TPS56837 can support both internal and external soft start. Leaving the SS pin floating leads to default internal soft start time. Connecting an external capacitor between SS and AGND leads to external soft start time based on Equation 1.

TPS56837 has a 18-nF internal capacitor C_ss inside the device, corresponding to T_ss of 1.8 ms with an internal pullup current I_ss of 6 uA. So, TPS56837 can allow the SS pin floating which can save one soft start capacitor and be easy for layout.

If an external capacitor C_ss between SS and AGND is connected, the internal pullup current charges both the internal and external capacitor, TPS56837 tracks the lower one between the internal soft start voltage and the external soft start voltage. In other words, T_ss follows the slower one. When the external capacitor C_ss is smaller than 18 nF, T_ss is the default internal soft start time, 1.8 ms. If only the external capacitor C_ss is greater than 18 nF, the soft-start time is adjusted to longer T_ss based on Equation 1.

Figure 2-3, Figure 2-4 and Figure 2-5 show the T_ss measurement results on TPS56837 EVM board with internal C_ss, external C_ss = 1 nF, and external C_ss = 47 nF, respectively. In Figure 2-3, when leaving the SS pin floating, the soft start time is approximately equal to default internal soft start time, 1.8 ms. Figure 2-4 shows the T_ss when connecting an external 1-nF capacitor between SS and AGND. The soft start time is still approximately equal to default internal soft start time, 1.8 ms. In Figure 2-5, an external 47-nF capacitor is connected to the SS pin. The soft start time is 4.7 ms, which equals the calculated results using Equation 1.

Figure 2-3 Soft Start Time with Internal C_ss

Figure 2-4 Soft Start Time with External C_ss = 1 nF

Figure 2-5 Soft Start Time with External C_ss = 47 nF