The BW pin programs the ratio between burst mode entry and exit thresholds as well as senses the output voltage through the bias winding and protects the power stage from over voltage. The nominal output voltage is 12 V. The bias winding has 3 turns, and the secondary side winding has 2 turns. Assuming there is a 0.5-V drop in the rectifier diodes (Vf) and a further 0.5-V drop due to other losses (Vloss), the nominal voltage of the bias winding is given by:
The desired OVP threshold in this design is 140% of the nominal value. The OVP threshold level in UCC25640x device is 4 V, so the nominal BW pin voltage is given by:
The required BW divider ratio is then given by:
In this design, the burst mode threshold ratio is chosen to be 0.6 (Option 6).The target programming resistance is then:
The lower BW resistor can be calculated by:
A standard value of 5.36 kΩ is chosen for the lower BW resistor.
The upper resistor can be calculated by:
A standard value of 30.9 kΩ is chosen for the upper BW resistor.