Constant voltage power supply circuit and method of controlling the same

A constant-voltage power supply circuit is disclosed that is able to prevent overshoot of an output voltage possibly occurring when changing a constant-voltage circuit in operation and is able to supply a constant output voltage. The constant-voltage power supply circuit includes a first constant-voltage circuit, having a first output transistor and a first output voltage controller, that generates a first reference voltage and generates a first proportional voltage in proportion to a voltage on an output terminal, and a second constant-voltage circuit having a second output transistor and a second output voltage controller that generates a second reference voltage and generates a second proportional voltage in proportion to the voltage on the output terminal. When the first output voltage controller or the second output voltage controller starts operations according to a control signal input from the outside, a rising edge of the first reference voltage or the second reference voltage is delayed so as to be later than a rising edge of the first proportional voltage or the second proportional voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a constant-voltage power supply circuit including plural constant-voltage circuits used for an electronic apparatus which requires a stabilized power supply circuit integrated into a semiconductor integrated circuit, and a method of controlling the constant-voltage power supply circuit; and particularly, to a constant-voltage power supply circuit able to be used in various electronic apparatuses which is able to change the constant-voltage power supply circuit to be used depending on system operating conditions so as to reduce power consumption, and a method of controlling the constant-voltage power supply circuit.

2. Description of the Related Art

FIG. 11is a circuit diagram illustrating a constant-voltage power supply circuit of the related art, which is able to switch over plural constant-voltage power supply circuits depending on operating situations.

As shown inFIG. 11, a constant-voltage power supply circuit100includes two constant-voltage circuits101and102, which share an input terminal IN and an output terminal OUT. The constant-voltage circuit101includes an output transistor111which is a PMOS transistor, resistors112,113, a reference voltage generator114, an error amplifier115, and switches116,117. The constant-voltage circuit102includes an output transistor121which is a PMOS transistor, resistors122,123, a reference voltage generator124, an error amplifier125, and switches126,127. The constant-voltage circuits101and102have the same circuit configuration, and form series regulators, respectively.

The switches116,117are controlled to be switched by an external control signal SCa, and the switches126,127are controlled to be switched by an external control signal SCb. When the external control signal SCa is input to stop operations of the constant-voltage circuit101, the switches116,117are switched off (disconnection state), thereby operations of the reference voltage generator114and the error amplifier115are stopped, and current supply to the resistors112,113is stopped.

Similarly, when the external control signal SCb is input to stop operations of the constant-voltage circuit102, the switches126,127are switched off (disconnection state); thereby operations of the reference voltage generator124and the error amplifier125are stopped, and current supply to the resistors122,123is stopped.

For example, Japanese Laid Open Patent Application No. 2004-180472 discloses a power supply switching circuit able to supply a constant output voltage under normal operating conditions without dependence on power supply selection while suppressing overshoot and undershoot of the output voltage during power supply switching.

However, when the constant-voltage circuits101and102share the output terminal OUT, as shown inFIG. 11, when switching the constant-voltage circuits101and102, overshoot of the output voltage may occur.

FIG. 12is a time chart illustrating waveforms of signals in the constant-voltage power supply circuit100as shown inFIG. 11.

As shown inFIG. 12, when switching from the constant-voltage circuit101to the constant-voltage circuit102, due to the control signal SCa, the switches116,117are switched off (disconnection state); meanwhile, due to the control signal SCb, the switches126,127are switched on (connection state). When the switches126,127are OFF, although a divisional voltage VFBb given by the resistors122and123and a reference voltage Vrb are connected to the grounding voltage GND, once the switches126,127are switched on, the reference voltage Vrb is raised to a preset voltage.

In this process, the time period required for the reference voltage Vrb from the reference voltage generator124to reach the preset voltage, and the time period required for the divisional voltage VFBb to reach the reference voltage Vrb are different. Because of this difference, an overshoot voltage occurs. Namely, the divisional voltage VFBb input to the error amplifier125is given by resistors122and123, and the error amplifier125turns ON the transistor121so as to operate in a saturation state until the divisional voltage VFBb reaches the reference voltage Vrb. At this moment, the output voltage Vout rises to a relatively high voltage due to the constant-voltage circuit101which is already in operation, and this causes the overshoot.

