The internal soft-start circuitry controls the output voltage slope during start-up. This action avoids excessive inrush current and makes sure of 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 to start operation, the device starts switching after a delay of about 200μs, then the internal reference and hence, V_OUT, rises with a slope controlled by an external capacitor connected to the SS/TR pin.

Leaving the SS/TR pin disconnected provides the fastest start-up ramp with typically 150µs. A capacitor connected from SS/TR to GND is charged with 2.5µA by an internal current source during soft start until the capacitor reaches the 0.6V reference voltage. The capacitance required to set a certain ramp-time (t_ramp) is:

Equation 6.

If the device is set to shutdown (EN = GND), undervoltage lockout, or thermal shutdown, an internal resistor pulls the SS/TR pin to GND to make sure of a proper low level. Returning from those states causes a new start-up sequence.

A voltage applied at SS/TR can be used to track a main 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. The SS/TR pin must not be connected to the SS/TR pin of other devices. An external voltage applied on SS/TR is internally clamped to the feedback voltage (0.6V). TI recommends to set the target for the external voltage on SS/TR slightly above the feedback voltage. Given the tolerances of the resistor divider R₅ and R₆ on SS/TR, this makes sure the device switches to the internal reference voltage when the power-up sequencing is finished. See Figure 9-57.