Hello there,
All you need is a triangle generator connected to one input of the comparator, but you dont even need that really. All you need is a cap and resistor and something to discharge the cap at the end of the PWM period. That will give you a pseudo ramp that will work almost as well as a true ramp for generating PWM.
So it would be a cap in series with a resistor with the resistor to +Vcc and the cap bottom connected to ground, and possibly an NPN transistor to discharge the cap at the end of the period, or another comparator with open collector output if the cap is small. I used this in a commercial battery charger long time ago.
**broken link removed**