JP2010231499A - Constant voltage power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a constant voltage power supply for properly suppressing variation of an output voltage according to the variation forms of a load. <P>SOLUTION: When overshoot is caused in an output voltage, a transistor M1 is quickly turned on, and a current mirror circuit configured of transistors M2 and M3 is driven by using the surplus accumulated charge of an external capacitor. Thus, it is possible to quickly set a switch circuit to an on-state, and to let the surplus accumulated charge of the external capacitor C1 as the factor of overshoot escape to the ground with large currents. When the operation of the switch circuit 1 causes any inconvenience, an enable signal corresponding to the operation/stop of the load is supplied to a transistor M4, and the switch circuit 1 is set to an inoperable state. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a technique for suppressing fluctuation of an output voltage of a constant voltage power supply with a large load fluctuation.

FIG. 2 shows a circuit diagram of an example of a conventional series type constant voltage power source. The constant voltage power supply of FIG. 2 has the following circuit configuration.
A main current path of the power transistor MP composed of an n-channel MOSFET is connected between an input terminal IN connected to an external power supply source and an output terminal OUT connected to an external load. The output terminal of the error amplifier EA is connected to the gate of the power transistor MP, and the non-inverting input terminal (+) of the error amplifier EA is connected to the reference voltage source VREF. The voltage detection resistors R1 and R2 are connected in series between the output terminal OUT and the ground (= the circuit reference potential point, hereinafter referred to as the ground), and the connection point between the resistors R1 and R2 is the inversion of the error amplifier EA. Connect to the input terminal (-). Note that C1 connected between the output terminal OUT and the ground is an external capacitor.

A series type constant voltage power supply having a circuit configuration as shown in FIG. 2 has a feature that a response speed to a load change is faster than a switching type constant voltage power supply, for example. However, no matter how fast the series-type constant voltage power supply has a fast response speed, if the load fluctuates suddenly and in large quantities, the response cannot catch up, and the output voltage may fluctuate.
For example, when the load changes from a light state to a heavy state, the output voltage of the constant voltage power supply may drop. In the case of the constant voltage power supply having the circuit configuration of FIG. 2, if the power transistor MP having a large current capacity is applied and the error amplifier EA having a high driving capability is applied, the load is changed from a light state to a heavy state. It is possible to suppress a decrease in output voltage at the time.

  Conversely, when the load changes from a heavy state to a light state, the output voltage of the constant voltage power supply may increase. In the case of the constant voltage power supply having the circuit configuration of FIG. 2, since the output voltage once increased does not decrease unless the power consumption in the load progresses, there is a possibility that the output voltage will be higher than usual for a long time. . Therefore, in a constant voltage power supply for supplying a voltage to a load that fluctuates sharply and in large quantities, for example, as shown in Patent Document 1, a switch circuit that is turned on when an overshoot occurs in an output voltage is connected to an output terminal OUT and a ground. Connected in between. (In the following discussion, “load fluctuation” refers to the case where the load changes from a heavy state to a light state in principle.)

JP 2005-092693 A

  A load supplied with a voltage from a constant voltage power supply does not always change by the same amount. In order to suppress fluctuations in the output voltage of the constant voltage power supply, the switch circuit connected between the output terminal OUT and the ground is quickly closed and the cause of the overshoot, especially when the load is abrupt and fluctuates greatly. Therefore, the excessive accumulated charge of the capacitor C1 must be discharged to the ground with a sufficiently large current.

Also, recent load driving modes are diversified. For example, the load may be driven in a burst mode in order to reduce power consumption. When supplying output voltage from a constant-voltage power supply to a load driven in burst mode, the switch circuit operates with a relatively small overshoot that occurs during the off period (vs .: on period) during the burst period (vs .: pause period). Then it may be inconvenient.
Accordingly, an object of the present invention is to provide a constant voltage power source that can appropriately suppress fluctuations in output voltage in accordance with a fluctuation form of a load.

  The present invention for solving the above-described problems is a power transistor whose main current path is connected between the input and output terminals of the power supply and a feedback signal supplied in accordance with the output voltage to stabilize the output voltage. In a constant voltage power supply comprising an error amplifier that supplies a drive signal to a power transistor and a switch circuit that is connected between the output terminal and the ground and that is turned on when an overvoltage occurs in the output voltage, the switch circuit includes: One end of the main current path is connected to the output terminal, and the first transistor is configured to be connected to be complementary to the power transistor, and is connected in series to the other end of the main current path of the first transistor. A second transistor connected to function as a current reference side transistor of the circuit, and a main current path of the first transistor Forming a current path parallel to the series circuit of the second transistor, a second transistor as a current output side transistor of the current mirror circuit, a third transistor constituting the current mirror circuit, and a second transistor; And a fourth transistor connected between the common connection point of the main current path of the third transistor and the ground, and driven by a signal supplied from the outside.

When overshoot occurs, the first transistor is turned on at a high speed, and the current mirror circuit including the second and third transistors is driven by using the excess accumulated charge of the external capacitor, thereby quickly The switch circuit can be turned on.
The excessive accumulated charge of the external capacitor that causes overshoot can be released to the ground by a large current generated by the current mirror circuit.
If it is inconvenient for the switch circuit to operate, for example, a signal corresponding to the operation / stop of the load can be supplied to the fourth transistor to keep the switch circuit in an inoperative state.
As a result, it is possible to provide a constant voltage power source that can appropriately suppress fluctuations in the output voltage in accordance with the fluctuation form of the load.

The circuit diagram of the Example of the constant voltage power supply by this invention. The circuit diagram of the conventional constant voltage power supply.