In an electronic apparatus driven by a battery, in order to extend the operating service life of the battery, it is necessary to reduce the current consumed by the circuit. For this purpose, it is attempted to switch over a number of constant-voltage circuits for operation according to operating situations, thereby reducing the consumption of current. For example, a constant-voltage circuit with large consumption of current is used when a load is large, and a constant-voltage circuit with small consumption of current is used when a load is small or in a standby state. In this way, the consumption of current can be reduced. However, if overshoot occurs as mentioned above when switching over the constant-voltage circuits, it may cause malfunction of the load connected to the circuit.

SUMMARY OF THE INVENTION

The present invention may solve one or more of the problems of the related art.

A preferred embodiment of the present invention may provide a constant-voltage power supply circuit able to prevent overshoot of an output voltage possibly occurring when changing a constant-voltage circuit in operation and able to supply a constant output voltage, and a method of controlling the constant-voltage power supply circuit.

According to a first aspect of the present invention, there is provided a constant-voltage power supply circuit that converts a voltage input to an input terminal to a predetermined constant voltage and outputs the constant voltage, comprising:

a first constant-voltage circuit includinga first output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, anda first output voltage controller that generates a predetermined first reference voltage and generates a first proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the first reference voltage and the first proportional voltage, and outputs the amplified difference to the control electrode of the first output transistor; and

a second constant-voltage circuit includinga second output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, anda second output voltage controller that generates a predetermined second reference voltage and generates a second proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the second reference voltage and the second proportional voltage, and outputs the amplified difference to the control electrode of the second output transistor,

wherein

the first output voltage controller and the second output voltage controller start operations or stop operations according to a first control signal and a second control signal input from the outside, respectively,

when starting operations, a rising edge of the corresponding one of the first reference voltage and the second reference voltage is delayed so as to be later than a rising edge of the corresponding one of the first proportional voltage and the second proportional voltage.

Preferably, the first output voltage controller comprises:

a first delay circuit that delays the first control signal by a predetermined first time period;

a first reference voltage generator that generates and outputs the first reference voltage;

a first divisional voltage circuit that divides the voltage output from the output terminal and generates and outputs the first proportional voltage;

a first error amplifier that controls operations of the first output transistor so that the first proportional voltage becomes equal to the first reference voltage, said first error amplifier starting operations or stopping operations according to the first control signal;

a first power supply switch that is switched according to an output signal from the first delay circuit, and controls power supply to the first reference voltage generator; and

a first output voltage supply switch that is switched according to the first control signal, and controls supply of the voltage on the output terminal to the first divisional circuit.

Alternatively, preferably, the first output voltage controller comprises:

a first reference voltage generator that generates and outputs the first reference voltage;

a first delay circuit that delays the first reference voltage by a predetermined first time period;

a first divisional voltage circuit that divides the voltage output from the output terminal and generates and outputs the first proportional voltage;

a first error amplifier that controls operations of the first output transistor so that the first proportional voltage becomes equal to the output voltage of the first delay circuit, said first error amplifier starting operations or stopping operations according to the first control signal;

a first power supply switch that is switched according to the first control signal, and controls power supply to the first reference voltage generator; and

a first output voltage supply switch that is switched according to the first control signal, and controls supply of the voltage on the output terminal to the first divisional circuit.

According to a second aspect of the present invention, there is provided a constant-voltage power supply circuit that converts a voltage input to an input terminal to a predetermined constant voltage and outputs the constant voltage, comprising:

a first constant-voltage circuit includinga first output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, anda first output voltage controller that generates a predetermined first reference voltage and generates a first proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the first reference voltage and the first proportional voltage, and outputs the amplified difference to the control electrode of the first output transistor; and

a second constant-voltage circuit includinga second output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, anda second output voltage controller that generates a predetermined second reference voltage and generates a second proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the second reference voltage and the second proportional voltage, and outputs the amplified difference to the control electrode of the second output transistor,said second constant-voltage circuit having a response speed to a voltage change on the output terminal faster than a response speed of the first constant-voltage circuit to a voltage change on the output terminal,

wherein

the first output voltage controller and the second output voltage controller start operations or stop operations according to a first control signal and a second control signal input from the outside, respectively,

when the second output voltage controller starts operations, a rising edge of the second reference voltage is delayed so as to be later than a rising edge of the second proportional voltage.

