A stacked high-voltage ESD protection clamp is provided that realizes the desired
triggering characteristics of a BJT or BSCR stacked snapback clamp. The operational
principle of the new circuit is based upon introduction of a middle node capacitor
into the stacked (cascoded) clamp. The capacitor (or driver) provides conditions
for a two-stage turn-on. At the beginning of an ESD pulse, the capacitor is discharged.
With the ESD voltage increase, part of the current is used to charge the capacitor,
thus shunting one of the BJTs (BSCRs). As a result, the other BJT (BSCR) will experience
fast turn-on. After turn-on, the current provides a fast charge of the capacitor
and the turn-on of the second device. Thus, the middle node capacitor allows the
triggering characteristics of the clamp to be controlled.