Supervisor with battery-backup switchover
The MAX691AEWE+T is a 1-channel voltage supervisor from Maxim Integrated that integrates a battery-backup circuit — it monitors the primary supply rail and, on a brownout or power loss, switches the load to a backup battery while asserting a reset signal to the processor. The reset output is available in both active-high and active-low polarities, with a push-pull stage that drives the reset line without requiring an external pull-up resistor.
Threshold, timeout, and temperature range
The voltage threshold is set at 4.65 V, typical for monitoring a 5 V supply rail — when VCC drops below this level, the reset output asserts and the battery-backup switch engages. Reset timeout is 140 ms minimum, giving the supply rail time to recover before the processor is released from reset; this prevents spurious toggling on a glitchy power bus.