Preferably, the second output voltage controller comprises:

a second delay circuit that delays the second control signal by a predetermined second time period;

a second reference voltage generator that generates and outputs the second reference voltage;

a second divisional voltage circuit that divides the voltage output from the output terminal and generates and outputs the second proportional voltage;

a second error amplifier that controls operations of the second output transistor so that the second proportional voltage becomes equal to the second reference voltage, said second error amplifier starting operations or stopping operations according to the second control signal;

a second power supply switch that is switched according to an output signal from the second delay circuit, and controls power supply to the second reference voltage generator; and

a second output voltage supply switch that is switched according to the second control signal, and controls supply of the voltage on the output terminal to the second divisional circuit.

Alternatively, preferably, the second output voltage controller comprises:

a second reference voltage generator that generates and outputs the second reference voltage;

a second delay circuit that delays the second reference voltage by a predetermined second time period;

a second divisional voltage circuit that divides the voltage output from the output terminal and generates and outputs the second proportional voltage;

a second error amplifier that controls operations of the second output transistor so that the second proportional voltage becomes equal to the output voltage of the second delay circuit, said second error amplifier starting operations or stopping operations according to the second control signal;

a second power supply switch that is switched according to the second control signal, and controls power supply to the second reference voltage generator; and

a second output voltage supply switch that is switched according to the second control signal, and controls supply of the voltage on the output terminal to the second divisional circuit.

According to a third aspect of the present invention, there is provided a constant-voltage power supply circuit that converts a voltage input to an input terminal to a predetermined constant voltage and outputs the constant voltage, comprising:

a first constant-voltage circuit includinga first output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, anda first output voltage controller that generates a predetermined first reference voltage and generates a first proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the first reference voltage and the first proportional voltage, and outputs the amplified difference to the control electrode of the first output transistor; and

a second constant-voltage circuit includinga second output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, anda second output voltage controller that generates a predetermined second reference voltage and generates a second proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the second reference voltage and the second proportional voltage, and outputs the amplified difference to the control electrode of the second output transistor,

wherein

the first output voltage controller and the second output voltage controller start operations or stop operations according to a first control signal and a second control signal input from the outside, respectively,

when starting operations, in a time period from a time of starting the operations, the corresponding one of the first output transistor and the second output transistor is turned OFF (disconnection state).

Preferably, the first output voltage controller comprises:

a first delay circuit that delays the first control signal by a predetermined first time period;

a first reference voltage generator that generates and outputs the first reference voltage;

a first divisional voltage circuit that divides the voltage output from the output terminal and generates and outputs the first proportional voltage;

a first error amplifier that controls operations of the first output transistor so that the first proportional voltage becomes equal to the first reference voltage, said first error amplifier starting operations or stopping operations according to the first control signal;

a first power supply switch that is switched according to a control circuit, and controls power supply to the first reference voltage generator; and

a first output voltage supply switch that is switched according to the first control signal, and controls supply of the voltage on the output terminal to the first divisional circuit.

According to a fourth aspect of the present invention, there is provided a constant-voltage power supply circuit that converts a voltage input to an input terminal to a predetermined constant voltage and outputs the constant voltage, comprising:

a first constant-voltage circuit includinga first output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, anda first output voltage controller that generates a predetermined first reference voltage and generates a first proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the first reference voltage and the first proportional voltage, and outputs the amplified difference to the control electrode of the first output transistor; and

a second constant-voltage circuit includinga second output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, anda second output voltage controller that generates a predetermined second reference voltage and generates a second proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the second reference voltage and the second proportional voltage, and outputs the amplified difference to the control electrode of the second output transistor,said second constant-voltage circuit having a response speed to a voltage change on the output terminal faster than a response speed of the first constant-voltage circuit to a voltage change on the output terminal,

wherein

the first output voltage controller and the second output voltage controller start operations or stop operations according to a first control signal and a second control signal input from the outside, respectively,

when the second output voltage controller starts operations, in a time period from a time of starting the operations, the second output transistor is turned OFF (disconnection state).

