LNK362P

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Hello!
In the Engineering Prototype Report for 2.0 W CV Adapter using LNK362P,I see the following words:
As soon as the voltage across the DRAIN and SOURCE pins of U1 exceeds 50 V, the internal high voltage current source (connected to the DRAIN pin of the IC) begins charging the capacitor (C3) connected to the Bypass (BP) pin.Once the voltage across C3 reaches 5.8 V, the controller enables MOSFET switching.
My question are:

Why the voltage across the DRAIN pin exceeds 50 V,the internal high voltage current source begins charging the capacitor ?
Why the initial voltage of the DRAIN pin is not 0 V?
I think the voltage of the DRAIN pin is increasing form 0 V.Is there a trigger level to trigger the system to charge the capacitor?
Thanks,