Need a simple and small PWM generator chip

You could use a suitable MCU, like a 16F88 PIC or a Tiny2313 AVR.

Leon

Comparator + sawtooth generator. DC set by using control voltage as threshold. Complexity depends on how linearly DC scould follow control voltage. Doable with an SO8 chip + a few discretes. Then of course there's the full range of UC* PWM control chips from TI (formerly Unitrode), but they won't do 0% or 100% DC because it doesn't make sense for power apps.

robert

There are PWM chips available that you can control digitally. Alternatively, you can use 2 op-amps to generate a variable frequency triangle wave and a comparator to generate the PWM.

Elektro wrote:

NE555

this sort of thing can be done with a dual op-amp, one half is setup to generate a sawtooth at 5Khz the other compares it to the input.

You mean like a PIC?

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Why throw software at something easely done by some simple analog circuitry?

It depends a lot on the application, but a software controlled PWM can be a little easier to implement if one needs things like a full power start for n seconds, a minimum/maximum duty cycle floor/ceiling, or a particular non-linear response. All certainly do-able with just silicon but, again depending on the app, possibly simpler or cheaper with a tiny microcontroller.