JAJSAY9L February 2009 – May 2018 LM26420
When the switch current reaches the current limit value, it is turned off immediately. This effectively reduces the duty cycle and therefore the output voltage dips and continues to droop until the output load matches the peak current limit inductor current. As the FB voltage drops below 480 mV the operating frequency begins to decrease until it hits full on frequency foldback, which is set to approximately 150 kHz for the Y version and 300 kHz for the X version. Frequency foldback helps reduce the thermal stress in the device by reducing the switching losses and to prevent runaway of the inductor current when the output is shorted to ground.
It is important to note that when recovering from a overcurrent condition the converter does not go through the soft-start process. There may be an overshoot due to the sudden removal of the overcurrent fault. The reference voltage at the non-inverting input of the error amplifier always sits at 0.8 V during the overcurrent condition, therefore when the fault is removed the converter bring the FB voltage back to 0.8 V as quickly as possible. The overshoot depend on whether there is a load on the output after the removal of the overcurrent fault, the size of the inductor, and the amount of capacitance on the output. The smaller the inductor and the larger the capacitance on the output the smaller the overshoot.
Overcurrent protection for each output is independent.