JAJSJ67E may 2020 – july 2023 UCC28782
PRODUCTION DATA
The VS pin is used to sense the positive voltage level of the auxiliary winding voltage (VAUX) to detect an over-voltage condition of VO. When an OVP event is triggered, the auto-recovery version of OVP stops switching and there is a 1.5-s fault recovery time (tFDR) before any VO restart attempt is made. As QL turns off, the settled VAUX is equal to (VO+VF) x NAS, where NAS is the auxiliary-to-secondary turns ratio of the transformer, NA / NS, and VF is the forward voltage drop of the secondary-side rectifier. The VS pin senses VAUX through a voltage divider formed by RVS1 and RVS2. The pin voltage (VVS) is compared with an internal OVP threshold (VOVP). If VVS ≥ VOVP condition is qualified for three consecutive PWML pulses, the controller stops switching, brings RUN pin low, and initiates the 1.5-s time delay. During this long delay time, only the UVLO-cycle of VVDD is active, and there are no test pulses of PWML. After the 1.5-s timeout is completed and VVDD reaches the next VVDD(OFF), a normal start sequence begins. The calculation of RVS2 is
The long tFDR timer (1.5 s) helps to protect the power stage components from the large current stress during every restart and allows some time for VO to discharge in the case of light-load or no-load, before attempting restart.