Emerson Segura, Lifescale Global Diagnostics Inc, Toronto, ON, Canada
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痴 LED, which in turn drives IC1痴 phototransistor into conduction and shuts off Q1, interrupting dc power to ESD-susceptible devices for several milliseconds. In addition, the system痴 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=(IT_R6)/VLED, in which ITe(VLED)/R6, and VCC>VLED.
The ESD-induced fault threshold current, IT, is greater than or equal to the optoisolator LED痴 conducting forward-voltage drop divided by the value of sense resistor R6. Also, the raw power-supply voltage must exceed the LED痴 forward-voltage drop. Resistor R1 provides a path for IC1痴 base-leakage current, and resistors R3 and R2 determine Q1痴 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 VCC when ESD-induced power-supply current exceeds approximately 300mA. Total cost of the six resistors, one MOSFET, and one optoisolator is approximately $1 (production quantities).
