SLVSDV8 July 2017 TPS54424
The TPS54424 regulates to the SS/TRK pin while its voltage is lower than the internal reference to implement soft start. A capacitor on the SS/TRK pin to ground sets the soft start time. The SS/TRK pin has an internal pull-up current source of 5 μA that charges the external soft start capacitor. Equation 4 calculates the required soft start capacitor value. The FB voltage will follow the SS/TRK pin voltage with a 25 mV offset up to 90% of the internal voltage reference. When the SS/TRK voltage is greater than 90% of the internal reference voltage the offset increases as the effective system reference transitions from the SS/TRK voltage to the internal voltage reference.
If during normal operation, VIN goes below the UVLO, EN pin pulled below 1.15 V, or a thermal shutdown event occurs, the TPS54424 stops switching and the SS/TRK pin floats. When the VIN goes above UVLO, EN goes above 1.20 V, or a thermal shutdown is exited, the SS/TRK pin is discharged to near ground before reinitiating a powering up sequence.
When the COMP pin voltage is clamped by the maximum COMP clamp in an overload condition the SS/TRK pin is discharged to near the FB voltage. When the overload condition is removed, the soft-start circuit controls the recovery from the fault output level to the nominal output regulation voltage. At the beginning of recovery a spike in the output voltage may occur while the COMP voltage transitions from the maximum clamp to the value determined by the loop.