Hi I am also working on a project with this IC and am having the same problem. The chip is just like the older UC3843. I tried following the charts in the datasheets but still don't have it right.
I want a max duty cycle of somewhere around 25%. Now I know there is an alternative chip in the same family that limits the duty cycle to 50% by using an internal f/f but I already have several UCC38C43's. I followed the charts and the equation (from the UC3843 datasheet) which is f= 1.72/(Rt * Ct). I am using a 470pF with 27k, it comes out to about 135kHz. Tested with a function generator and in simulation that this works. However, the duty cycle of the circuit is at max, near 98%. It kills my switching.
Can someone please explain to me how limiting the duty cycle is accomplished? If I can't do it at that frequency on the IC is there a way to do it outside of the chip on the OUTput pin? Thanks!