Preferably, the second output voltage controller comprises:

a second delay circuit that delays the second control signal by a predetermined second time period;

a second reference voltage generator that generates and outputs the second reference voltage;

a second divisional voltage circuit that divides the voltage output from the output terminal and generates and outputs the second proportional voltage;

a second error amplifier that controls operations of the second output transistor so that the second proportional voltage becomes equal to the second reference voltage, said second error amplifier starting operations or stopping operations according to the output signal from the second delay circuit;

a second power supply switch that is switched according to the second control signal, and controls power supply to the second reference voltage generator; and

a second output voltage supply switch that is switched according to the second control signal, and controls supply of the voltage on the output terminal to the second divisional circuit.

Preferably, the predetermined first time period equals the delayed time of the rising edge of the first reference voltage when the first output voltage controller starts operations, which delayed time makes the rising edge of the first reference voltage later than the rising edge of the first proportional voltage.

Preferably, the predetermined second time period equals the delayed time of the rising edge of the first reference voltage when the second output voltage controller starts operations, which delayed time makes the rising edge of the first reference voltage later than the rising edge of the first proportional voltage.

Preferably, the first constant-voltage circuit and the second constant-voltage circuit are integrated in one IC chip.

According to a fifth aspect of the present invention, there is provided a method of controlling a constant-voltage power supply circuit, said constant-voltage power supply circuit converting a voltage input to an input terminal to a predetermined constant voltage and outputting the constant voltage, said constant-voltage power supply circuit including a first constant-voltage circuit having a first output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, and a first output voltage controller that generates a predetermined first reference voltage and generates a first proportional voltage in proportion to a voltage on the outputs terminal, amplifies a difference between the first reference voltage and the first proportional voltage, and outputs the amplified difference to the control electrode of the first output transistor; and a second constant-voltage circuit having a second output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, and a second output voltage controller that generates a predetermined second reference voltage and generates a second proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the second reference voltage and the second proportional voltage, and outputs the amplified difference to the control electrode of the second output transistor,

said method comprising the step of:

when the first output voltage controller or the second output voltage controller starts operations according to a control signal from the outside, a rising edge of the corresponding one of the first reference voltage and the second reference voltage is delayed so as to be later than a rising edge of the corresponding one of the first proportional voltage and the second proportional voltage.

According to a sixth aspect of the present invention, there is provided a method of controlling a constant-voltage power supply circuit, said constant-voltage power supply circuit converting a voltage input to an input terminal to a predetermined constant voltage and outputting the constant voltage, said constant-voltage power supply circuit including a first constant-voltage circuit having a first output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, and a first output voltage controller that generates a predetermined first reference voltage and generates a first proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the first reference voltage and the first proportional voltage, and outputs the amplified difference to the control electrode of the first output transistor; and a second constant-voltage circuit having a second output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, and a second output voltage controller that generates a predetermined second reference voltage and generates a second proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the second reference voltage and the second proportional voltage, and outputs the amplified difference to the control electrode of the second output transistor, said second constant-voltage circuit having a response speed to a voltage change on the output terminal faster than a response speed of the first constant-voltage circuit to a voltage change on the output terminal,

said method comprising the step of:

when the second output voltage controller starts operations according to a control signal input from the outside, a rising edge of the second reference voltage is delayed so as to be later than a rising edge of the second proportional voltage.

According to a seventh aspect of the present invention, there is provided a method of controlling a constant-voltage power supply circuit, said constant-voltage power supply circuit converting a voltage input to an input terminal to a predetermined constant voltage and outputting the constant voltage, said constant-voltage power supply circuit including a first constant-voltage circuit having a first output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, and a first output voltage controller that generates a predetermined first reference voltage and generates a first proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the first reference voltage and the first proportional voltage, and outputs the amplified difference to the control electrode of the first output transistor; and a second constant-voltage circuit having a second output transistor that outputs a current corresponding to a signal input to a control electrode thereof from the input terminal to the output terminal, and a second output voltage controller that generates a predetermined second reference voltage and generates a second proportional voltage in proportion to a voltage on the output terminal, amplifies a difference between the second reference voltage and the second proportional voltage, and outputs the amplified difference to the control electrode of the second output transistor,

said method comprising the step of:

when the first output voltage controller or the second output voltage controller starts operations according to a control signal from the outside, in a time period from a time of starting the operations, the corresponding one of the first output transistor and the second output transistor is turned OFF (disconnection state).

