DAC to run a 555 Timer
I cannot I need to keep generation of PWM independent of the
  microcontroller. Thats the requirement


Why don't you use something like this - it's cheap and reliable and directly takes a 0 to 1V signal and produces PWM at a frequency defined by Rset. 



You'll still need a DAC if you are totally sure you are not allowed to put the pot onto the chip above.