JP2864050B2 - Power switching circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a power supply switching circuit for switching between a main power supply and a sub power supply.

[Summary of the Invention]

The present invention relates to a conventional circuit having a function of switching between two power supplies formed as a monolithic IC. The transistor connected between the substrate and the drain of the switch transistor is driven by the inverter supplied with power from the output terminal of the circuit, and the transistor connected between the substrate and the drain of the circuit is driven by the input voltage of the inverter, so that the sub-power supply is shifted to the main power supply side. The present invention realizes an efficient power supply switching circuit that prevents the current from flowing out and prolongs the battery life of the sub power supply.

[Conventional technology]

FIG. 2 shows a conventional power supply switching circuit diagram. An auxiliary power is input from an input terminal 1 to a main power input terminal 2. When the main power is input, the output of the comparator 3 is
It goes high, and the gate of the switch transistor 5 goes low via the inverter 4. Then, the switch transistor 5 is turned on, and the voltage of the main power supply is output to the output terminal 6. On the other hand, when the main power is turned off and only the sub power is supplied, the switch transistor 5 is turned off, and the voltage of the sub power is generated at the output terminal 6 through the Schottky diode 7.

[Problems to be solved by the invention]

In this case, since the source and the substrate of the switch transistor 5 are connected, there is a parasitic diode having the drain terminal as the anode and the source terminal as the cathode between the source and the drain of the switch transistor 5, and the current of the sub power supply is reduced. There is a drawback that the power flows into the input terminal 1 side and is consumed, thereby significantly shortening the battery life of the sub power supply.

[Means for solving the problem]

SUMMARY OF THE INVENTION The present invention has been made to solve the problems of the prior art, and has made it possible to prevent the current from flowing from the auxiliary power supply to the input terminal 1 side, thereby extending the battery life.

Specifically, power of the inverter is supplied from an output terminal 6, transistors are provided between the source and the substrate of the switch transistor and between the drain and the substrate, and one of the transistors and the switch transistor 5 are driven by the inverter 4, The other transistor is driven by the input voltage of the inverter 4.

[Action]

The power supply switching circuit of the present invention, as shown in FIG.
In order to drive the transistors connected between the source and drain of the switch transistor and the substrate by the inverter supplied with positive power from the output terminal, the direction of the parasitic diode present in the switch transistor is switched depending on the presence of the main power and the sub power. I was able to.

〔Example〕

Hereinafter, embodiments of the power supply switching circuit of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a power supply switching circuit diagram according to the present invention. The description of the same parts as those in FIG. 2 is omitted. A transistor 8 whose source is on the substrate side is connected between the substrate and the source of the switch transistor 5, and a transistor 9 whose source is on the substrate side is connected between the substrate and the drain. The gate of the transistor 8 is connected to the gate of the switch transistor 5 and supplied from the output terminal of the inverter 4. On the other hand, the gate of the transistor 9 is supplied from the input terminal of the inverter 4. Further, the positive power supply terminal of the inverter 4 is connected to the output terminal 6.

Next, the operation will be described. When the main power is applied to the input terminal 1, as described above, the output of the inverter 4 becomes LOW level and the switch transistor is turned ON. At this time, the transistor 8 is also turned on at the same time. However, since the gate of the transistor 9 is applied from the input terminal of the inverter 4, the transistor 9 is turned off. As a result, the electrode on the input terminal 1 side of the switch transistor 5 and the substrate of the switch transistor 5 are short-circuited by the transistor 8, so that the substrate effect does not work on the switch transistor 5, the switch transistor 5 has a low ON resistance, and the switch transistor 5 has a low ON resistance. 5 has a small voltage drop. Therefore, almost the voltage of the main power supply is output to the output terminal 6.

A case where the main power supply of the input terminal 1 is removed and only the sub-electrode is applied to the input terminal 2 will be described. Since the main power supply of the input terminal 1 is removed, the power is not supplied to the comparator 3 and the output of the comparator 3 is in a high impedance state. On the other hand, the voltage of the sub power supply applied to the input terminal 2 is output to the output terminal 6 through the Schottky diode 7 and supplies the voltage to the inverter 4. Therefore, the output of the inverter 4 is inverted to the high level. As a result, the transistor 8 turns off and the transistor 9 turns on. Also, the switch transistor 5
Turn off. Since the transistor 9 is turned on, the electrode on the output terminal 6 side of the switch transistor 5 and the substrate are short-circuited, and a parasitic diode having the anode on the input terminal 1 side and the cathode on the output terminal 6 side is formed. However, this diode is reverse-biased because the output terminal 6 is at the positive potential and the input terminal 1 is at the ground potential, so that no current flows from the input terminal 2 to the input terminal 1.

〔The invention's effect〕

As described above, according to the present invention, the positive power supply of the inverter is supplied from the output terminal, the transistor is provided between the source and drain of the switch transistor and the substrate, and the switch transistor and the respective transistor are connected to the input or the inverter of the inverter. Driving at the output terminal has the effect of extending the battery life of the sub-power supply applied to one input terminal.

[Brief description of the drawings]

FIG. 1 is a power supply switching circuit diagram according to the present invention, and FIG. 2 is a conventional power supply switching circuit diagram. 1,2 Input terminal 5,8,9 Transistor 4 Inverter 7 Schottky diode 10 Resistor

Claims (1)

(57) [Claims] 1. A power supply switching circuit for switching one of an input terminal of a main power supply and an input terminal of a sub power supply and outputting to an output terminal of a switching circuit, wherein the power supply switching circuit is connected between the input terminal of the main power supply and the output terminal. MO
An S-type switch transistor; a second transistor connected between a source electrode of the MOS-type switch transistor and a substrate forming the MOS-type switch transistor; and a second transistor connected between a drain electrode of the MOS-type switch transistor and the substrate. A third transistor connected to the power supply switching circuit;
An inverter driven by the presence or absence of a voltage at an input terminal of a main power supply, wherein the first and second transistors are driven by voltages at an input terminal of the inverter and an output terminal of the inverter. Power supply switching circuit.