The current-sense pin is connected through a series resistor (R_LC) to the current-sense resistor (R_CS). The current-sense threshold is 0.75 V for I_PP(max) and 0.25 V for I_PP(min). The series resistor R_LC provides the function of feed-forward line compensation to eliminate change in I_PP due to change in di/dt and the propagation delay of the internal comparator and MOSFET turn-off time. There is an internal leading-edge blanking time of 235 ns to eliminate sensitivity to the MOSFET turn-on current spike. It should not be necessary to place a bypass capacitor on the CS pin. The value of R_CS is determined by the target output current in constant-current (CC) regulation. The values of R_CS and R_LC can be determined by the equations below. The term η_XFMR is intended to account for the energy stored in the transformer but not delivered to the secondary. This 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 1.5%. The η_XFMR value is approximately: 1 - 0.05 - 0.035 - 0.015 = 0.9.

Equation 3.

where

• V_CCR is a current regulation constant (see the Section 6.5 table),
• N_PS is the transformer primary-to-secondary turns ratio (a ratio of 13 to 15 is recommended for 5-V output),
• I_OCC is the target output current in constant-current regulation,
• η_XFMR is the transformer efficiency.

Equation 4.

where

• R_S1 is the VS pin high-side resistor value,
• R_CS is the current-sense resistor value,
• T_D is the current-sense delay including MOSFET turn-off delay, add ~50 ns to MOSFET delay,
• N_PA is the transformer primary-to-auxiliary turns ratio,
• L_P is the transformer primary inductance,
• K_LC is a current-scaling constant (see the Section 6.5 table).