I find it quite odd that the feedback is applied to pin 2, and not pin 1. I mean, what is the error amplifier doing now? It's not connected to anything, except for its own feedback R and C. Maybe "FB" on pin 2 is deliberately ambiguous; it may refer to feedback for the error amplifier itself, or feedback for the whole PWM control system, if you happen to want the feedback to work in the opposite direction.
In the example schematic from the manufacturer, feedback is applied to pin 1, where too high an output will pull pin 2 low.
In the OBC circuit, too high an output will turn on the TL431, which will turn on the opto, which will pull pin 2 low directly, regardless of what happens with pin 1 and the error amplifier. The datasheet doesn't seem to suggest this as normal operation. If the error amplifier output isn't designed to be shorted low (e.g. open drain with a pull-up resistor), then it seems to me that they are using this part outside its design parameters.
I'm also wondering if I got the purpose of pin 6, "CS", wrong. Perhaps it's part of the "soft start" circuit; a time constant of around 500 ms makes sense for that too. The "C" would be for C
apacitor, and the "S" for S
oft Start. It's so hard to follow the internal diagram when you don't know whether inputs to a box inhibit with positive or inhibit with negative. Similarly, the UVLO (under-voltage lock-out) block outputs: are then alternatively positive? If so, which one is positive normally? Often you can guess, but it's possible to make a plausible theory that is completely wrong.
Edit: since this is the start of a new page, I'll duplicate the reference schematic from the datasheet here: