Patent ID: 7545610

Claim:
A constant-voltage power supply circuit for converting an input voltage applied to an input terminal into a predetermined constant voltage for output from an output terminal, comprising: an output transistor to supply from the input terminal to the output terminal an output current responsive to an applied control signal; a reference voltage generating circuit unit to produce a predetermined reference voltage; an output voltage detecting circuit unit to detect an output voltage at the output terminal to produce a proportional voltage proportional to the detected output voltage; an error amplifying circuit unit to receive a predetermined bias current to control an operation of the output transistor such that the proportional voltage becomes equal to the reference voltage; a bias current adjusting circuit unit to supply the error amplifying circuit unit with the bias current responsive to the output current output from the output transistor; and an overcurrent protection circuit unit to control the output transistor to reduce the output voltage and the output current such that the output current becomes a predetermined short-circuited current amount upon lowering of the output voltage to a ground potential in response to exceeding of the output current over a predetermined overcurrent protection current amount when the output voltage is at rated voltage, wherein the error amplifying circuit unit is configured such that a response speed thereof responsive to voltage fluctuation of the output voltage changes in response to the received bias current, and the bias current adjusting circuit unit is configured to suspend the supply of the bias current to the error amplifying circuit unit in response to lowering of the output voltage to a predetermined voltage.