The Maxim Integrated MAX821PUS is a simple reset/power-on reset voltage supervisor. It monitors a single supply rail and asserts a reset signal when the voltage drops below the 4V threshold, holding the reset for an adjustable/selectable timeout period after the supply recovers. The active-low, push-pull output drives directly into the reset input of an MCU, FPGA, or other logic device without needing an external pull-up resistor. The SOT-143-4 package keeps the footprint small — roughly the size of a small-signal transistor — so it fits tight layouts on multi-layer boards.
Threshold and reset timing — what matters for the BOM
The 4V threshold is the trip point: when the monitored supply falls below 4V, the reset output goes active low. For a 5V rail, this gives 1V of headroom before a brownout triggers a reset. The reset timeout is adjustable or selectable via an external capacitor or resistor (depending on the variant configuration). That lets you tailor the reset hold time to match the power-up settling of your system — longer for a slow-starting power supply, shorter for a fast-responding MCU.
It is not RoHS compliant (per the listing), so factor that into your compliance review if your BOM requires RoHS.