RS1 determines the start-up input voltage. For this value use:
where
• NPA is the transformer primary-to-auxiliary turns-ratio,
• VIN(run) is the AC RMS voltage to enable turnon of the controller (run),
(in case of DC input, leave out the √2 term in the equation),
• IVSL(run) is the run-threshold for the current pulled out of the VS pin during the switch on-time
RS2 determines the output over-voltage threshold. For this use:
where
• VOV is the maximum allowable peak voltage at the converter output,
• VF is the output-rectifier forward drop at near-zero current,
• NAS is the transformer auxiliary-to-secondary turns-ratio,
• RS1 is the VS divider high-side resistance,
• VOVP is the overvoltage detection threshold at the VS input
