A semiconductor device is equipped with a step-up circuit having a series of
multiple
charge pump units. Each of the units has a well separation type MOS transistor.
The separation well of the transistor is coupled to a high potential so as to form
double reverse biases between the N-type well and a P-type substrate and between
the N-type well and a P-type well. This permits the threshold Vth of the MOS transistor
to be held at low level. The units are provided with a clock whose current supply
capability is limited until a predetermined condition (that a predetermined period
of time has elapsed after the onset of the step-up circuit by a startup signal
or that the output voltage has reached a predetermined level). This limitation
of the clock facilitates suppression of power consumption by the step-up circuit
during a startup, thereby reducing changes in amplitude of a supply voltage.
