UCC35705

Reviewing the UCC35705 data sheet, it seems to me the FB pin should be driven by an externally compensated network providing voltage reference too. I suggest to you the use of a TL431 or similar shunt regulator (very cheap) to drive the diode section of an optoisolated coupler and connect the coupled phototransistor on the FB pin providing suitable pull-up to correctly bias the same FB pin (remember the UCC35705 is a voltage mode regulator which compares the feedback from a compensated network to the oscillator ramp to determine the right duty cycle of the converter). In the case you do not need isolation between primary and secondary of your power supply, it is not necessary to use an optocoupled feedback network, but a simple bjt driven stage (think of an unexpensive BC807 / BC817 for instance). Please, pay attention the TL431 aplifier behaves an inherent zero in the Bode plot in the case its biasing is directly derived from the same output voltage you have to regulate. To avoid this, you can use a zener diode to derive a biasing voltage and decouple it from the output voltage. Hope it will help!

Thanks for attention! it helped me to understand the operation of this IC.