The MAX5921/MAX5939 from Maxim Integrated (now part of Analog Devices) solve the problem every -48 V telecom shelf faces: how to plug a card into a live backplane without the inrush current arcing the connector or browning out the supply. The MAX5921 is a controlled turn-on hot-swap controller - it ramps an external N-channel MOSFET on gently, holding the inrush within a programmed limit while the load's bulk capacitance charges. MAX5921EESA+ is the RoHS ("+") option in the 8-pin SO; the part is a drop-in for the industry-standard LT1640/LT4250.

The current path is built around an external R_SENSE, which the designer sizes for the card's current, and the controller watches the voltage across it through the SENSE pin. Two limits act on it: a programmable inrush/circuit-breaker level for normal turn-on, and a fast short-circuit response that slams the GATE down with 450 mA of pulldown current - the high pulldown is what gets a big MOSFET off quickly when the load shorts, before the device or the connector is damaged. An overcurrent fault integrator distinguishes a real fault from a momentary spike, and the part withstands -100 V input transients with no external components.

Supervision rounds it out. Programmable overvoltage (OV) and undervoltage (UV) comparators, each with a built-in glitch filter, keep the card off until the -48 V rail is in range; an open-drain PWRGD̅ output tells the system the card is powered and in regulation; and a thermal-shutdown circuit protects the external MOSFET. The MAX5921 operates from -20 V to -80 V over the extended -40 °C to +85 °C range, with the input steps and spikes of a real backplane designed into its immunity.

Note: Values are taken from the Maxim MAX5921/MAX5939 datasheet. Confirm every parameter against the latest official documentation before committing a design.

MAX5921EESA+ ships in the 8-pin SO (0.150-inch narrow SOIC, package outline 21-0041) - the standard small-outline body that sits next to the external MOSFET and R_SENSE on a -48 V line card. The controller handles only gate-drive and sense signals; the load current flows in the external MOSFET and sense resistor, so layout is the design: Kelvin-connect SENSE across R_SENSE, keep the GATE trace short to the MOSFET, route the high-voltage VEE/DRAIN nodes with adequate clearance, and bypass V_DD at the pin. The dimensional drawing below gives the full mechanical detail.

The 8-pin SO groups the supervisory inputs on one side and the MOSFET drive/sense on the other (per the MAX5921/MAX5939 datasheet):

• Pin 1 - PWRGD̅: Power-good output (open-drain); asserts when the card is powered and in regulation.
• Pin 2 - OV: Overvoltage programming input; sets the upper backplane-voltage threshold.
• Pin 3 - UV: Undervoltage programming input; sets the lower threshold (card stays off below it).
• Pin 4 - V_EE: Negative supply (-48 V return / most-negative rail).
• Pin 5 - SENSE: Current-sense input; Kelvin-connect across the external R_SENSE.
• Pin 6 - GATE: Gate drive for the external N-channel MOSFET; 450 mA pulldown.
• Pin 7 - DRAIN: MOSFET drain-sense / output monitor input.
• Pin 8 - V_DD: Positive supply for the controller; bypass to V_EE.

The closest options are the family sibling and Maxim's other -48 V / negative hot-swap controllers. Match voltage range, gate-drive and sense method to your line card before substituting.

Selection Tips: Stay with the MAX5921 for a controlled-turn-on -48 V hot-swap with external R_SENSE and high gate pulldown in 8-SO, and as a drop-in for the LT1640/LT4250. Step to the MAX5939 family companion or the MAX5910 for related -48 V designs, or the integrated MAX5950/MAX5957/MAX5978 when a higher-integration power-management controller fits the system better.

Maxim Integrated, now part of Analog Devices, Inc. (ADI), built the MAX5921/MAX5939 for the -48 V telecom and datacom infrastructure where cards are inserted and removed live - giving designers a drop-in for the established LT1640/LT4250 with the high gate pulldown and transient immunity those rugged backplanes demand.

ADI maintains the datasheet, the R_SENSE and timing-component selection guidance, and the MOSFET SOA application notes for the family - the documentation that turns the external sense resistor and MOSFET into a safe, repeatable hot-swap on a live -48 V shelf.