8.4.5 Prebiased Output Start-up

The TPS543B20 device prevent current from being discharged from the output during start-up, when a pre-biased output condition exists. No SW pulses occur until the internal soft-start voltage rises above the error amplifier input voltage, if the output is pre-biased. As soon as the soft-start voltage exceeds the error amplifier input, and SW pulses start, the device limits synchronous rectification after each SW pulse with a narrow on-time. The low-side MOSFET on-time slowly increases on a cycle-by-cycle basis until 128 pulses have been generated and the synchronous rectifier runs fully complementary to the high-side MOSFET. This approach prevents the sinking of current from a pre-biased output, and ensures the output voltage start-up and ramp-to regulation sequences are smooth and monotonic.