What this monitor does that a simple reset IC cannot
The TPS3899PL31DSER is a single-channel voltage monitor from Texas Instruments that asserts a reset when the monitored rail drops below 3.1 V. The reset output stays low for 40 µs typical after the rail recovers, giving the supply time to stabilise before the processor starts. The push-pull output drives the reset line high or low without an external pull-up resistor, saving one resistor per rail in the BOM.
Threshold and timeout: sizing the reset window
The 3.1 V threshold is a fixed value — no resistor divider needed. That simplifies the layout but locks the monitor to a single rail voltage. If your system uses a 3.3 V supply, this part triggers a reset when the rail sags to 3.1 V, which is about 6 % below nominal. The 40 µs typical reset timeout is short compared to many general-purpose reset ICs that use 33 ms or 130 ms minimum. That 40 µs window suits fast-recovering supplies like a local LDO, but if the main regulator needs tens of milliseconds to settle, you will need to add an external RC delay or choose a part with a longer timeout.
Package and footprint: 6-WSON layout notes
The 6-WSON package measures 1.5 mm x 1.5 mm with a 0.50 mm pitch. That small footprint fits tight layouts but demands a well-planned fan-out — the centre pad is the thermal and electrical ground connection. Ensure the PCB land pattern follows the TI recommended footprint for the 6-WSON to avoid solder bridging.