To minimize inrush current and output voltage overshoot during start up, the device has a soft start. At turn on, the input current raises monotonically until the output voltage reaches regulation. The TPS6305x devices charge the soft start capacitor, at the SS pin, with a constant current of typically 1 µA. The input current follows the current used to charge the capacitor at the SS pin. The soft start operation is completed once the voltage at the SS pin has reached typically 1.3 V. Figure 3 shows the value of the soft start capacitor in respect to the soft-start time.
The soft-start time is the time from when the EN pin is asserted to when the output voltage has reached 90% of its nominal value. There is typically a 100-µs delay time from EN pin assertion to the start of the switching activity. The soft-start time depends on the load current, the input voltage, and the output capacitor. The soft-start time in boost mode is longer then the time in buck mode and it also depends on the load current, input voltage and output capacitor.
The soft-start time in Figure 3 is referred to typical application with 10-µF effective output capacitance.
The inductor current is able to increase and always assure a soft start unless a real short circuit is applied at the output.