I'm designing an LED driver circuit that requires very fast PWM switching at a high resolution (10bit at 30khz requiring a minimum pulse width ~30ns). In order to achieve the pulse timing required, I'm using a FET to shunt the LED driving current to ground during the "off" periods. I'm trying to determine the worst-case power dissipation for the FET, so I can plan heat mitigation strategies when designing my board.
One of the FETs I am considering is the BUK9M52-40E, because it works on logic-level voltages and on/off times in the single ns range. It is not a power MOSFET, however, and does not appear to have a substantial heat sink build in, so I would presumably need to rely on copper pours and thermal vias for heat management.
I will be driving 8 LEDs in series at 24V at a constant average current of 360mA with a peak current of 540mA.
The FET has a maximum RDSon of 52mO at 5V, though I hope to drive it with a 3.3V signal, which from what I can determine would result in an RDSon of closer to 100mO. So given a constant average current of 360mA, and assuming 100mO of resistance, (0.36A x 0.01O) the voltage across the drain-source will be 0.0036V. So the power dissipation in the FET will be 0.0036V x 0.36A = 0.001296W.
This number seems... really low. And given that the FET is rated to 31W, I assume that this is well within its capabilities, and I could probably get away with very little thermal management, and potentially a much smaller FET. Am I correct in this assumption?