Patent ID: 7183755

Claim:
A constant-voltage power supply circuit configured to perform at least a first operation to input an input voltage to an input terminal, a second operation to convert said input voltage into a predetermined constant voltage, and a third operation to output said predetermined constant voltage as a first voltage output from an output terminal, comprising: an output transistor configured to output a first current from said input terminal to said output terminal according to a signal input to a control electrode; an output voltage control unit configured to perform a fourth operation to generate said predetermined reference voltage, a fifth operation to generate a second voltage proportional to said first voltage, a sixth operation to amplify a difference between said predetermined reference voltage and said second voltage proportional to said first voltage to obtain an amplified difference, and a seventh operation to input said amplified difference to said control electrode; an over-current protection circuit configured to perform a first control operation on said output transistor, in a case when said first current from said output terminal reaches a predetermined current value or greater for a rated output voltage, to decrease said first current as well as said first voltage; and a second control operation on said output transistor, in a case when said first voltage from said output terminal is decreased to a ground potential, to output a predetermined short-circuit current from said output terminal; said over-current protection circuit comprising (a) a current detection transistor configured to perform an eighth operation to detect said first current from said output transistor, and a ninth operation to output a second current proportional to said first current from said output transistor, a control electrode of said current detection transistor being connected to the control electrode of said output transistor, a current input terminal of said current detection transistor being connected to said input terminal as well as said output transistor; (b) a current-voltage conversion circuit configured to convert said second current from said current detection transistor into a third voltage to obtain a converted voltage; (c) an error amplification circuit, first and second input terminals of said error amplification circuit being input with said second current proportional to said first current output from said output transistor and said converted voltage, respectively; (d) an amplitude expander configured to expand an amplitude of a fourth voltage output from said error amplification circuit, so as to range from the ground potential approximately to said input voltage; and (e) a control circuit configured to control an operation of said output transistor according to an output signal from said amplitude expander.