There is a need for preventing a MOS transistor from being destroyed due
to an inrush current from an input terminal when a boost operation starts
from a boost disabling state. During the boost operation, a third MOS
transistor (M3) turns off and a fourth MOS transistor (M4) turns on to
prevent a current leak from an output terminal (Vout) to an input
terminal (Vin) due to a parasitic diode of a second MOS transistor (M2).
In the boost disabling state, the third MOS transistor turns on and the
fourth MOS transistor turns off to prevent a current leak from the input
terminal to the output terminal due to the parasitic diode of the second
MOS transistor. When the boost operation starts from the boost disabling
state, an electrode toward the output terminal of the second MOS
transistor is charged before changing a substrate bias state of this
transistor. In this manner, an inrush current is prevented from flowing
from the input terminal to the output terminal via the parasitic diode of
the second MOS transistor.