According to the present invention, the first constant-voltage circuit and the second constant-voltage circuit share the output terminal, when the first constant-voltage circuit or the second constant-voltage circuit starts operations, a rising edge of the first reference voltage or the second reference voltage in operation is delayed so as to be later than a rising edge of the first proportional voltage or the second proportional voltage. In this way, overshoot of the output voltage is preventable, which possibly occurs when switching over the constant-voltage circuits, and it is possible to supply a constant output voltage.

In addition, since the first constant-voltage circuit and the second constant-voltage circuit share the output terminal, when the first constant-voltage circuit or the second constant-voltage circuit starts operations, in a time period from a time of starting the operations, the first output transistor or the second output transistor is turned OFF (disconnection state). In this way, overshoot of the output voltage is preventable, which possibly occurs when switching over the constant-voltage circuits, and it is possible to supply a constant output voltage.

These and other objects, features, and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments given with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, preferred embodiments of the present invention are explained with reference to the accompanying drawings.

First Embodiment

FIG. 1is a circuit diagram illustrating an example of a constant-voltage power supply circuit according to a first embodiment of the present invention.

As shown inFIG. 1, a constant-voltage power supply circuit1converts an input voltage Vin input to an input terminal IN to an output voltage Vout output from an output terminal OUT.

The constant-voltage power supply circuit1includes a constant-voltage circuit2and a constant-voltage circuit3, which have the same circuit configuration, and each of the constant-voltage circuits2and3forms a series regulator.

The constant-voltage circuit2includes a reference voltage generator11which generates and outputs a reference voltage Vr1, resistors R11, R12which divide the output voltage Vout and generate and output a divisional voltage VFB1, an output transistor M11which is a PMOS transistor that controls a current io1corresponding to a signal input to a gate of the transistor M11and outputs the current io1to the output terminal OUT, an error amplifier A11which controls the output transistor M11so that the divisional voltage VFB1becomes equal to the reference voltage Vr1, switches12,13, and a delay circuit14.

The constant-voltage circuit3includes a reference voltage generator21which generates and outputs a reference voltage Vr2, resistors R21, R22which divide the output voltage Vout and generate and output a divisional voltage VFB2, an output transistor M21which is a PMOS transistor that controls a current io2corresponding to a signal input to a gate (electrode) of the transistor M21and outputs the current io2to the output terminal OUT, an error amplifier A21which controls the output transistor M21so that the divisional voltage VFB2becomes equal to the reference voltage Vr2, switches22,23, and a delay circuit24.

Here, the constant-voltage circuit2corresponds to the first constant-voltage circuit in claims; the output transistor M11corresponds to the first output transistor in claims; and the reference voltage generator11, the resistors R11, R12, the error amplifier A11, switches12,13, and the delay circuit14correspond to the first output voltage controller in claims.

Similarly, the constant-voltage circuit3corresponds to the second constant-voltage circuit in claims; the output transistor M21corresponds to the second output transistor in claims; and the reference voltage generator21, the resistors R21, R22, the error amplifier A21, switches22,23, and the delay circuit24correspond to the second output voltage controller in claims.

In addition, the reference voltage generator11corresponds to the first reference voltage generator in claims, the resistors R11, R12correspond to the first divisional voltage circuit in claims, the error amplifier A11corresponds to the first error amplifier in claims, the switch12corresponds to the first power supply switch in claims, the switch13corresponds to the first output voltage switch in claims, and the delay circuit14corresponds to the first delay circuit in claims.

Similarly, the reference voltage generator21corresponds to the second reference voltage generator in claims, the resistors R21, R22correspond to the second divisional voltage circuit in claims, the error amplifier A21corresponds to the second error amplifier in claims, the switch22corresponds to the second power supply switch in claims, the switch23corresponds to the second output voltage switch in claims, and the delay circuit24corresponds to the second delay circuit in claims.

Additionally, a control signal SC1corresponds to the first control signal in claims, and a control signal SC2corresponds to the second control signal in claims.

In the constant-voltage circuit2, the output transistor M11is connected between the input terminal IN and the output terminal OUT, and the switch13and the resistors R11, R12are connected in series between the output terminal OUT and a grounding voltage GND.

An output terminal of the error amplifier A11is connected to a gate of the output transistor M11, the divisional voltage VFB1is input to a non-inverted input terminal of the error amplifier A11, and the reference voltage Vr1is input to an inverted input terminal of the error amplifier A11.

