The desired 10-ms soft start time is achieved following the procedure in Section 9.3.3. The procedure is replicated below for convenience.

First, use Equation 13 to determine the needed value of C_SS. This results in a calculated C_SS value of 67.7 nF. A 68-nF ±10% capacitor is selected.

Equation 13.

where

• t_SS = 10 ms = 10 × 10^–3 s
• I_SS(TYP) = 65 μA = 65 × 10^–6 A
• V_OUT(NOM) = 12 V

Next determine the resulting slew rate using Equation 14. Using the minimum value for the 10% tolerance C_SS and the maximum value for I_SS results in the worst case (fastest) slew rate of 1,356 V/s.

Equation 14.

where

• I_SS(MAX) = 83 μA = 83 × 10^–6 A
• C_SS = 68 nF × (1 – 10%) = 61.2 × 10^–9 F

Finally, determine if the resulting slew rate is less than the maximum allowed by Equation 15. I_{L_trip} is typically 8.5 A, but in order to select a conservative value it is suggested to let I_{L_trip} = 5.4 A (which is also the absolute maximum rating for continuous switch current). The 1,356-V/s slew rate is less than the maximum acceptable slew rate of 14,109 V/s. Therefore, this soft start capacitor is acceptable.

Equation 15.

where

• I_{L_trip} = 5.4 A
• I_OUT(NOM) = 3.0 A
• C_OUT = 170.1 μF = 170.1 × 10^–6 F

While it is typically trivial to meet the slew rate restrictions, note that for space applications a large output capacitor is often utilized. This results in a lower maximum acceptable slew rate and additional care must be taken in order to ensure the expected slew rate is not too fast.