SLUSBL5A February 2015 – June 2019 UCC28730
The VS pin connects to a resistor-divider from the auxiliary winding to ground and is used to sense input voltage, output voltage, event timing, and Wait-state wake-up signaling. The auxiliary voltage waveform is sampled at the end of the transformer secondary current demagnetization time to provide an accurate representation of the output voltage. The waveform on the VS pin determines the timing information to achieve valley-switching, and the timing to control the duty-cycle of the transformer secondary current when in Constant-Current Mode. Avoid placing a filter capacitor on this input which interferes with accurate sensing of this waveform.
During the MOSFET on-time, this pin also senses VS current generated through RS1 by the reflected bulk-capacitor voltage to provide for AC-input Run and Stop thresholds, and to compensate the current-sense threshold across the AC-input range. For the AC-input Run/Stop function, the Run threshold on VS is 225 µA and the Stop threshold is 80 µA.
At the end of off-time demagnetization, the reflected output voltage is sampled at this pin to provide regulation and overvoltage protection. The values for the auxiliary voltage-divider upper-resistor, RS1, and lower-resistor, RS2, are determined by Equation 2 and Equation 3.
When the UCC28730 is operating in the Wait state, the VS input is receptive to a wake-up signal superimposed upon the auxiliary winding waveform after the waveform meets either of two qualifying conditions. A high-level wake-up signal is considered to be detected if the amplitude at the VS input exceeds VWU(high) (2 V) provided that any voltage at VS has been continuously below VWU(high) for the wake-up qualification delay tWDLY (8.5 us) after the demagnetization interval. A low-level wake-up signal is considered to be detected if the amplitude at the VS input exceeds VWU(low) (57 mV) provided that any voltage at VS has been continuously below VWU(low) for the wake-up qualification delay tWDLY (8.5 us) after the demagnetization interval. The high-level threshold accommodates signals generated by a low-impedance secondary-side driver while the low-level threshold detects signals generated by a high-impedance driver.