SLVSD02E March 2015 – August 2021 TPS65982
When configured as a sink, the SS pin provides a soft start function for each of the high-voltage power path supplies (P_HV and external PP_EXT path) up to 5.5 V. The SS circuitry is shared for each path and only one path will turn on as a sink at a time. The soft start is enabled by application code or via the host processor. The SS pin is initially discharged through a resistance RSS_DIS. When the switch is turned on, a current ISS is sourced from the pin to a capacitance CSS. This current into the capacitance generates a slow ramping voltage. This voltage is sensed and the power path FETs turn on and the voltage follows this ramp. When the voltage reaches the threshold VTHSS, the power path FET will be near being fully turned on, the output voltage will be fully charged. At time TSSDONE, a signal to the digital core indicates that the soft start function has completed. The ramp rate of the supply is given by Equation 4:
The maximum ramp voltage for the supply is approximately 16.2 V. For any input voltage higher than this, the ramp will stop at 16.2 V until the firmware disables the soft start. At this point, the voltage will step to the input voltage at a ramp rate defined by approximately 7 μA into the gate capacitance of the switch. The TSSDONE time is independent of the actual final ramp voltage.