5V rail watchdog in a three-pin SOT-23
The Texas Instruments TPS3809I50DBVR is a simple reset supervisor that monitors a single 5V supply rail and pulls the reset output low when the voltage drops below the 4.55V threshold.
The 4.55V threshold sits about 9% below a nominal 5V rail, providing enough margin to catch a brownout without false-triggering on normal ripple. The 120 ms reset timeout is long enough to let the main supply and any local bulk capacitors charge fully before the processor comes out of reset. Active-low, push-pull output means no external pull-up resistor is needed — the output drives high when the supply is good and sinks to ground on a fault.
Swapping it in the field
The SOT-23-3 package is small enough to fit tight layouts but large enough to hand-solder with a fine-tip iron — no hot-air station required for a field swap.