The TPS54388C-Q1 device regulates to the lower of the SS/TR pin and the internal reference voltage. A capacitor on the SS/TR pin to ground implements a slow-start time. The TPS54388C-Q1 device has an internal pullup current source of 2 μA, which charges the external slow-start capacitor. Equation 4 calculates the required slow-start capacitor value, where t_(SS/TR) is the desired slow start time in ms, I_(SS/TR) is the internal slow start charging current of 2 μA, and V_ref is the internal voltage reference of 0.8 V.

Equation 4.

If during normal operation, VIN goes below UVLO, the EN pin goes below 1.2 V, or a thermal shutdown event occurs, the TPS54388C-Q1 device stops switching. On VIN going above UVLO, the release or pulling high of EN, or exit from a thermal shutdown, SS/TR discharges to below 60 mV before re-initiation of a power-up sequence. The VSENSE voltage follows the SS/TR pin voltage with a 50-mV offset up to 85% of the internal voltage reference. When the SS/TR voltage is greater than 85% of the internal reference voltage, the offset increases as the effective system reference transitions from the SS/TR voltage to the internal voltage reference.