Occurrence of power supply
noise arising in connection with a step-down action at the time of turning on power supply is to be restrained. A step-down unit is provided with a
switched capacitor type step-down circuit and a series
regulator type step-down circuit, and stepped-down
voltage output terminals of the step-down circuits are connected in common. The common connection of the stepped-down
voltage output terminals of both step-down circuits makes possible parallel driving of both, selective driving of either or consecutive driving of the two. In the consecutive driving, even if the
switched capacitor type step-down circuit is driven after driving the series
regulator type step-down circuit first to supply a stepped-down
voltage to loads, the
switched capacitor type step-down circuit will need only to be compensated for a
discharge due to the loads, and a peak of a
charge current for capacitors can be kept low. When operation of the switched
capacitor type step-down circuit is started, no large rush current arises, and occurrence of
noise is restrained.