A soft-start capacitor can be used to control the start-up time of the LM27402. The start-up time is estimated by the following equation:

Equation 9.

I_SS is nominally 3 µA. For the evaluation board, the soft-start time has been designed to be approximately 10 ms, resulting in a C_ss capacitor value of 47 nF. The LM27402 defaults to a 1.28-ms start-up ramp time if C_ss is not used.