Patent ID: 7012791

Claim:
A constant-voltage power supply unit, comprising: an output circuit that includes a primary control transistor circuit having a conductivity controlled by an output-controlling signal and adapted to convert a source voltage to a predetermined output voltage, thereby providing said predetermined output voltage along with an output current, and a voltage detection circuit for generating a feedback voltage in accord with said output voltage; a current detection circuit for generating an output current detection voltage in accord with said output current; a voltage control circuit for comparing said feedback voltage with a reference voltage and for generating a voltage control signal in accord with the difference between said feedback voltage and reference voltage, said voltage control signal serving as a basis of said output-controlling signal; and an over-current limiting circuit adapted to compare the sum voltage of said feedback voltage and offset voltage with said output current detection voltage, and, when said output current detection voltage exceeds said sum voltage, control said voltage control signal so as to bring said primary control transistor circuit towards its turned-off state, thereby reducing said output voltage and output current, wherein said offset voltage is large when said output current detection voltage is small but becomes smaller as said output current detection voltage becomes larger.