I had a suitable PCB design with a TOP224, but I needed to control ON/OFF so I converted to a TOP256 using the same xfmr.
ON/OFF works nicely and I get 50 watts out. But I observe TOP256 operating frequency to be about 35 kHz, regardless of whether F pin is connected to S or C. It's not a stable 35 kHz and varies with load.
When I go back to the TOP224, operating freq is a stable 100 kHz.
Please help soon, as I need to get this design released in a couple days.