Power-supply interrupter fights ESD-induced device latch-up

Under certain conditions, ESD events can damage digital circuits by causing latch-up. For example, when ESD triggers them, parasitic transistors normally formed as parts of a CMOS device can behave as an SCR (silicon-controlled rectifier). Once ESD triggers, the SCR presents a low-resistance path between portions of the CMOS device and conducts heavily. Damage to the device can result unless you immediately remove power from the circuit. ESD from human interaction presents a significant problem for mobile industrial and medical devices. For adequate ESD protection, most medical and industrial devices require a grounded return path for ESD currents. In the real world, mobile devices may serve in environments in which properly grounded power outlets are unavailable.

To protect expensive equipment from latch-up failures even when no ESD ground is present, you can add the power-interruption circuit shown in Figure 1 to prevent damage when ESD-induced latch-up occurs. Under normal conditions, current drawn by ESD-susceptible devices develops a small voltage across sense resistor R₆. A voltage divider formed by R₄ and R₅ defines a reset-current threshold for the LED portion of optoisolator IC₁, and, under normal operational current consumption, the LED remains dark.

The output of IC₁ controls the gate bias applied to MOSFET Q₁, which is normally on. When latch-up occurs, power-supply current drain rapidly increases by an order of magnitude or more. The large voltage drop developed across R₆ forward-biases IC₁'s LED, which in turn drives IC₁'s phototransistor into conduction and shuts off Q₁, interrupting dc power to ESD-susceptible devices for several milliseconds. In addition, the system's firmware design must allow for automatic recovery from a power interruption.

The following describes the relationship between the reset-current threshold and the values of R₄ and R₅: (R₄+R₅)/R₄=(I_T×R₆)/V_LED, in which I_T≥(V_LED)/R₆, and V_CC>V_LED.

The ESD-induced fault threshold current, I_T, is greater than or equal to the optoisolator LED's conducting forward-voltage drop divided by the value of sense resistor R₆. Also, the raw power-supply voltage must exceed the LED's forward-voltage drop. Resistor R₁ provides a path for IC₁'s base-leakage current, and resistors R₃ and R₂ determine Q₁'s gate-shutoff bias.

In Figure 1, the optoisolator presents an LED forward-voltage drop of 1.2V. For the component values shown, the circuit momentarily interrupts V_CC when ESD-induced power-supply current exceeds approximately 300 mA. Total cost of the six resistors, one MOSFET, and one optoisolator is approximately $1 (production quantities).

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