WEBENCH® Tools/TPS54824EVM-779: Application problem

Hi Yadong,

I'm sorry, I'm not sure I understand the question. Are you asking why safe startup into pre-biased output is needed? Or, are you looking for more details on how this feature works in this part?

Let me know and I should be able to help.

Anthony

Dear Anthony

Sorry, I did not describe my problem. I understand the function of pre-biasd. I just want to further understand that the SS pin voltage is greater than the FB pin voltage. Is the purpose to compare the relationship between input and output? Is there a formula for comparison between them?

Hi Yadong,

The error amplifier is designed such that it will compare the FB pin to either the internal reference or the SS/TRK 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.

Hope this helps explain.

Anthony 

Dear Anthony

1) I want to confirm whether during the startup process and whether the SS/TRK pin voltage is higher than the FB pin voltage means that the input voltage is greater than the output voltage?
2) Whether the low-side MOSFET participates in the switching of the high-side MOSFET during the first few cycles of startup. Because I think that in a cycle at startup, if there is a voltage in the subsequent circuit, the low-side MOSFET is allowed to sink current. If the SS/TRK pin voltage is higher than the FB pin voltage, which means that the input voltage is greater than the output voltage, the low-side MOSFET that is allowed to sink current during the first on-cycle may not be particularly large.

Excuse me, is my understanding reasonable

Hi Yadong,

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 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. Below is a link to the page with this waveform.

Let me know if more clarification is needed.

https://www.ti.com/lit/ds/symlink/tps54824.pdf#page=31

Dear Anthony

During the pre-bias test, I enabled EN and found that the Vcomp voltage was pulled high, but there was no immediate output. I want to verify whether it is because the SS/TRK pin is lower than the FB voltage, which causes the subsequent clock circuit to be disabled.