The internal soft-start circuitry controls the output voltage slope during start-up. This avoids 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 50 µs, and V_OUT rises with a slope controlled by an external capacitor connected to the SS/TR pin. See Figure 9-34 and Figure 9-35 for typical start-up operation.

Using a very small capacitor (or leaving the SS/TR pin disconnected) provides fastest start-up behavior. There is no theoretical limit for the longest start-up time. The TPS6215x can start into a pre-biased output. During monotonic pre-biased start-up, both the power MOSFETs are not allowed to turn on until the internal ramp of the device 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 start-up 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 follows this voltage in both directions, up and down (see Section 9).