Usually bulk capacitors are placed on the output due to various reasons, such as uninterrupted operation during power interruption or micro-short at the input, hold-up requirements for doing a memory dump before turning of the module and filtering requirements as well. This design considers minimum bulk capacitors requirements for meeting functional status "A" during LV124 E10 test case 2 100-µs input interruption. To achieve functional pass status A, acceptable voltage droop in the output of LM74721-Q1 is based on the UVLO settings of downstream DC/DC converters. For this design, 1-V drop in output voltage for 100 µs is considered and the minimum hold-up capacitance required is calculated by:

Minimum hold-up capacitance required for 1-V drop in 100 µs is 500 µF. 3 × 220-µF electrolytic capacitors are selected.

Also during ISO7637-2 pulse 1 transient event, LM74721-Q1 operates external MOSFET in active clamp mode, allowing reverse current to flow from output to back to the input source. The output hold-up capacitor also ensures output voltage does not swing negative when device is operating VDS clamp mode.