Patent ID: 7501852

Claim:
A tolerant input circuit for connection to a power supply and an input pad, the tolerant input circuit comprising: an input circuit; a step-down device including a first N-channel MOS transistor connected between the input pad and the input circuit, the first N-channel MOS transistor of the step-down device having a gate supplied with voltage from the power supply, wherein voltage of a high voltage signal provided to the input pad is decreased to a value lower than or equal to that of the voltage of the power supply by the step-down device and provided to the input circuit, the first N-channel MOS transistor including a back gate; and a back gate voltage control circuit, connected to the back gate of the first N-channel MOS transistor in the step-down device, for increasing back gate voltage of the first N-channel MOS transistor in the step-down device when the input pad is provided with the high voltage signal, wherein the back gate voltage control circuit includes: a second N-channel MOS transistor connected between the input pad and the back gate of the first N-channel MOS transistor in the step-down device, the second N-channel MOS transistor including a back gate; and a third N-channel MOS transistor connected between the input pad and the back gate of the second N-channel MOS transistor.