The soft-start circuitry controls the output voltage slope during start-up. This action avoids excessive inrush current and ensures a controlled output voltage rise time. This action also prevents unwanted voltage drops from high impedance power sources or batteries. When EN is set high, the device starts switching after a delay of about 270 μs. Then V_OUT rises with a slope controlled by an external capacitor connected to the SS/TR pin.

A capacitor connected from SS/TR to GND is charged with 10 µA by an internal current source during soft start until it reaches the reference voltage of 0.6 V. After reaching 0.6 V, the SS/TR pin voltage is clamped internally while the SS/TR pin voltage keeps rising to a maximum of about 3.3 V. The capacitance required to set a certain ramp-time (t_ramp) is:

Leaving the SS/TR pin un-connected provides the fastest start-up ramp of 150 µs typically. If the device is set to shutdown (EN = GND), undervoltage lockout, or thermal shutdown, an internal resistor of about 1.1 kΩ pulls the SS/TR pin to GND to ensure a proper low level. Returning from those states causes a new start-up sequence.

A voltage applied at the SS/TR pin can also be used to track a master voltage. The output voltage follows this voltage in both directions up and down in forced PWM mode. In PSM mode, the output voltage decreases based on the load current. An external voltage applied on SS/TR is internally clamped to the feedback voltage (0.6 V). TI recommends to set the final value of the external voltage on SS/TR to be slightly above 0.6 V to make sure the device operates with its internal reference voltage when the power-up sequencing is finished. See Voltage Tracking.