A capacitance connected between SS/TR pin and AGND allows a user programmable start-up slope of the output voltage. A constant current source supports 2.5µA to charge the external capacitance. The capacitor required for a given soft-start ramp time for the output voltage is given by:
CSS is the capacitance (F) required at the SS/TR pin and
tSS is the desired soft-start ramp time (s).
DC Bias effect: High capacitance ceramic capacitors have a DC Bias effect, which will have a strong influence on the final effective capacitance. Therefore the right capacitor value has to be chosen carefully. Package size and voltage rating in combination with dielectric material are responsible for differences between the rated capacitor value and the effective capacitance.