Patent ID: 7176745

Claim:
A semiconductor integrated circuit device comprising: an interface circuit including a first MOSFET having a first threshold voltage; a main circuit including a second MOSFET having a second threshold voltage lower than the first threshold voltage; a substrate bias voltage generator to generate a substrate bias voltage for the second MOSFET; a step-down voltage power supply circuit; a substrate bias monitor circuit to monitor a value of the substrate bias voltage; and a third MOSFET to control a power supply to the main circuit, wherein the interface circuit is arranged to interface with the external of the semiconductor integrated circuit device and with the main circuit, wherein the interface circuit is arranged to be supplied with a first supply voltage and the main circuit is arranged to be supplied with a second supply voltage lower than the first supply voltage, wherein the substrate bias voltage generator is arranged to be supplied with the first supply voltage; wherein the step-down voltage power supply circuit produces the second supply voltage from the first supply voltage, and wherein the first supply voltage is supplied from an external device of the semiconductor integrated circuit device; and: wherein the third MOSFET is controlled to turn on when the substrate bias monitor circuit detects that the substrate bias voltage is ensured to be sufficiently stable.