The internal soft start circuitry controls the output voltage slope during startup, avoiding excessive inrush current and ensures a controlled output voltage rise time. It also prevents unwanted voltage drops from high-impedance power sources or batteries. When EN is set to start device operation, the device starts switching after a delay of about 50µs and VOUT rises with a slope controlled by an external capacitor connected to the SS/TR pin. See Figure 38 and Figure 39 for typical startup operation.
Using very small capacitor (or leaving SS/TR pin un-connected) provides fastest startup behavior. The TPS6215xA-Q1 can start into a pre-biased output. During monotonic pre-biased startup, both of the power MOSFETs are not allowed to turn on until the device's internal ramp sets an output voltage above the pre-bias voltage. If the device is set to shutdown (EN=GND), undervoltage lockout or thermal shutdown, an internal resistor pulls the SS/TR pin down to ensure a proper low level. Returning from those states causes a new startup sequence as set by the SS/TR connection.
A voltage supplied to SS/TR can be used for tracking a master voltage. The output voltage will follow this voltage in both directions up and down (see Application and Implementation section).