4.2V threshold — what it monitors
The ROHM BD45422G-TR is a single-channel voltage detector with a 4.2V threshold, designed to assert a reset when the monitored supply drops below that level. With an active-low, open-drain output, it pulls the reset line low during undervoltage and releases it after the supply recovers and the 180ms minimum reset timeout expires. This part is from the BD45xxx series and comes in an SC-74A (SOT-753) package for surface-mount assembly.
180ms reset timeout — why it matters
The 180ms minimum reset timeout gives the system power rail time to stabilize before the processor or logic starts executing code. This prevents spurious boot-ups during supply glitches or slow ramp-up. For a 5V rail, the 4.2V threshold provides roughly 16% undervoltage detection margin — enough to catch a sagging supply before logic errors occur, but not so tight that normal ripple triggers false resets.
Open-drain output — wired-OR flexibility
The open-drain (open-collector) output lets you connect multiple voltage detectors to a single shared pull-up resistor, creating a wired-OR reset bus. This is useful in multi-rail systems where any rail dropping out should assert a global reset. The output is active low, so the reset line is held low during the fault and released high after the timeout.
The SC-74A package (SOT-753 footprint) is a common 5-pin small-outline transistor package, easy to place and reflow.
ROHM lists the BD45422G-TR as Active. The open-drain output and 4.2V threshold are fixed — no pin-strapping options.