The TPS61253x integrates an internal circuit that controls the ramp up of the output voltage during start-up and prevents the converter from the large inrush current. When the device is enabled, the high-side rectifying switch turns on to charge the output capacitor linearly which is called the pre-charge phase. During the pre-charge phase, the output current is limited to the pre-charge current limit ILIM_DC. The pre-charge phase terminates until the output voltage getting close to the input voltage.

Once the output capacitor has been biased close to the input voltage, the device starts switching which is called the soft-start phase. During the soft start phase, there is a soft-start voltage controlling the FB pin voltage, and the output voltage rising slope follows the soft-start voltage slope. The device finishes the soft-start phase and operates normally when the nominal output voltage is reached.