A constant voltage supply unit having a high-speed load response, equipped with
a fold-back type over-current protection function in which an output current detection
voltage indicative of the output current is compared with the sum of a feedback
voltage indicative of the output voltage and an offset voltage. The over-current
protection function has a characteristic that the offset voltage is inversely proportional
to the output-current detection voltage, so that the offset voltage is large when
the output-current detection voltage (or the output current) is low, and decreases
with the output-current detection voltage. In addition, the constant-voltage power
supply unit allows enhance feedback of ac components in the feedback loop so as
to enhance the ESR of the load-side capacitor, thereby securing phase compensation
to prevent oscillations in the feedback loop.