Circuit protection device

The invention provides for a circuit protection device in which a threshold voltage is selectively raised above the power supply voltage (VDD) by the inclusion of level shifter (20) connected between the power supply (VDD) and a discharge reference rail (15). The level shifter (20) conducts when a voltage applied to the rail (15) is greater than the power supply voltage (VDD) by an amount equal to the sum of the various diode voltage drops across the level shifter (20). The various input pads of an integrated circuit including FETs to be protected are connected to an anode of an associated diode (D1, D3) wherein the cathode of the associated diode (D1, D3) is connected to the discharge reference rail (15), rather than directly to VDD as in the prior art. The anode of the diode (D1) is also connected to the circuit to be protected. Thus, when a voltage applied to the input pad is approximately one diode drop above the voltage on the discharge reference rail (15), the voltage at the input pad will be shunted to the power supply voltage (VDD) generator through the level shifter (20.) The level shifter (20) may include conventional diodes or zener diodes. Diode D5 serves to maintain reference rail (15) at one diode drop below VDD during normal operation. The inclusion of the level shifter 20 permits the input voltages at nodes 1,2 to rise significantly above VDD for performing various test functions on the integrated circuit.