Hi all I have a question regarding using a MOSFET for a constant current source. I have found this question with some answers but it wasn't clear to me yet.
I want to create a constant current LED driver with a MOSFET shunt and opamp configuration. I have designed the schematic below with a 1 ohm sense resistor and a BUK6D43-60E MOSFET. The forward voltage of the LED's will be between 34.5 and 45 volt and the current will be 1 ampere. So the voltage over the MOSFET will be around 12.5 volt maximum. The LED's will be pulsed with a frequency of <100 Hz and a pulse duration with a maximum of 300 microseconds, this will be a dutycycle of 3%. From the datasheet below I got a Zja of around 4-6 K/W. I know Tjunction = Tambient + Rj-a * P, now Tambient has a maximum of 60C and Rj-a = Zja = 4-6 K/W. Now what do I have to use for P? Is this (Vcc-Vleds-Vshunt) * If or is this (Vcc-Vleds-Vshunt) * If * dutycycle? Because this would be the difference between 135C and 62.25C.