The reference voltage generator11receives an input voltage Vin as a power voltage through the switch12, controls the switch13according to a control signal SC1from outside, and controls the operations of the error amplifier A11. In addition, the control signal SC1is delayed by the delay circuit14, and the delayed control signal SC1dcontrols the switching operations of the switch12.

Similarly, in the constant-voltage circuit3, the output transistor M21is connected between the input terminal IN and the output terminal OUT, and the switch23and the resistors R21, R22are connected in series between the output terminal OUT and a grounding voltage GND.

An output terminal of the error amplifier A21is connected to a gate of the output transistor M21, the divisional voltage VFB2is input to a non-inverted input terminal of the error amplifier A21, and the reference voltage Vr1is input to an inverted input terminal of the error amplifier A21.

The reference voltage generator21receives the input voltage Vin as a power voltage through the switch22, controls the switch23according to a control signal SC2from outside, and controls the operations of the error amplifier A21. In addition, the control signal SC2is delayed by the delay circuit24, and the delayed control signal SC2dcontrols the switching operations of the switch22.

FIG. 2is a circuit diagram exemplifying the delay circuit14inFIG. 1.

The delay circuit24has essentially the same circuit configuration except that a different resistor and a different condenser are used to have a different time constant, and thus, the overlapping descriptions are omitted.

As shown inFIG. 2, the delay circuit14includes a resistor R14and a condenser C14, the resistor R14and the condenser C14are connected in series between the control signal SC1and the grounding voltage GND, and the delayed control signal SC1dis extracted from the connection point between the resistor R14and the condenser C14.

Operations of the constant-voltage power supply circuit1with the above-described circuit configuration are described with reference toFIG. 3.

FIG. 3is a time chart illustrating waveforms of signals in the constant-voltage power supply circuit1as shown inFIG. 1.

InFIG. 3, when the control signal SC1is at a high level, the constant-voltage circuit2starts to operate, and when the control signal SC1is at a low level, the constant-voltage circuit2stops operation. Similarly, when the control signal SC2is at the high level, the constant-voltage circuit3starts to operate, and when the control signal SC2is at the low level, the constant-voltage circuit3stops operation.

When switching from a state in which only the constant-voltage circuit3is in operation to a state in which the operation of the constant-voltage circuit3is stopped and only the constant-voltage circuit2is in operation, the control signal SC1changes to the high level while the control signal SC2, being at the high level, after a certain time period changes to the low level.

Similarly, when switching from a state in which only the constant-voltage circuit2is in operation to a state in which the operation of the constant-voltage circuit2is stopped and only the constant-voltage circuit3is in operation, the control signal SC2changes to the high level while the control signal SC1, being at the high level, after a certain time period changes to the low level.

Namely, when the control signal SC1is at the high level, the switch13is turned ON (connection state); at the same time, the error amplifier A11starts to operate, and after the delay time Td1, which is set in advance in the delay circuit14, the switch12is turned ON (connection state). Additionally, when the control signal SC1is at the low level, the switch13is turned OFF (disconnection state); at the same time, the error amplifier A11stops operation. After the delay time Td1, the switch12is turned OFF (disconnection state).

Similarly, when the control signal SC2is at the high level, the switch23is turned ON (connection state); at the same time, the error amplifier A21starts to operate, and after the delay time Td2set in the delay circuit24, the switch22is turned ON (connection state). Additionally, when the control signal SC2is at the low level, the switch23is turned OFF (disconnection state); at the same time, the error amplifier A21stops operation. After the delay time Td2, the switch22is turned OFF (disconnection state).

For example, when the control signal SC1rises to the high level to drive the constant-voltage circuit2to operate, the divisional voltage VFB1, which is input to the non-inverted input terminal of the error amplifier A11, rises gradually. In this process, the switch12is turned OFF, and the reference voltage Vr1from the reference voltage generator11becomes the grounding voltage GND. Due to this, the output transistor M11is still in the OFF state, but the constant-voltage circuit3is in operation; therefore, the output voltage Vout is at a constant value. Next, after the preset delay time Td1, the switch12is turned ON, and the reference voltage generator11outputs the preset reference voltage Vr1.

Here, the delay time Td1is equal to the time required for the divisional voltage VFB1to rise as the division of the output voltage Vout after the control signal SC1is changed to the high level. Additionally, the control signal SC2changes to the low level after the control signal SC1is changed to the high level.

