[ LM3409HV ] Possible Damage with PFET or LED

The main causes of failure are heating, over-voltage, and over-current. Assuming the LM3409 circuit is designed and working properly and over-current of the FET and/or the LEDs will never happen. It is hysteretic which introduces an inherent current limit. The current cannot exceed the peak current set by the current sense pins and resistor. Over-voltage only happens if you choose a FET with very little headroom over the maximum input voltage. In that case spiking on the switch node can over-voltage the FET. It is also possible if the layout is not well done you could have enough negative ringing at the switch off edge to damage the FET, but it would have to be a lot. Just practice good layout and that should never be an issue. So you just need to choose a FET with some voltage margin and shoot for low Rds(on) and low gate charge. You also generally want to keep the max switching frequency at 800kHz or below for coolest operation (the lower the better to reduce AC losses in the FET).