The TPS61088 has an adjustable soft start function to prevent high inrush current during start-up. To minimize the inrush current during start-up, an external capacitor, connected to the SS pin and charged with a constant current, is used to slowly ramp up the internal positive input of the error amplifier. When the EN pin is pulled high, the soft-start capacitor C_SS (C7 in Figure 8-1) is charged with a constant current of 5 μA typically. During this time, the SS pin voltage is compared with the internal reference (1.204 V), the lower one is fed into the internal positive input of the error amplifier. The output of the error amplifier (which determines the inductor peak current value) ramps up slowly as the SS pin voltage goes up. The soft-start phase is completed after the SS pin voltage exceeds the internal reference (1.204 V). The larger the capacitance at the SS pin, the slower the ramp of the output voltage and the longer the soft-start time. A 47-nF capacitor is usually sufficient for most applications. When the EN pin is pulled low, the voltage of the soft-start capacitor is discharged to ground.

Use Equation 1 to calculate the soft-start time.

Equation 1.

where

• t_SS is the soft start time
• V_REF is the internal reference voltage of 1.204 V
• C_SS is the capacitance between the SS pin and ground
• I_SS is the soft-start charging current of 5 µA