9.2.2.2 Soft Start

Once R_ON is defined, the charge current of the soft-start capacitor is calculated with Equation 13.

Equation 13.

During soft start, C_SS charges from 0 to 5 V by the calculated I_SS current. The actual control range of the soft-start capacitor voltage is between 2.5 and 4.5 V. Therefore, the soft-start capacitor value must be based on this narrower control range and the required startup time (t_SS) according to Equation 14.

Equation 14.

Note that t_SS defines a time interval to reach the maximum-current capability of the converter and not the time required to ramp the output voltage from 0 V to the nominal regulated level. Using an open-loop start-up scheme does not allow accurate control over the ramp-up time of the output voltage. In addition to the I_SS and C_SS values, the time required to reach the nominal output voltage of the converter is a function of the maximum-output current (current limit), the output capacitance of the converter and the actual load conditions. If it is critical to implement a tightly-controlled ramp-up time at the output of the converter, the soft start must be implemented using a closed-loop technique. Closed-loop soft-start implements with the error amplifier of the voltage regulation loop when the voltage reference is ramped from 0 V to the final steady-state value during the required t_SS startup-time interval.