In case of an overcurrent condition on the isolated output or outputs, the output voltage drops lower than the regulation level since the maximum power delivered is limited by the peak current capability on the primary side. The peak primary current is maintained at 0.75 A (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 LM5181 assumes the output cannot be recovered and re-calibrates its switching frequency to 9 kHz until the overload condition is removed. The LM5181 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, where η is the efficiency. The typical threshold value for I_SW-PEAK from Section 7 is 0.75 A.

Equation 13.

A failsafe current limit set at 1.2 A, or 1.6 times the nominal peak current limit, provides redundant fault protection in case of transformer short circuit or saturation effects. This initiates a 7.5-ms hiccup interval after eight overcurrent events.