The device has a low-side current sense through the CS pin. The PWM controller and the cycle-by-cycle peak current limit protection circuit use the low-side current sense. If required, a small external RC filter at the CS pin can be added to overcome the leading edge noise of the current sense signal.

In case of an overcurrent condition on the isolated output or outputs, the output voltage drops lower than the regulation level because the maximum power delivered is limited by the peak current capability on the primary side. The peak primary current is maintained at 100 mV on V_CS (plus an amount related to the 100-ns propagation delay of the current limit comparator) until the output decreases to the secondary diode voltage drop to impact the reflected signal on the primary side. At this point, the LM25185 assumes the output cannot be recovered and re-calibrates the switching frequency to 9 kHz until the overload condition is removed. The LM25185 responds with similar behavior to an output short circuit condition.

For a given input voltage, Equation 13 gives the maximum output current prior to the engagement of overcurrent protection. The typical threshold value for I_SW-PEAK from the Electrical Characteristics is 100 mV / R_CS.