The soft-start feature helps the converter gradually reach the steady state operating point, thus reducing start-up stresses and surges. The device regulates the FB pin to the SS pin voltage or the internal reference, whichever is lower.
At start-up, the internal 10-μA soft-start current source (ISS) turns on 50 µs after the VCC voltage exceeds the 2.85-VCC UV threshold, or if the VCC voltage is greater than 4.5 V, whichever comes first. The soft-start current gradually increases the voltage on an external soft-start capacitor connected to the SS pin. This results in a gradual rise of the output voltage. The SS pin is pulled down to ground by an internal switch when the VCC is less than VCC UVLO threshold, the UVLO is less than the UVLO threshold, during hiccup mode off-time or thermal shutdown.
In boost topology, soft-start time (tSS) varies with the input supply voltage. The soft-start time in boost topology is calculated as shown in Equation 3.
In SEPIC topology, the soft-start time (tSS) is calculated as follows.
TI recommends choosing soft-start time long enough so that the converter can start up without going into an overcurrent state. See Section 9.3.11 for more detailed information.
Figure 9-8 shows an implementation of primary side soft start in flyback topology.
Figure 9-9 shows an implementation of secondary side soft start in flyback topology.