1.1 What is Latch-Up?

Latch-Up is a condition where a low impedance path is created between a supply pin and ground. This condition is caused by a trigger (current injection or overvoltage), but once activated, the low impedance path remains even after the trigger is no longer present. This low impedance path may cause system upset or catastrophic damage due to excessive current levels. The Latch-Up condition typically requires a power cycle to eliminate the low impedance path. CMOS and BiCMOS circuits use NMOS and PMOS transistors to create the circuit functions. In the design of the CMOS integrated circuit, the proximity of the PN junctions that form the NMOS and PMOS transistors create inherent parasitic transistors and diodes. These parasitic structures create PNPN Thyristors, also called silicon-controlled rectifiers (SCRs). Excursions (overshoots and undershoots) outside the normal operating voltage and current levels can trigger PNPN Thyristors and may cause Latch-Up. Latch-Up is not a risk if the voltage and current levels applied to the device adhere to the absolute maximum ratings.