Patent ID: 7548044

Claim:
A portable electronic device which employs a constant-voltage power supply circuit 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 amplify a difference between a predetermined reference voltage and a second voltage to obtain an amplified difference and 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 when said first current from said output terminal reaches a predetermined current value or greater and a second control operation on said output transistor when said first voltage from said output terminal is decreased to a ground potential; said over-current protection circuit comprising: (a) an error amplification circuit, first and second input terminals of said error amplification circuit being input with a second current proportional to said first current output from said output transistor and a converted voltage, respectively; and (b) an amplitude expander configured to expand an amplitude of a third voltage output from said error amplification circuit, wherein said third voltage is in a range from the ground potential approximately to said input voltage.