I am driving the Gate of the N-channel MOSFET with a 3.3V, 100KHz PWM from a microcontroller.
The CREE LED which I am using is rated for 1A and after testing it (With a 5V constant DC supply and resistor in series with the LED), I am happy with the brightness I got from the LED when the average current passing through it is around 350mA. So I want to drive this LED around this current rating(also because of thermal considerations for long life. 1/3rd max current rating). So for a current of 350mA, 5V supply and 3.05V forward voltage of the LED, the current limiting resistor comes to around 6Ω(1W resistor).
Now when I wire up the PWM circuit as shown in the image above, with
R1=6Ω and a 3.3V, 100KHz, 90% duty cycle signal at the gate and 5V DC supply, the average current measured(directly from the DC supply or reverse calculation from voltage across the current limiting resistor, R1) comes to around 60mA and not the 350mA I designed for. So the LED is dimmer than when it is driven at an average current of 350mA. I presume this is due to the PWM drive.
Now question is how should I mathematically calculate
R1 for a particular average current and a particular duty cycle so that I get my desired brightness(when average current is 350mA).
I can practically keep changing R1 to lower values till I find the correct value, but shouldn't there be a better way?
So if I use a lower R1 value, Avg current will drop in case of PWM drive, but I am sure impulse current will be higher. Can this potentially damage the LED in the long run?(Switching Freq: 100KHz) or is it acceptable?
PS: I am not looking for a constant current driver circuit solution.