High voltage protection circuits

Abstract

Integrated circuits are provided for protecting a device from high voltage signals, such as caused by ESD, at an external pin (12) of a device on an integrated circuit. A first circuit has a voltage reference terminal (24), and a pin resistor (13) connected in series with the pin (12) and an input terminal (14) to a functional circuit. An SCR (30) has an anode, cathode, anode-gate, and cathode-gate terminals. The anode of the SCR (30) is connected to the input terminal, while the cathode of the SCR is connected to the voltage reference terminal (24). A shunt resistor (19) connects across the anode and anode-gate of the SCR (30), and an another shunt resistor (20) connects across the cathode and cathode-gate of the SCR. A zener diode (22) is provided for setting a breakdown voltage of the SCR (30) between its anode-gate and cathode-gate. This integrated circuit protects a device against high voltages having a positive polarity, and also protects the device from ESD voltages having a negative polarity by switching the anode connections with the cathode connections. In operation, the SCR (30) triggers in response to a voltage at the pin (12) above the zener's breakdown voltage, and the SCR automatically turns off when the voltage across the pin resistor (13) has returned to within a hold voltage level that may be near or within the operational range of signals at the pin. The relative size of the first resistor (13) and the one of shunt resistors (19 or 20) having the lowest resistance controls the holding current of the SCR at either the anode-gate or cathode-gate of the SCR (30). Another integrated circuit provides for protecting a device from high voltage signals having either a positive or negative polarity using two SCRs (28a, 28b).