7.3.8 Soft-Start Pin

The TPS54618-Q1 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 TPS54618-Q1 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.

vertical spacer

Equation 4.

where

• Tss is the desired slow-start time in ms
• Iss is the internal slow-start charging current of 2 μA
• Vref is the internal voltage reference of 0.799 V

If, during normal operation, the VIN goes below the UVLO, the EN pin pulls below 1.2 V, or a thermal shutdown event occurs, the TPS54618-Q1 stops switching. When the VIN goes above UVLO, EN is released or pulled high, or a thermal shutdown is exited, then SS/TR is discharged to below 40 mV before reinitiating a powering-up sequence. The VSENSE voltage follows the SS/TR pin voltage with a 54-mV offset up to 85% of the internal voltage reference. When the SS/TR voltage is greater than 85% on the internal reference voltage the offset increases as the effective system reference transitions from the SS/TR voltage to the internal voltage reference.