Abstract: A comparator monitors an output voltage V.sub.out. When an output voltage V.sub.out decreases to a predetermined output voltage or less, the comparator sends a signal that represents this state to a controlling circuit. When the controlling circuit receives the signal, it causes a switching device to be turned on. Thus, a coil current I.sub.L is increased. A hysteresis (window) comparator monitors the coil current I.sub.L. When the coil current I.sub.L reaches a comparison current I.sub.LP, the controlling circuit causes the switching device to be turned off. When the coil current I.sub.L decreases to the comparison current I.sub.LB (where I.sub.LP >I.sub.LB), the controlling circuit references an output signal of the comparator. When the output voltage V.sub.out is lower than the predetermined output voltage, the controlling circuit causes the switching device to be turned on.