With the SS/TR pin, it is possible to adjust the soft-start behavior and track an external voltage. See Section 8.2.2.4 for operation details.

The internal soft-start circuitry controls the output voltage slope during start-up. This avoids excessive inrush current and makes sure there is a controlled output voltage rise time. It also prevents unwanted voltage drops from high impedance power sources or batteries. When EN is set high to start operation, the device starts switching after a delay, then the internal reference, and hence V_O, rises with a slope controlled by an external capacitor connected to the SS/TR pin.

Leaving the SS/TR pin unconnected provides the fastest start-up, limited internally (the pin must not be pulled LOW externally).

If the device is set to shut down (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 to track a primary voltage. The output voltage follows this voltage up and down in forced PWM mode. In PFM mode, the output voltage decreases based on the load current.