In this way, as shown inFIG. 3, the overshoot does not occur in the output voltage Vout.

The process is the same when the control signal SC2rises to the high level to drive the constant-voltage circuit3to operate, and detailed descriptions are omitted.

FIG. 4is a circuit diagram illustrating another example of the constant-voltage power supply circuit according to the first embodiment of the present invention.

In theFIG. 1, the power supplies to the reference voltage generators11,21are delayed by delay circuits14and24, respectively. Instead, the reference voltage Vr1from the reference voltage generator11may be delayed by a delay circuit, and input into an inverted input terminal of the error amplifier.

FIG. 4shows such an example of the constant-voltage power supply circuit. InFIG. 4, the same reference numbers are assigned to the same elements as described inFIG. 1, and only the differences betweenFIG. 1andFIG. 4are described with overlapping descriptions being omitted.

The circuit diagram of the constant-voltage power supply circuit shown inFIG. 4differs from that inFIG. 1in that the switch12is controlled by the control signal SC1, while the switch22is controlled by the control signal SC2. The delay circuit14is connected between the output terminal of the reference voltage generator11and an inverted input terminal of the error amplifier A11, and the delay circuit24is connected between the output terminal of the reference voltage generator21and an inverted input terminal of the error amplifier A21.

Operations of the constant-voltage power supply circuit1as shown inFIG. 4are described with reference toFIG. 5.

FIG. 5is a time chart illustrating waveforms of signals in the constant-voltage power supply circuit as shown inFIG. 4.

When the control signal SC1is at the high level, the switches12and13are turned ON (connection state); at the same time, the error amplifier A1and the reference voltage generator11start to operate, and after the delay time Td1, which is set beforehand in the delay circuit14, the reference voltage Vr1is input to the inverted input terminal of the error amplifier A11as a reference voltage Vr1d. When the control signal SC1is at the low level, the switches12and13are turned OFF (disconnection state); at the same time, the error amplifier A11and the reference voltage generator11stop operations, and the level of the reference voltage Vr1decreases according to the time constant of the delay circuit14.

Similarly, when the control signal SC2is at the high level, the switches22and23are turned ON (connection state); at the same time, the error amplifier A21and the reference voltage generator21start to operate, and after the delay time Td2, which is set beforehand in the delay circuit24, the reference voltage Vr2is input to the inverted input terminal of the error amplifier A21as a reference voltage Vr2d. When the control signal SC2is at the low level, the switches22and23are turned OFF (disconnection state); at the same time, the error amplifier A21and the reference voltage generator21stop operations, and the level of the reference voltage Vr1decreases according to the time constant of the delay circuit24.

In this way, the same effects as those shown with reference toFIG. 1are obtainable.

FIG. 6is a circuit diagram illustrating another example of the constant-voltage power supply circuit according to the first embodiment of the present invention.

When the constant-voltage circuit3has a faster response to the change of the output voltage Vout than the constant-voltage circuit2, as shown inFIG. 6, the delay circuit14as shown inFIG. 1may be omitted, and a delay circuit for delaying the control signal SC2may be provided only in the constant-voltage circuit3. With such a circuit configuration, similar to that shown inFIG. 3, the overshoot does not occur in the output voltage Vout.

FIG. 7is a circuit diagram illustrating another example of the constant-voltage power supply circuit according to the first embodiment of the present invention.

Similar toFIG. 6, when the constant-voltage circuit3has a faster response to the change of the output voltage Vout than that of the constant-voltage circuit2, as shown inFIG. 7, the delay circuit14as shown inFIG. 4may be omitted, and a delay circuit for delaying the reference voltage from the reference voltage generator and outputting the delayed reference voltage to the error amplifier A11may be provided only in the constant-voltage circuit3. With such a circuit configuration, similar to that shown inFIG. 5, the overshoot does not occur in the output voltage Vout.

According to the present embodiment, the constant-voltage circuit2and the constant-voltage circuit3share the output terminal OUT; when the constant-voltage circuit2or the constant-voltage circuit3starts operations, the rising edge of the reference voltage, which is to be input to the error amplifier of the constant-voltage circuit in operation, is delayed so as to be later than the rising edge of the divisional voltage input to the error amplifier. Due to this, the overshoot of the output voltage is preventable, which possibly occurs when switching over the constant-voltage circuits, and thus it is possible to supply a constant output voltage.

