SLUSCC7C July 2016 – June 2018 TPS546C23
To control the inrush current required to charge the output capacitor bank during start up, the devices implement a soft-start time. When the device is enabled, the feedback reference voltage, VREF, ramps from 0 V to the final level defined by Equation 3 at a slew rate defined by the TON_RISE command. The specified rise times are defined by the slew rate required to ramp the reference voltage from 0 V to the final value at each given rise time.
The actual rise time of the converter output is slightly less than the rise time defined by the TON_RISE command. This difference occurs because switching does not occur until the error-amplifier output reaches the valley of the PWM ramp. During the soft-start time, the error-amplifier output voltage starts at 0 V and then begins switching again only when the VSHARE voltage reaches the valley of the PWM ramp which is 1.23 V (typical). When the VSHARE voltage reaches the valley of the PWM ramp, the converter output voltage rises quickly until the feedback voltage, VFB, reaches the VREF level, at which point they track through the end of the soft-start period.
The devices support several soft-start times from 1 ms to 100 ms which are selected by the TON_RISE command.