The ON duration of an N channel MOS transistor is set based on a target voltage at a node between a resistor and a light receiving element of a photocoupler. A comparator compares the target voltage with a reference voltage at a node between a variable resistor and a resistor. A normal mode and a low frequency operation mode are switched from one to the other based on an output signal from the comparator. The resistance of the variable resistor becomes low when an input voltage from a power source is high. Even when the input voltage is high, therefore, the ON duration of the N channel MOS transistor at the transition between the normal mode and the low frequency operation mode becomes short. This reduces the switching energy in low frequency operation mode, thereby suppressing noise.