13 the external microcomputer turns on the m 5 for example for a predetermined period such as the same period as the period t 4 in the first embodiment after activation of the power supply vcc this can achieve the same effect as in the first embodiment referring to fig2 fig3 and fig5 a step down voltage output circuit using a charge pump circuit in accordance with a third embodiment of the present invention will be described fig5 is a block diagram of the step down voltage output circuit using the charge pump circuit in accordance with the third embodiment of the present invention the third embodiment fig5 is different from the first embodiment fig1 in that a second oscillating circuit 14 which is different from a first oscillating circuit 1 is added in fig5 the same reference numerals are assigned to the same components in fig1 first embodiment and description thereof is omitted fig5 shows the oscillating circuit 1 which oscillates at a predetermined frequency and outputs a clock signal v 1 a control logic 2 which receives the clock signal v 1 as an input signal an m 1 3 an m 2 4 an m 3 5 an m 4 6 a capacitor 7 an inv 1 8 a vcc terminal 9 a vsub terminal 10 a timer circuit 11 an n channel mos transistor m 5 12 and a second oscillating circuit 14 which is different from the oscillating circuit 1 fig2 is a view for showing of the timing of operation of the step down voltage output circuit using the charge pump circuit in accordance with the third.