The voltage of an input signal end is divided by the voltage division circuit and then the voltage division voltage is output, the signal control circuit outputs a switch control signal of a first level when the voltage division voltage is larger than a first threshold voltage and the voltage division voltage is smaller than a second threshold voltage, and therefore the output control circuit canrespond to the switch control signal of the first level and conduct the input signal end and the output signal end. The signal control circuit may output a switch control signal of a second level whenthe divided voltage is less than the first threshold voltage or the divided voltage is greater than the second threshold voltage, and the output control circuit may conduct the input signal terminalwith the ground terminal in response to the switch control signal of the second level.