Patent ID: 7800433

Claim:
A power supply switching circuit, comprising: a first power source terminal to which a first power supply is connected; a second power source terminal to which a second power supply is connected; a first MOS transistor of a second conductive type which is formed on a semiconductor substrate of a first conductive type, and includes a drain connected to the first power source terminal and a source connected to a load terminal; a second MOS transistor of the second conductive type which is formed on the semiconductor substrate of the first conductive type, and includes a drain connected to the second power source terminal and a source connected to the load terminal; a first power supply detection circuit for detecting that the first power supply is connected to the first power source terminal, and delivering a detection signal; a control circuit for delivering, upon receiving the detection signal, a control signal to a gate of the first MOS transistor and a gate of the second MOS transistor; a step-up circuit for stepping up one of a voltage of the first power supply and a voltage of the second power supply, to deliver a stepped-up voltage; a first level shifter which is disposed between the control circuit and the gate of the first MOS transistor and performs a level shift operation on the control signal to the stepped-up voltage; and a second level shifter which is disposed between the control circuit and the gate of the second MOS transistor and performs a level shift operation on the control signal to the stepped-up voltage.