Embodiments of a constant voltage power source according to the present invention will be described below with reference to the drawings.
FIG. 1 is a circuit diagram of a constant voltage power source according to the present invention. 1 is a constant voltage power source in which a power transistor MP, an error amplifier EA, a reference voltage source VREF, and resistors R1 and R2 are connected to each other as in the conventional example of FIG. Is connected to a switch circuit 1 having the following configuration.

  The gate of the transistor M1 formed of a transistor having a conductivity type opposite to that of the power transistor MP is connected to the output terminal of the error amplifier EA so as to perform a complementary operation with the power transistor MP. The source of the transistor M1 is connected to the output terminal OUT, and the drain of the transistor M1 is connected to the drain of the transistor M2 having the same conductivity type as the power transistor MP. The gate of the transistor M3 is commonly connected to the gate of the transistor M2 so as to form a current mirror circuit with the transistor M2.

  Here, the drain and gate of the transistor M2 are short-circuited so that the transistor M2 becomes a current reference side transistor of the current mirror circuit. Further, the drain of the transistor M3 is connected to the output terminal OUT so that the transistor M3 becomes a current output side transistor of the current mirror circuit, and the source of the transistor M3 is commonly connected to the source of the transistor M2. The main current path of the transistor M4 is connected between the common connection of the sources of the transistors M2 and M3 and the ground. Note that the gate of the transistor M4 is connected to an enable terminal 2 which receives an enable signal from the outside.

In the switch circuit 1 in which the transistors M1 to M4 are connected and configured as described above, when an overshoot occurs in the output voltage of the constant voltage power supply, the peak of the overshoot is suppressed by the following operation action.
When the load connected to the output terminal OUT is steep and fluctuates in a large amount (however, the load is reduced), the current drawn from the external capacitor C1 to the load decreases. At this time, the current passing through the power transistor MP does not suddenly decrease due to an operation delay of the feedback loop including the power transistor MP, the resistor R1, and the error amplifier EA. For this reason, an overshoot occurs in which the output voltage rises greatly exceeding the set value.

  When an overshoot occurs in the output voltage, the error amplifier EA is sized so that the drive signal supplied to the power transistor MP makes the passing current of the power transistor MP substantially zero, in other words, the power transistor MP is turned off. , To. At this time, the transistor M1 shifts to a conductive state due to the overshoot output voltage applied to the source and the drive signal applied to the gate. When current flows through the transistor M1, the transistor M2 is positively biased by the output voltage in the overshoot state, and current flows through the main current paths of the transistors M2 and M3 constituting the current mirror circuit.

  Here, if the mirror ratio of the transistors M2 and M3 is increased, the excessively accumulated charge of the external capacitor C1 causing overshoot with a sufficiently large current is discharged to the ground through the switch circuit 1. it can. Further, since only the current flowing through the main current path of the transistor M1 does not discharge the excessive accumulated charge of the external capacitor C1, a device having a high operation speed can be applied to the transistor M1 even though the current capacity is small.

  Until the transistors M2 and M3 shift from the cutoff state to the conductive state, the input parasitic capacitance in the parallel state of the two transistors acts so as to draw the surplus accumulated charge of the external capacitor C1 through the transistor M1. To do. Therefore, if the operation speed of the switch circuit 1 is considered as the time until a predetermined amount of charge is drawn from the external capacitor C1 to the switch circuit 1, applying a transistor having a high operation speed to the transistor M1 (applying a transistor having a high operation speed) Therefore, the delay amount of the operation as the switch circuit 1 that tends to be increased by increasing the number of stages of the transistors can be made equal to or less than that of the conventional switch circuit.

  Incidentally, as described above, depending on the drive mode of the load, it may be inconvenient if the switch circuit 1 operates. When there is a request to deactivate the switch circuit 1 in a specific state in the circuit of FIG. 1, a signal having a low voltage level is supplied to the enable terminal 2 when the specific condition occurs, and the transistor M4 is turned off. It should just be in a state. However, normally, the voltage level at the position of the enable terminal 2 is increased to make the switch circuit 1 operable, and a signal having a low voltage level is supplied to the enable terminal 2 as necessary to make the switch circuit 1 inoperable. Needless to say, it should be in a state.

1: Switch circuit 2: Enable terminal IN: Input terminal OUT: Output terminal MP: Power transistor M1: Transistor (first)
M2: Transistor (second)
M3: Transistor (third)
M4: Transistor (fourth)
EA: Error amplifier VREF: Reference voltage source (reference voltage)
C1: External capacitor

Claims (2)

A power transistor whose main current path is connected between the input and output terminals of the power supply, and an error in which a feedback signal corresponding to the output voltage is supplied and a drive signal for stabilizing the output voltage is supplied to the power transistor In a constant voltage power supply comprising an amplifier and a switch circuit that is connected between the output terminal and the ground and is turned on when an overshoot occurs in the output voltage, the switch circuit includes:
A first transistor having one end of a main current path connected to the output terminal and connected to be complementary to the power transistor;
A second transistor connected in series to the other end of the main current path of the first transistor and connected to function as a current reference transistor of a current mirror circuit;
The main current path forms a current path parallel to the series circuit of the first transistor and the second transistor, and the second transistor and the current mirror circuit are used as current output side transistors of the current mirror circuit. A third transistor comprising;
A fourth transistor connected between a common connection point of main current paths of the second transistor and the third transistor and the ground, and driven by a signal supplied from the outside;
A constant voltage power supply comprising:   2. The constant voltage power supply according to claim 1, wherein a signal supplied from the outside that drives the fourth transistor is an enable signal corresponding to an operating state of a load.

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