Hi Tim and thank's for a very quick reply,
I think I have to use a different terminology, or rather better explain what I mean with primary and secondary side: Primary side is the wall triac dimmer. Secondary side is me injecting a signal somewhere into the control loop or feedback network to dim the LED independent of the wall triac dimmer (but when doing so the primary dimmer should be 100%). It has nothing to do with an isolation barrier, even though that is the convention. Sorry for the confusion.
Since LM3445 behaves best in our tests we will try to implement a similar circuit to cut off its VCC to get 0%. Hopefully that can be tested tomorrow.
Best Regards,
Göran Nordahl