Problem with LT3999

Hi

Iam using the LT3999 in series with a push Pull Trafo from würth (750315371) to generate 12VDC isolated.

After a powerup, the first few seconds all works fine but then the output voltage begins to oscillate and as a result, the LT3999 is getting quite hot.

Do you have any ideas why this happens?

Maybe I should reduce the frequency (500 kHz)

In addition, i left the ILIM and RDC pins floating and UVLO and OVLO are disabled (connected to VIN, GND resp.)

 PDF

Thank you for the help

Best

Andy