Patent ID: 7215180
Filing Date: 2007-05-08
Classification: G05F

Abstract:
1. A constant voltage circuit, comprising: an output control transistor configured to control a current output from a predetermined output terminal so that a voltage output from the output terminal remains constant at a predetermined value; and an overcurrent protection circuit configured to control an operation of the output control transistor so as to prevent an output current of the output control transistor from exceeding a predetermined value, wherein the overcurrent protection circuit includes a proportional current generation circuit part configured to generate and output a current proportional to the output current of the output control transistor, a current division circuit part configured to divide the output current of the proportional current generation circuit part in a predetermined division ratio, a division ratio control circuit part configured to control the division ratio of the current division circuit part, a current-voltage conversion circuit part configured to convert a predetermined one of divided currents obtained as a result of dividing the current in the current division circuit part into a voltage and output the voltage, and an output current control circuit part configured to perform output current control on the output control transistor in accordance with the output voltage of the current-voltage conversion circuit part, wherein when the output voltage of the current-voltage conversion circuit part reaches a predetermined voltage, the output current control circuit part controls an increase in the output current of the output control transistor so as to reduce the voltage output from the output terminal, wherein when the voltage output from the output terminal is reduced to a predetermined first limit voltage, the division ratio control circuit part changes the division ratio of the current division circuit part so that the current supplied to the current-voltage conversion circuit part increases so as to reduce the output current of the output control transistor.