SNVS690I January 2011 – August 2021 LMZ14201H
Programmable soft-start permits the regulator to slowly ramp to its steady-state operating point after being enabled, thereby reducing current inrush from the input supply and slowing the output voltage rise-time to prevent overshoot.
Upon turnon, after all UVLO conditions have been passed, an internal 8uA current source begins charging the external soft-start capacitor. The soft-start time duration to reach steady-state operation is given by the formula:
This equation can be rearranged as follows:
Use of a 4700-pF capacitor results in 0.5-ms soft-start duration. This is a recommended value. Note that high values of CSS capacitance will cause more output voltage droop when a load transient goes across the DCM-CCM boundary. Use Equation 18 below to find the DCM-CCM boundary load current for the specific operating condition. If a fast load transient response is desired for steps between DCM and CCM mode the soft-start capacitor value should be less than 0.018 µF.
Note that the following conditions will reset the soft-start capacitor by discharging the SS input to ground with an internal 200-μA current sink: