A switching control IC is provided with a circuit for generating a
triangular wave using a frequency that is dependent on a capacitor
connected to a CT terminal of the switching control IC, and a resistor
connected to an RT terminal thereof. A synchronizing signal input circuit
for externally receiving a synchronizing signal that is a pulse voltage
signal is connected to the CT terminal. A charging voltage for the
capacitor connected to the CT terminal is changed by the synchronizing
signal, which synchronizes the frequency of the triangular wave with the
synchronizing signal, whereby the frequencies of a plurality of DC-DC
converters are synchronized.