The VS pin is connected to a resistor divider from the auxiliary winding to ground. The output-voltage feedback information is sampled at the end of the transformer secondary current demagnetization time to provide an accurate representation of the output voltage. Timing information to achieve valley-switching and to control the duty cycle of the secondary transformer current is determined by the waveform on the VS pin. Avoid placing a filter capacitor on this input which would interfere with accurate sensing of this waveform.

The VS pin also senses the bulk capacitor voltage to provide for AC-input run and stop thresholds, and to compensate the current-sense threshold across the AC-input range. This information is sensed during the MOSFET on-time. For the AC-input run/stop function, the run threshold on VS is 220 µA and the stop threshold is 80 µA. The values for the auxiliary voltage divider upper-resistor R_S1 and lower-resistor R_S2 can be determined by the equations below.

Equation 1.

where

• N_PA is the transformer primary-to-auxiliary turns ratio,
• V_IN(run) is the AC RMS voltage to enable turn-on of the controller (run),
• I_VSL(run) is the run-threshold for the current pulled out of the VS pin during the MOSFET on-time. (see the Section 6.5 table)

Equation 2.

where

• V_OCV is the converter regulated output voltage,
• V_F is the output rectifier forward drop at near-zero current,
• N_AS is the transformer auxiliary to secondary turns ratio,
• R_S1 is the VS divider high-side resistance,
• V_VSR is the CV regulating level at the VS input (see the Section 6.5 table).