What this -100V hot swap controller does on a networking card
The MAX5900AAEUT+ is a single-channel hot swap controller from the Simple Swapper™ family, designed to manage inrush current and protect the backplane when a line card is inserted into a live -48V or -72V telecom shelf. It handles a supply range of -9V to -100V, drawing just 500 µA of quiescent current, so the controller itself does not load the rail during normal operation.
Package and rework — SOT-23-6 on the bench
Housed in a 6-lead SOT-23 surface-mount package, the MAX5900AAEUT+ is a small-footprint part that reflows cleanly with a standard hot-air profile — no exposed paddle, no underfill needed. Pin 1 is clearly marked on the package top, and the 0.95 mm lead pitch gives enough room for a fine-tip iron touch-up if the reflow leaves a cold joint.
Protection features that matter on a live -48V rail
Built-in UVLO (undervoltage lockout) is programmable via an external resistor divider, letting you set the turn-on threshold so the controller stays off until the backplane voltage is stable. The fault response includes auto retry with a timeout — after a fault clears, the controller attempts a restart without latching off permanently, which suits redundant power architectures where a momentary glitch should not kill the card. Thermal limit protection shuts down the external pass FET if the die temperature exceeds the safe operating area, a safeguard when airflow is lost in a crowded chassis.
Temperature grade and environment
The internal switch is external — the controller itself does not carry the load current, so the thermal budget is set by the external MOSFET you pair it with, not the SOT-23 package.