Patent ID: 7917786

Claim:
A voltage regulating circuit for a motherboard comprising a selecting switch and a switch module, the selecting switch comprising a first NMOS transistor and a PMOS transistor, the switch module comprising second to fourth NMOS transistors; wherein the source of the PMOS transistor is arranged to receive a standby power provided by a power supply, the gate of the PMOS transistor is connected to the standby power and the drain of the second NMOS transistor, the drain of the first NMOS transistor is arranged to receive a system power, the gate of the first NMOS transistor is arranged to receive a starting voltage and connected to the drain of the third NMOS transistor, the drain of the PMOS transistor is connected to the source of the first NMOS transistor as an output terminal of the selecting switch; wherein the gate of the second NMOS transistor is connected to the gate of the third NMOS transistor and the drain of the fourth NMOS transistor and the standby power, the source of the second NMOS transistor is connected to the standby power and arranged to receive a state signal from the motherboard, the gate of the fourth NMOS transistor is arranged to receive a power good signal generated by the power supply, the sources of the third and fourth NMOS transistors are grounded; wherein when the motherboard is turned off, the state signal is at a high level, the power good signal is at a low level; when the motherboard is turned on, the state signal is at a high level, the power good signal is at a high level; when the motherboard is in a standby state, the state signal is at a low level, the power good signal is at a low level.