SLUSBL5A February 2015 – June 2019 UCC28730
The current-sense pin connects to a series resistor (RLC) to the current-sense resistor (RCS). The maximum current-sense threshold (VCST(max)) is approximately 0.74 V for IPP(max) and minimum current-sense threshold (VCST(min)) is approximately 0.25 V for IPP(min). RLC provides the function of feed-forward line compensation to eliminate changes in IPP with input voltage due to the propagation delay of the internal comparator and MOSFET turn-off time. An internal leading-edge blanking time of 225 ns eliminates sensitivity to the MOSFET turn-on current spike. It should not be necessary to place a bypass capacitor on the CS pin. The target output current in constant-current (CC) regulation determines the value of RCS. The values of RCS and RLC are calculated by Equation 4 and Equation 5. The term VCCR is the product of the demagnetization constant, 0.432, and VCST(max). VCCR is held to a tighter accuracy than either of its constituent terms. The term ηXFMR accounts for the energy stored in the transformer but not delivered to the secondary. This term includes transformer resistance and core loss, bias power, and primary-to-secondary leakage ratio.
Example: With a transformer core and winding loss of 5%, primary-to-secondary leakage inductance of 3.5%, and bias-power to output-power ratio of 0.5%, the ηXFMR value at full-power is: 1 - 0.05 - 0.035 - 0.005 = 0.91.