The time period (alloWable ON time) from When the boost capacitor CB is fully charged until When the voltage of the boost capacitor CB drops to the minimum voltage required
to hold the sWitch S1 ON as a result of the operation that the driver D1 performs to hold the sWitch S1 ON, can be calculated beforehand. Additionally, the maximum amount
of the time period (maximum ON time) from When the sWitch S1 is turned from the OFF to the ON state till When a reset pulse is generated can be regulated based on the rising speed of the ramp signal Ramp11. Accordingly, if the maximum ON time is designed to be shorter than the above described allowable ON time, the driver D1 alWays drives the sWitch S1 according to the state of the latch circuit 5. This is because the voltage of the boost capacitor CB never drop to the minimum voltage required to hold the sWitch S1
ON, as shoWn in FIG. 6.