9.2.2.2 Selecting Soft Start Setting: C_SS and C_SS,VEE

First, compute the minimum inrush current where the timer will trip using equation below.

Equation 12.

To avoid running the timer the inrush current needs to be sufficiently low. Target 0.5 A of inrush current to allow margin, and compute the target C_SS using equation below. Note that a 10% total tolerance capacitance was assumed on Cout.

Equation 13.

Chose C_SS close to 58 nF. For this example 55 nF was used, which assumes a 33 nF and 22 nF cap in parallel. This results in an inrush current of 0.53 A at max C_OUT (1452 µF) and inrush current of 0.48 A at typical C_OUT (1320 µF). Also it is recommended to add a capacitor between the soft start pin and VEE (C_SS,VEE) to improve immunity to input voltage noise during soft start. It's recommended to chose a capacitor that's 3x larger than C_SS. In this case a 150 nF capacitor was chosen.

Finally the start-up time at maximum input voltage can be computed using the equation below:

Equation 14.