Patent ID: 7973595

Claim:
A power switch circuit, comprising: a first switch transistor connected to a main power supply that supplies a first voltage; a second switch transistor connected in series to the first switch transistor and to a backup power supply that supplies a second voltage; and a switch control unit that controls activation and deactivation of the first and second switch transistors so that a voltage corresponding to one of the first voltage and the second voltage is selectively output to a connection node between the first and second transistors; wherein the first switch transistor includes a first diode, which is formed so that a direction from the main power supply toward the connection node defines a forward direction, and the second switch transistor includes a second diode, which is formed so that a direction from the connection node toward the backup power supply defines a forward direction, and wherein the switch control unit includes: a comparator, connected to the main power supply, that compares a level of the first voltage with a reference voltage and generates a comparison result signal having one of a level of the first voltage and a ground level in accordance with the level of the first voltage; a switch signal generation unit that generates first and second switch signals respectively controlling the first and second switch transistors; and a level shifter, connected to the main power supply and the backup power supply, that generates a logic control signal having one of a level of the second voltage and the ground level in accordance with the level of the comparison result signal, and wherein the switch signal generation unit generates the first and second switch signals in accordance with the level of the logic control signal.