The voltage on the respective SS pin controls the start-up of the buck output. When the voltage on the SS pin is less than the internal 0.6V reference, the TPS65261, TPS65261-1 regulates the internal feedback voltage to the voltage on the SS pin instead of 0.6V. The SS pin can be used to program an external soft-start function or to allow output of the buck to track another supply during start-up. The device has an internal pull-up current source of 5µA (typical) that charges an external soft-start capacitor to provide a linear ramping voltage at the SS pin. The TPS65261, TPS65261-1 regulates the internal feedback voltage to the voltage on the SS pin, allowing VOUT to rise smoothly from 0V to its regulated voltage without inrush current. The soft-start time can be calculated approximately by Equation 8.

Equation 8.

Many of the common power supply sequencing methods can be implemented using the SSx and ENx pins. Figure 7-6 shows the method implementing ratio-metric sequencing by connecting the SSx pins of three buck channels together. The regulator outputs ramp up and reach regulation at the same time. When calculating the soft-start time, the pull-up current source must be tripled in Equation 8.

Figure 7-6 Ratio Metric Power Up Using SSx Pins

Simultaneous power supply sequencing can be implemented by connecting capacitor to SSx pin, shown in Figure 7-7. The capacitors can be calculated using Equation 8 and Equation 9.

Equation 9.

Figure 7-7 Simultaneous Startup Sequence Using SSx Pins