Easiest way to go from analog input to PWM output?
Amusingly enough, I was literally just looking at single-chip PWM generators for my job.

If interest to you might be the Linear LTC6992 - TimerBlox: Voltage-Controlled Pulse Width Modulator (PWM)

You give it a 0-1V input, and you get a PWM pulsetrain out. It can also internally limit the duty cycle to 5%-95%, or 0-100%.





You would need a op-amp or two to scale your input to the chip's input range, but you would need to do that anyways, because very few ADCs are bi-polar (assuming you were going the microprocessor route).

Looking at the reference schematic you provided, I would also recommend using a MOSFET driver of some sort, to improve switching times, if nothing else.
It would also likely provide level-shifting for the gate-voltage.