Patent ID: 6876180

Claim:
A power supply circuit comprising: a main transistor, provided in a current path extending from a power input terminal to a power output terminal, for lowering a source voltage applied from a power supply circuit through the power input terminal in accordance with a given drive signal; a voltage detecting circuit for detecting an output voltage outputted from the main transistor at said power output terminal and outputting the detected output voltage; a voltage control circuit, operable in response to said output voltage of the main transistor, for performing a closed-loop control to supply a first drive signal to said main transistor as a given drive signal so that the output voltage detected by said voltage detecting circuit is equalized to a target voltage; and a startup circuit, operable in response to the output voltage of the main transistor during a low output voltage period where the output voltage is lower than a predetermined voltage, for supplying a second drive signal to said main transistor as the given drive signal to control said main transistor to be turned on such that the output voltage is substantially equal to the source voltage of the power supply circuit.