Voltage controlled PWM generator
Take a look at these slides (starting about slide 13). It details creating a PWM generator of a fixed frequency (which you can choose) using only a 555 and a comparator. The level shift and amplify portion of the circuit isn't required; it is only to compensate for a slow input AC waveform of about 100mVpp. 

Overall, the circuit looks like this:


All in all, it's a relatively simple circuit. The 555 creates the sawtooth waveform for you, and the comparator compares your input voltage to this waveform. Depending on the level of the input as compared to the level of VCC on the 555, the duty cycle will be altered. The equations for the frequency and Ton are even included, so no need to derive.