Buck driver do not stabilize curent

[stevetto]

I have been working on a LED buck driver for a while but unfortunately without any succes.
The design is based on a MAX 16820 chip.
I use an attiny13 to generate the pwm signal for the MAX 16820. This part seems to be ok, I can change the pwm's duty cycle.
As the duty cycle changes the LED current also changes.
But if I change the input voltage the led current also changes, so the driver do not stabilize it.

At first I tested it with an XHP70 led, than I changed it to an xm-l2 (the current sense resistor was also changed) but I still have the same issue.
I have attached my calculations and the schematics.

I would really apprieciate any suggestions.

[adamcord]

It looks like the MAX16820 limits LED current to some maximum based on the sense resistor, but if your supply voltage isn't high enough to push this max current during the LED's on state, they will be drawing some current <= max current. That's my guess as to why the average current changes as it does with supply voltage.