HVLED001A - FB pull-up current suddenly increases from cca 0.6mA to 1mA.

[JHarm]

Hi all, I am confused about FB pull-up current. At start, ZCD detects overvoltage, so I understand that FB is pulled down.

After 300ms, FB is pulled by 0.6mA and after next 500ms FB is suddenly pulled by 1mA.

How does overvoltage protection work? How long is overvoltage opAmp active? How long S&H circuitry hold sampled value? There is another transisotor in the datasheet schematic diagram. What is its purpose?

On attached scope Yellow is FB voltage, Blue is output voltage, Red is TLV431 cathode. The second scope is zoomed. At third scope (zoomed more) you can see even ZCD pin voltage (Green) - cca 1.8V pulses each microsecond in burst mode. What is the reason for such change?

Please help.

Thank you very much.

Jan