The general principle of soft start is to use constant pullup current (I_ss) to charge the internal or external soft start capacitor (C_ss) so as to regulate the capacitor voltage (V_ss) ramp-up with designed slope. The reference voltage (V_REF) follows V_ss until V_REF reaches the setting reference voltage. The time rumping to V_REF is soft start time (T_ss). For TPS56837, I_ss is 6 uA, V_REF is equal to 600 mV, T_ss can be set by internal or external C_ss.

Equation 1 shows Tss based on described general principle:

Figure 2-1 and Figure 2-2 show Simplis circuit and Simplis simulation results of soft start principle, respectively. In Figure 2-1, U7 voltage is clamped to 600 mV as V_REF. According to Equation 1, with 6-uA I_ss charging 22-nF C_ss to achieve 600-mV V_REF, T_ss is calculated as 2.2 ms which is the same as Figure 2-2 simulation results.

Figure 2-1 Simplis Circuit of Soft Start

Figure 2-2 Simplis Results of Soft Start