=====================CMOS_LATCHUP=============================== ^ /_\ Vcc | _/\ /\________________ |_________/\ /\____ | \/ | | | \/ | | Gate | _|_ | Gate | | _|_ | \\\ | _|_ | _|_ _|_ |___| _|_ _|_ |___| _|_ _|_ _|___|___|___|_|___|_|___|_____|___|_|___|_|___|___|___|___ | P+ | | N+ |<--| N+ | | | P+ |++>| P+ | | N+ | | \___/ \____/ \____/ | \____/ \____/ \___/ | Bulk Drain Source | Source Drain Bulk | \ NWELL / SUBSTRATE \_________________________/ In a CMOS circuit, it is common to have a Pchannel source connected to Vcc while a Nchannel source is connected to ground. An example of this would be an invertor. The Pchannel device would sit in a Nwell bulk that is also biased up to Vcc. Likewise, the Nchannel sits in ground biased substrate. If the resistance to the biasing of the both the Nwell and substate are no lower, there exists an opportunity for a latch up to occur. ^ /_\ +5V _____|__________ | | -> | PNP`|______/\ /\_| _'| | \/ | |________ | | |_________ | | _| __/\ /\_|__|' NPN | \/ |`-> |________________| | _|_ \\\ The circuit above shows the "classical" SCR schematic. When on, the loop gain is the product of the two betas. Emitter base resistors are commonly added to define the conditions under which the SCR is On or off. ^ /_\ Vcc | _/\ /\________________ |_________/\ /\____ | \/ | | | \/ | | Gate | _|_ | Gate | | _|_ | \\\ | _|_ | _|_ _|_ |___| _|_ _|_ |___| _|_ _|_ _|___|___|___|_|___|_|___|_____|___|_|___|_|___|___|___|___ | P+ | | N+ | | N+ | | | P+ | | P+ | | N+ | | \___/ \____/ \____/->|<+\____/ \____/ \___/ | 700mV \ `-->|<++' / Vcc-700mV | electrons`--->\<+++' holes NWELL / SUBSTRATE \__________________________/ Latch up is usually tested by forward biasing an ESD diode with 100mA. The latch up requires that both a P+ and N+ junction needs to be forward biased. The P+ junction will eject holes into substate. The N+ junction ejects electrons into the Nwell. When the IR drop due to the ejection rates reachs a diode voltage, the SCR will latch up. It is standard linear practice to provide as many substate and Vcc tub connections as possible. This practice should discourage latchup.