Power supply circuit and method for controlling power supply voltage

Abstract

A power supply circuit is provided. The power supply circuit includes an output terminal, a first switching circuit, and a second switching circuit. The first switching circuit is used to conduct to provide a first voltage level to the output terminal. The second switching circuit includes a first transistor, a second transistor, and a third transistor. The first transistor is coupled to a second voltage level. The second voltage level is lower than the first voltage level. The second transistor is coupled between the first transistor and the output terminal. The first and second transistors are used to conduct to provide the second voltage level to the output terminal. The third transistor is coupled between the second transistor and ground. The second and third transistors are used to conduct to discharge the output terminal.

Claims

1. A power supply circuit, comprising: an output terminal; and a first switching circuit for conducting to provide a first voltage level to the output terminal; And a second switching circuit, comprising: a first transistor coupled to a second voltage level, the second voltage level being lower than the first voltage level; and a second transistor coupled between the first transistor and the output terminal, wherein the first and second transistors are configured to be turned on to provide the second voltage level to the output terminal; And a third transistor coupled between the second transistor and a ground, wherein the second and third transistors are used to conduct to discharge the output terminal.

2. The power supply circuit as claimed in claim 1, wherein a source of the first transistor is coupled to a source of the second transistor.

3. The power supply circuit as claimed in claim 2, wherein one source of the third transistor is grounded and one drain of the third transistor is coupled to the source of the first transistor.

4. The power supply circuit as claimed in claim 1, wherein the second transistor is a laterally diffused metal-oxide-semiconductor transistor.

5. The power supply circuit as claimed in claim 1, wherein the first switching circuit and the first transistor are turned off when the second and third transistors are turned on to discharge the output terminal.

6. The power supply circuit as claimed in claim 1 further comprises: a comparator for comparing the second voltage level and the voltage at the output terminal to generate an output signal; and a gate coupled to the output signal and for generating a control signal to a gate of the third transistor based on the output signal.

7. The power supply circuit as claimed in claim 1 further comprises: a control circuit for controlling a gate voltage of the second transistor, wherein the control circuit comprises: a current source coupled to the second voltage level; a fourth transistor coupled between the current source and the second switching circuit; and a fifth transistor coupled between a gate of the fourth transistor and the second switching circuit.

8. The power supply circuit as claimed in claim 7, wherein the gate of the fourth transistor is coupled to the current source, a source of the fourth transistor is coupled to a source of the first transistor and a source of the second transistor, wherein a source of the fifth transistor is coupled to the gate of the fourth transistor, and a drain of the fifth transistor is coupled to the gate of the second transistor.

9. The power supply circuit as claimed in claim 8, wherein the fifth transistor is turned on to generate a current on the second transistor to discharge the output terminal.

10. A method for controlling a power supply voltage, comprising: turning on a first switching circuit to provide a first voltage level to an output terminal; turning off the first switching circuit and turning on a first transistor and a second transistor to provide a second voltage level to the output terminal, wherein the second voltage level is lower than the first voltage level; and turning off the first switching circuit and the first transistor, and turning on the second transistor and a third transistor to discharge the output terminal, wherein the third transistor is coupled between the second transistor and a ground.

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