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I read the Christophe Basoo article The TL431 in Switch-Mode Power Supplies loops: part II

About how to choose the resistor I have some questions I would like to ask.

On page 13 and page 14 (Watch the LED series resistor value) There are my questions

1.) Why the VTL431 voltage is 2.5V, how to know this value

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2.) I can't understand the equation(25), what is this sentence meaning. Maybe is my English skill is not good so I can't get it, can someone explain it to me? enter image description here

Jitter456
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  • @winny I see the datasheet said the Output voltage Level is 2V~Vsup. – Jitter456 Apr 17 '21 at 09:10
  • @winny on page 14, describes equation 25. – Jitter456 Apr 17 '21 at 09:12