JP2010231498A - Constant voltage power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a constant voltage power supply for improving ripple rejection characteristics in a relatively high frequency domain by improving circuit configurations. <P>SOLUTION: A transistor M3 operating when receiving supply of a fixed bias from a constant voltage source VB is installed between a transistor M1 configuring an active load and a transistor 5 configuring a differential amplifier circuit. Then, a power transistor MP is driven by using a signal appearing at the common connection point of the transistor M3 and the transistor M5 so that the transistor M3 can be interposed between the gate of the power transistor MP and the active load M1. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an improvement in a circuit configuration for improving ripple rejection characteristics in a high frequency region of a constant voltage power supply.

  The series type constant voltage power supply is characterized in that the circuit configuration is simpler than that of, for example, a switching type constant voltage power supply, and the response speed to load fluctuations is fast. Therefore, the series type constant voltage power supply is used as a power supply circuit for supplying a highly stable voltage to a load with relatively small power consumption.

FIG. 2 is a circuit diagram of a conventional general constant voltage power supply, in which each circuit element is connected and configured as follows.
The main current path of the power transistor MP is connected between the input terminal IN and the output terminal OUT of the constant voltage power source, and the detection resistors R1 and R2 are connected between the output terminal OUT and a reference potential point (hereinafter referred to as a grant) of the circuit. Connect in series. The sources of the transistors M1 and M2 whose gates are commonly connected are respectively connected to the input terminal IN, and the gate and drain of the transistor M2 are short-circuited. The drain of the transistor M1 is connected to the drain of the transistor M5 via the main current path of the transistor M3, and the drain of the transistor M2 is connected to the drain of the transistor M6 via the main current path of the transistor M4.

  The sources of the transistors M5 and M6 are commonly connected so as to constitute a differential amplifier circuit, and the common connection point of the sources is connected to the ground via the current source CS1. The gates of the transistors M3 and M4 are each connected to a constant voltage source VB. The gate of the transistor M5 is connected to the reference voltage source VREF, and the gate of the transistor M6 is connected to the common connection point of the detection resistors R1 and R2. The main current path of the transistor M7 and the current source CS2 are connected in series between the input terminal IN and the ground, and the gate of the transistor M7 is connected to the common connection point of the transistors M1 and M3. A phase compensation resistor RF and a phase compensation capacitor CF are connected in series between the gate and drain of the transistor M7.

  In the circuit of FIG. 2 configured as described above, the transistors M1 to M7, the current sources CS1 and CS2, and the constant voltage source VB constitute an error amplifier. Here, the transistors M1 and M2 constitute an active load in the error amplifier, and the transistors M3 and M4 constitute a cascode circuit with the transistors M5 and M6, respectively. Transistors M5 and M6 constitute a differential amplifier circuit in the error amplifier, and transistor M7 and current source CS2 constitute an output circuit. Note that the power transistor MP and the transistors M3, M4, M5, and M6 are N-channel type, and the transistors M1, M2, and M7 are P-channel type.

  The constant voltage power source shown in FIG. 2 detects the voltage at the position of the output terminal OUT (hereinafter referred to as the output voltage) by dividing it with detection resistors R1 and R2, and this is detected by a reference voltage in an error amplifier comprising transistors M1 to M7 and others. Comparison is made with the reference voltage of the source VREF. 2 drives the power transistor MP connected to the output circuit based on the difference between the two voltages. At that time, the current passing through the main current path of the power transistor MP is controlled so that the voltage appearing at the common connection point of the detection resistors R1 and R2 becomes equal to the reference voltage of the reference voltage source VREF. As a result, the constant voltage power supply of FIG. 2 operates so that its output voltage is stabilized at a value that makes the voltage at the common connection point of the detection resistors R1 and R2 equal to the reference voltage of the reference voltage source VREF.

Patent WO2003-091817

  As discussed in Patent Document 1, since the series type constant voltage power supply is often used connected to a load that requires a highly stable voltage, its ripple rejection characteristics are emphasized. In the case of the constant voltage power supply having the circuit configuration shown in FIG. 2, there is a problem in the ripple rejection characteristics in a relatively high frequency region.

  In the constant voltage power supply of FIG. 2, the transistor M1 has parasitic capacitances C1a and C1b between its gate and source and between its drain and gate, respectively. The power transistor MP has a parasitic capacitance CP between its gate and source. Here, when a relatively high-frequency ripple is superimposed on a voltage supplied to the input terminal IN (hereinafter referred to as input voltage), the ripple components are parasitic capacitances C1a and C1b, a phase compensation resistor RF, and a phase compensation capacitor CF. Then, it leaks through the path of the parasitic capacitance CP to the output terminal OUT. It is estimated that this is one of the causes that the ripple rejection characteristic of the constant voltage power supply having the configuration of FIG.

  Accordingly, an object of the present invention is to provide a constant voltage power supply having improved ripple rejection characteristics in a relatively high frequency region by improving the circuit configuration.

  The present invention for solving the above-described problems is a constant voltage power source that drives a power transistor connected between input and output terminals by an error amplifier that operates according to an output voltage and a reference voltage, and obtains the stable output voltage. A first and a second transistor having one end of a main current path commonly connected so as to form a differential amplifier circuit in the error amplifier, and the other end of the main current path of the first transistor and an active load A constant bias is supplied to the third transistor connected to the second transistor, the fourth transistor connected between the other end of the main current path of the second transistor and the active load, and the third and fourth transistors. A power source is driven by using a signal obtained at a common connection point between the other end of the main current path of the first transistor and the third transistor. That.

  A third transistor that operates with a constant bias supplied is interposed between the gate of the power transistor and the active load, thereby reducing a ripple component in a relatively high frequency region that leaks to the output terminal through the active load. . As a result, a constant voltage power supply with improved ripple rejection characteristics can be provided.

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. The circuit of FIG. 1 is characterized in that the gate of the transistor M7 and one end of the phase compensation resistor RF are connected to the common connection point of the transistors M3 and M5. The presence of other circuit elements in FIG. 1 and the connection configuration thereof are the same as those of the circuit of FIG.

With such a circuit configuration, the ripple rejection characteristic of the constant voltage power source is improved mainly by the following two actions.
First, in the circuit of FIG. 1, a transistor M3 that receives a constant bias is interposed between the input terminal IN and the gate of the power transistor MP together with the transistor M1, the phase compensation resistor RF, and the phase compensation capacitor CF. For this reason, the transistor M3 limits the ripple in the high-frequency region that leaks in the gate direction of the power transistor MP through the parasitic capacitances C1a and C1b of the transistor M1.

Similarly to the transistor M1, there is a parasitic capacitance between the gate and the source of the transistor M7, and it is conceivable that a ripple in a high frequency region leaks in the gate direction of the power transistor MP through the parasitic capacitance. In the circuit configuration of FIG. 1, a part of the ripple leaked through the gate-source parasitic capacitance of the transistor M7 is in the direction of the ground via the gate-source parasitic capacitance of the transistor M3 and the constant voltage source VB. To leak.
With these actions, the ripple rejection characteristic of the constant voltage power source of FIG. 1 is improved as compared with the circuit of FIG.

IN: input terminal OUT: output terminal MP: power transistors M1, M2: transistors (active load)
M3: Transistor (third)
M4: Transistor (fourth)
M5: Transistor (first)
M6: Transistor (second)
VB: Constant voltage source (constant bias)
VREF: Reference voltage source (reference voltage)
R1, R2: detection resistors C1a, C1b: parasitic capacitance CP: parasitic capacitance

Claims (2)

In a constant voltage power source that drives a power transistor connected between input and output terminals with an error amplifier that operates according to an output voltage and a reference voltage, and obtains the stable output voltage,
The first and second transistors having one end of the main current path commonly connected so as to form a differential amplifier circuit in the error amplifier, the other end of the main current path of the first transistor, and the active load A third transistor connected in between, a fourth transistor connected between the other end of the main current path of the second transistor and the active load, and a constant bias on the third and fourth transistors A constant voltage source for supplying
A constant voltage power supply, wherein the power transistor is driven using a signal obtained at a common connection point between the other end of the main current path of the first transistor and the third transistor.   The constant connection according to claim 1, wherein a common connection point between the other end of the main current path of the first transistor and the third transistor is connected to a gate of the power transistor through an output circuit. Voltage power supply.

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