, the capacitor C 3  can be charged instantly. As a result, an input signal to the inverter  18  becomes an H-level with little delay. Therefore, an output of the inverter  18  changes to an L-level soon after the output CMPo of the comparator  12  changes to an L-level. Therefore, the output D of the inverter  19  which receives the output of the inverter  18  changes to an H-level with little delay ( FIG. 4(   c )). When the output D of the inverter  19  becomes an H-level, the switch S 4  is turned on. However, the current of the current source I 3  is not supplied to the resistor R 21  since the switch S 3  is off at this time. 
     When the output CMPo of the inverter  22  becomes an L-level, the inverter  13  outputs an H-level signal. Since the current source I 4  is connected between a power source terminal on a positive side of the inverter  13  and the input terminal Vdd, it takes time to charge the capacitor C 1 , causing a delay time of Td 2  ( FIG. 4(   c )). 
     Therefore, the switch S 2  is turned off at a time t 7  after the delay time Td 2  has passed after the output CMPo of the inverter  22  becomes an L-level. As a result, the current supply of the current source I 2  as a bias current to the differential amplifier  11  is blocked, thereby only the current of the current source I 1  is supplied as the bias current of the differential amplifier  11 . Further, the output B of the inverter  15  which receives the L-level output A from the inverter  14  becomes an H-level. Then, since the high side of the inverter  15  has low impedance, the capacitor C 2  is charged instantly. Therefore, when the output A of the inverter  14  becomes an L-level, the output C of the inverter  16  becomes an L-level immediately. As a result, the switch S 1  is turned off ( FIG. 4(   c )), blocking a connection between the output of the differential amplifier  11  and a gate of the second output transistor M 2 . On the other hand, since the switch S 6  is turned on, the gate voltage Vm 2   g  of the second output transistor M 2  is pulled-up to the input terminal Vdd by the resistor R 3  to be as high as the input voltage Vdd ( FIG. 4(   a )). Moreover, since the gate of the first output transistor M 1  is connected to the output of the differential amplifier  11 , the gate voltage Vm 1   g  drastically drops as shown in  FIG. 4(   a ). When the connection between the differential amplifier  11  and the gate of the second output transistor M 2  is blocked, the differential amplifier  11  charges only the gate capacitance of the first output transistor M 1 . Since the gate capacitance of the first output transistor M 1  is small, the output voltage Vout is not changed even when the bias current is changed to only the current source I 1  at the same time as blocking the connection between the differential amplifier  11  and the gate of the second output voltage Vout. 
     As described above, in the constant voltage circuit of this embodiment, the bias current of the differential amplifier  11  is changed in accordance with the output current. Therefore, a driving efficiency of the constant voltage circuit is improved when the output current is small. At the same time, a driving property of the constant voltage circuit is changed by connecting or blocking the second output transistor M 2  in accordance with the output current. As a result, the constant voltage circuit is capable of high speed response when the output current is small and can also receive a large output current. 
     Similarly to the first embodiment, a small output current mode and a large output current mode are switched in accordance with the output current value in the second embodiment. As a countermeasure for a defect in a period when the circuit operation becomes unstable (the period from the time t 1  to t 4  in  FIG. 4(   b )), a predetermined voltage corresponding to the current source I 3  is added to the voltage Vb at the timing of the time t 1 . As a result, the voltage Vb does not fall lower than the voltage Vt even when the voltage Vb is unstable (specifically, in a manner similar to the corresponding description in the first embodiment). 
     In the second embodiment, an equivalent function to the first embodiment can be obtained with a simpler circuit configuration. On the other hand, although the circuit is not operated unless the current of the current source I 21  of the constant current inverter  23  is supplied, the NMOS transistor M 51  (the NMOS transistor  51  having a gate which receives the voltage Vb. See  FIG. 5 ) of the differential amplifier circuit which forms the comparator  12  is off when the voltage Vb is low. Therefore, the first embodiment has an advantage in that the current is not unnecessarily consumed and thus the power consumption is suppressed. In this manner, the first and second embodiments have different advantages. 
     In general, an operational amplifier has a capacitor connected in an amplifier stage for phase compensation. Next, a phase compensation of the constant voltage circuit of this embodiment is described. 
     In  FIG. 6 , an area surrounded by a solid line denotes an integrated stabilizing power source circuit, which is a circuit around the first output transistor M 1  and the second output transistor M 2  in the constant voltage circuit shown in  FIG. 1 . In  FIG. 6 , the first output transistor M 1  and the second output transistor M 2  when the output current is large in  FIG. 1  are combined and shown as one output transistor M. The same components as those in  FIG. 1  are denoted by the same reference numerals and the description will not be repeated.  FIG. 7  shows a small signal equivalent circuit of an area  24  surrounded by a broken line in  FIG. 6 . 
     In  FIG. 6 , MA denotes a transistor included as an internal circuit of the differential amplifier  11 , and I denotes a current source. In addition, a load resistor RL and a capacitor CL for stabilizing an output signal are connected to an output terminal Vout. 
     In  FIG. 7 , reference numeral Ro 1  denotes resistance between a source and a drain of an output transistor M; Ro 2  denotes resistance between a source and a drain of the transistor MA; gm 1  denotes transconductance of the output transistor M; gm 2  denotes transconductance of the transistor MA; Vi 1  denotes a gate voltage of the output transistor M; Vi 2  denotes a gate voltage of the transistor MA; C 1  denotes capacitance between the gate and drain of the output transistor M; C 2  denotes capacitance between the gate and drain of the transistor MA; CL denotes capacitance of a capacitor for stabilizing the output signal, which is connected to this stabilizing power source circuit; and RL denotes variable load resistance connected to this stabilizing power source circuit. 
     In the equivalent circuit of  FIG. 7 , there are two poles, namely a pole p 1  and a pole p 2 . Frequencies Fp 1  and Fp 2  at which the poles p 1  and p 2  are generated respectively are approximately obtained by the formulas below.
 
 Fp 1=1/(2 πgm 1 ·Ro 2 ·RL·C 1)  Formula 1
 
 Fp 2=1/(2π ·CL·RL )  Formula 2
 
     Here, when the load resistance RL increases (that is when the output current becomes small), the frequencies Fp 1  and Fp 2  of the two poles p 1  and p 2  are both shifted to the low frequency side and become close to each other as in Formulas 1 and 2. Then, the output of the differential amplifier  11  is fed back to the input before the gain is sufficiently decreased. Since the input and output have opposite phases at this time, the circuit oscillates. In the constant voltage circuit of this embodiment, only the first output transistor M 1  is used when the output current is small. Therefore, C 1  in Formula 1 becomes a small value. As a result, it can be prevented that the frequencies of the poles p 1  and p 2  become close to each other, therefore, the circuit does not oscillate. 
     In this manner, according to the present invention, oscillation of the circuit can be suppressed and power consumption can be reduced to be small when the output current is small. In addition, since the second output transistor M 2  is additionally used when the output current is large, the circuit can perform a high speed operation. 
     According to one embodiment, the constant voltage circuit of the present invention can operate stably for a wide range of load current without decreasing a response speed even when the load current is small. 
     This patent application is based on Japanese Priority Patent Application No. 2008-025194 filed on Feb. 5, 2008, and Japanese Priority Patent Application No. 2008-081336 filed on Mar. 26, 2008, the entire contents of which are hereby incorporated herein by reference.