What this hot-swap controller does on your board
The MAX5903NNEUT+T from Analog Devices (Simple Swapper™ series) is a single-channel hot-swap controller that manages the inrush current when you plug a card into a live backplane. It works with an external N-channel MOSFET — the controller itself doesn't carry the load current, so the pass transistor is sized to the actual load. Supply range runs 9V to 72V, which covers 12V, 24V, and 48V industrial and telecom rails without a separate regulator. The chip draws 1 mA from the rail, so it won't eat into your power budget on a densely populated card.
The undervoltage lockout (UVLO) threshold is programmable via an external resistor divider. That lets you set the turn-on voltage so the controller doesn't try to power up the card until the backplane rail is stable. On a 48V telecom shelf, you might set UVLO at 42V to avoid browning out during a battery transfer. If the input dips below that threshold, the controller holds the external MOSFET off — no partial turn-on, no glitches. The thermal-limit feature is the fail-safe that a simpler latch-off controller lacks. If the external MOSFET overheats — say from a sustained short or a load that's pulling more than the heatsink can handle — the controller shuts it down and lets it cool before retrying. That's useful on a card that gets hot-swapped into a live chassis where the load capacitance is higher than expected; the thermal limit keeps the MOSFET from letting the magic smoke out while the inrush timer sorts itself out.
The six-pin footprint is small enough to tuck near the input connector, which is exactly where you want the hot-swap controller — short traces to the external MOSFET gate and the sense resistor keep the loop tight and reduce parasitic inductance that can cause gate oscillation.