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[FAQ] TPS54824: What is the startup process in terms of FB, SS/TRK and COMP pins?

Part Number: TPS54824

The device has been designed to prevent the low-side MOSFET from discharging a pre-biased output. During monotonic pre-biased startup, the low-side MOSFET is not allowed to sink current until the SS/TRK pin voltage is higher than the FB pin voltage and the high-side MOSFET begins to switch. The one exception is if the BOOTS voltage is below the UVLO threshold. While in BOOT-SW UVLO, the low-side MOSFET is allowed to turn on to charge the BOOT capacitor. The low-side MOSFET reverse current protection provides another layer of protection for the device after the high-side MOSFET begins to switch.

The error amplifier is designed such that it will compare the FB pin to either the internal reference or the SS/TRK (soft start/tracking) pin, whichever is lowest. During soft-start, the SS/TRK pin is lower than the internal reference so the TPS54824 will be regulating the FB pin to the SS/TRK pin voltage. When the FB pin is greater than the reference the error amplifier is using, the error amplifier will pull the COMP voltage down. When the COMP voltage is pulled down enough, the TPS54824 is prevented from switching. Once the SS/TRK pin voltage passes the FB voltage, the error amplifier will pull up on the COMP voltage and the TPS54824 will start switching.

The SS/TRK voltage will not be influenced by the input voltage. When the TPS54824 turns on, it will pull down the SS/TRK voltage so that it ramps up from near 0V.The SS/TRK pin is used to minimize inrush currents or provide power supply sequencing during power up. A small value capacitor or resistor divider should be coupled to the pin for soft start or critical power supply sequencing requirements.

The low-side MOSFET is allowed to switch as soon as the high-side FET starts switching. The first switching cycle may have a relatively short on-time and as a result the LS FET can sink a little bit of current until the next switching cycle. However the amount of current is very small and is usually not enough to pull down the output voltage. This can be seen in the pre-bias startup waveform in the datasheet, Figure 47 (