Patent ID: 7268523

Claim:
A constant voltage power supply circuit for converting an input voltage received via an input terminal into a predetermined constant voltage that is output via an output terminal to a load which is coupled to the output terminal, comprising: a constant voltage circuit part configured to convert the input voltage into the predetermined constant voltage; a first excessive current protection circuit part configured to control the constant voltage circuit part so as to reduce the output voltage while maintaining an output current that is output via the output terminal to a predetermined maximum value if the output current is greater than or equal to the predetermined maximum value when the output voltage is a rated voltage; and a second excessive current protection circuit part configured to control the constant voltage circuit part so as to reduce the output voltage and the output current and to output a short-circuit current via the output terminal if the output voltage decreases to a ground voltage when the output voltage is decreased to a predetermined value by the first excessive current protection circuit part, wherein the second excessive current protection circuit part is disabled in response to a first test signal that is active.