BLK pin senses the LLC input voltage and determines when to turn on and off the LLC converter. Different versions of UCC25640x have different BLK thresholds.
Choose bulk startup voltage at 365 V, then the BLK resistor divider ratio can be calculated as below:
The desired power consumption of the BLK pin resistor divider is PBLKsns = 10 mW. The BLK sense resistor total value is given by:
The lower BLK divider resistor value is given by:
A standard value of 41.2 kΩ is selected for RBLKlower.The higher BLK divider resistor value is given by:
A standard value of 3x 4.99 MΩ in series is selected for RBLKupper. The actual bulk voltage thresholds can be calculated: