A power supply is provided, which includes a DC-DC converter being
supplied with an external DC input voltage and a first switching control
signal and outputting a duty sensing signal of which a magnitude is
varied in accordance with the first switching control signal, the duty
sensing signal being indicative of a duty ratio of the first switching
control signal, and the DC-DC converter converting the input voltage into
a DC output voltage of a predetermined magnitude based on the first
switching control signal; and a feedback controlling unit comparing the
duty sensing signal with a first reference signal to adjust the duty
ratio of the first switching control signal.