SLUSBL5A February 2015 – June 2019 UCC28730
The VS divider resistors determine the output voltage regulation point of the flyback converter. Also, the high-side divider resistor, RS1, determines the line voltage at which the controller enables continuous DRV operation. RS1 is initially determined based on the transformer primary to auxiliary turns ratio and the desired input voltage operating threshold.
The low-side VS divider resistor, RS2, is selected based on the desired constant-voltage output regulation target, VOCV.
The UCC28730 can maintain tight constant-current regulation over input line by utilizing the line compensation feature. The line compensation resistor value, RLC, is determined by various system parameters and the combined gate-drive turn-off and MOSFET turn-off delays, tD. Assume a 50-ns internal propagation delay in the UCC28730.
The UCC28730 provides adjustable cable compensation of up to approximately +8% of VOCV by connecting a resistor between the CBC terminal and GND. This compensation voltage, VOCBC, represents the incremental increase in voltage, above the nominal no-load output voltage, needed to cancel or reduce the incremental decrease in voltage at the end of a cable due to its resistance. The programming resistance required for the desired cable compensation level at the converter output terminals can be determined using the equation below. As the load current changes, the cable compensation voltage also changes slowly to avoid disrupting control of the main output voltage. A sudden change in load current will induce a step change of output voltage at the end of the cable until the compensation voltage adjusts to the required level. Note that the cable compensation does not change the overvoltage protection (OVP) threshold,VOVP (see Electrical Characteristics), so the operating margin to OVP is less when cable compensation is used. If cable compensation is not required, CBC may remain unconnected.