xr2209 duty cycle
I needed an oscillator like this before. I began with a triangular wave oscillator from which I controlled the frequncy then compared this triangular wave to a variable DC voltage (using a comparator) to obtain the duty cycle control. You can find out an example of a triangular wave oscillator in the Art of Electronics.
I also used the TL494 to do the job. It works alomst on the same principle (the triangular is replaced with a sawtooth). The frequency of the TL494 is controlled via Rt and Ct while the duty is varied from the DTC input.
Good Luck