30 V, 4.2 mΩ — what the headline numbers mean for your switching node
The STD96N3LLH6: It is rated for a 30 V drain-source voltage and delivers a maximum Rds(on) of 4.2 mΩ at 10 V gate drive with 40 A drain current.
Gate charge and drive voltage — the switching-speed lever
The drive voltage range spans 5.5 V to 10 V for the best Rds(on); at 4.5 V the on-resistance roughly doubles from the 10 V value, so a 5 V logic supply still works but you lose some conduction efficiency. The input capacitance of 2200 pF at 25 V drain-source means the gate-drive loop needs a clean layout — a 10 Ω series resistor and a Schottky catch diode on the gate trace are cheap insurance against ringing.
Thermal budget — 70 W dissipation in a DPAK
Maximum power dissipation is 70 W at the case, but that number assumes an infinite heatsink. In a real board with 1 oz copper and no forced air, the junction-to-ambient thermal resistance of a DPAK lands around 50–60 °C/W, so the practical continuous dissipation is closer to 2–3 W before the junction hits 175 °C. The 175 °C junction rating gives margin for short overloads, but sustained high-current operation needs a copper pour on the drain tab and preferably a thermal via array to an inner plane.