Second Embodiment

In the first embodiment, the rising edge of the reference voltage is delayed by using a delay circuit so as to be later than the rising edge of the divisional voltage. Instead, the operation start timing of the error amplifier may be delayed by a delay circuit. This is described in the present embodiment.

FIG. 8is a circuit diagram illustrating an example of the constant-voltage power supply circuit according to a second embodiment of the present invention.

InFIG. 8, the same reference numbers are assigned to the same elements as described inFIG. 1, and only the differences betweenFIG. 1andFIG. 8are described with overlapping descriptions being omitted.

The circuit diagram of the constant-voltage power supply circuit shown inFIG. 8differs from that inFIG. 1in that the switch12is controlled by the control signal SC1, while the switch22is controlled by the control signal SC2, the error amplifier A11is controlled by the delayed control signal SC1dfrom the delay circuit14, and the error amplifier A21is controlled by the delayed control signal SC2dfrom the delay circuit24.

Operations of the constant-voltage power supply circuit1as shown inFIG. 8are described with reference toFIG. 9.

FIG. 9is a time chart illustrating waveforms of signals in the constant-voltage power supply circuit1as shown inFIG. 8.

When the control signal SC1is at the high level, the switches12and13are turned ON (connection state); at the same time, the error amplifier A11starts to operate, and after the delay time Td1, which is set beforehand in the delay circuit14, the reference voltage generator11starts to operate. When the control signal SC1is at the low level, the switches12and13are turned OFF (disconnection state); at the same time, the reference voltage generator11stops operations, and after the delay time Td1, the error amplifier A11stops the operations.

Similarly, when the control signal SC2is at the high level, the switches22and23are turned ON (connection state); at the same time, the error amplifier A21starts to operate, and after the delay time Td2, which is set beforehand in the delay circuit24, the reference voltage generator21starts to operate. When the control signal SC2is at the low level, the switches22and23are turned OFF (disconnection state); at the same time, the reference voltage generator21stop operations, and after the delay time Td2, the error amplifier A21stops the operations.

For example, when the control signal SC1rises to the high level to drive the constant-voltage circuit2to operate, the reference voltage generator11starts to operates immediately, and the reference voltage Vr1, which is input to an inverted input terminal of the error amplifier A11, rises gradually up to a preset level. The divisional voltage VFB1, which is input to the non-inverted input terminal of the error amplifier A11, rises gradually.

In this process, the error amplifier A11is not in operation and the output transistor M11is still in the OFF state, but the constant-voltage circuit3is in operation; therefore, the output voltage Vout is at a constant value. Next, after the preset delay time Td1, and the error amplifier A11starts to operate.

In this way, as shown inFIG. 9, the overshoot does not occur in the output voltage Vout.

The process is the same when the control signal SC2rises to the high level to drive the constant-voltage circuit3to operate, and detailed descriptions are omitted.

FIG. 10is a circuit diagram illustrating another example of the constant-voltage power supply circuit according to the second embodiment of the present invention.

When the constant-voltage circuit3has a faster response to the change of the output voltage Vout than the constant-voltage circuit2, as shown inFIG. 10, the delay circuit14as shown inFIG. 8may be omitted, and a delay circuit for delaying the control signal SC2may be provided only in the constant-voltage circuit3. With such a circuit configuration, similar to that shown inFIG. 9, the overshoot does not occur in the output voltage Vout.

According to the present embodiment, the constant-voltage circuit2and the constant-voltage circuit3share the output terminal OUT; when the constant-voltage circuit2or the constant-voltage circuit3is to start operations, the operation start timing of the error amplifier in the constant-voltage circuit to be in operation may be delayed by a preset delay time with a delay circuit. Due to this, the overshoot of the output voltage is preventable, which overshoot possibly occurs when switching over the constant-voltage circuits, and thus it is possible to supply a constant output voltage.

The constant-voltage power supply circuits described in the above embodiments can be integrated into one chip.

While the present invention is described above with reference to specific embodiments chosen for purpose of illustration, it should be apparent that the invention is not limited to these embodiments, but numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.

This patent application is based on Japanese Priority Patent Application No. 2005-159764 filed on May 31, 2005, and the entire contents of which are hereby incorporated by reference.