Update - Some tuning performed, more stabilised frequency, I think using of LM358N(1MHz) or TL062(1Mhz) generally available Op.amps with good prices would be fine.
Run with open swith and at 10us/s for best performance. Device works at aproximately 100kHz, compares voltage on capacitor, when below set point, charges capacitor throught coil. My very own design, feel free to ask. LED shows pwm duty.
Tweaked a little
