SLUSBL5A February 2015 – June 2019 UCC28730
PRODUCTION DATA.
The current-sense pin connects to a series resistor (R_{LC}) to the current-sense resistor (R_{CS}). The maximum current-sense threshold (V_{CST(max)}) is approximately 0.74 V for I_{PP(max)} and minimum current-sense threshold (V_{CST(min)}) is approximately 0.25 V for I_{PP(min)}. R_{LC} provides the function of feed-forward line compensation to eliminate changes in I_{PP} 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 R_{CS}. The values of R_{CS} and R_{LC} are calculated by Equation 4 and Equation 5. The term V_{CCR} is the product of the demagnetization constant, 0.432, and V_{CST(max)}. V_{CCR} is held to a tighter accuracy than either of its constituent terms. The term η_{XFMR} 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 η_{XFMR} value at full-power is: 1 - 0.05 - 0.035 - 0.005 = 0.91.
where
where