PREVENTIVE CIRCUIT FOR ELECTROSTATIC BREAKDOWN

PURPOSE:To prevent the electrostatic breakdown of an IC by connecting a diode in the forward direction to an external input terminal from the grounding side and connecting a MOSFET conducted and controlled by the diode and a power supply in series in the forward direction to the power supply side from the external input terminal. CONSTITUTION:When a surge -Vr is applied to an external input terminal Pi, the base voltage of a transistor Q1 for an input section is kept at a fixed value or less by a forward diode D1, and a circuit 1 is protected from a high- voltage surge. When +Vr is applied, bias currents pass through the Vcc side through a forward diode D3, and a base input to Q1 is clamped at fixed potential or less and protected. When normal Vcc is not charged, a FETM1 is turned OFF, the input terminal Pi and an internal power supply line L are interrupted, and dummy currents do not flow into the internal power supply line L how an external circuit 3 changes. Accordingly, a malfunction due to the order of the charging of Vcc is not also generated, thus acquiring a breakdown preven tive effect sufficient to positive and negative high-voltage surges.