LM25143-Q1: LM25143-Q1 Buck power-up sequence when it used as primary power block

Hi Xiangling,

Can you please share your schematic and application conditions so I can better help you diagnose this issue?

Additionally, is there a load attached to this circuit? If so, what is the load at?

Thank you,

Joshua Austria

Hello Joshua,

Sure, please refer to attached schematic file.

When I tested the power-up sequence, I didn't connect external load, but the board have 0.7A~1.3A load.Primary_buck_LM25143_SCH_Review_20251110.pdf

Personally, I think the SYS_3V3 output has a plateau of around 1.8V due to soft start. Could you help me analyze it?

Thanks,

Xiangling

Hi Xiangling,

I agree, I am seeing the plateau but not the drop as originally mentioned. Do you have another waveform to show the drop?

Can you also please fill out this calculator with your circuit inputs? https://www.ti.com/tool/LM5143DESIGN-CALC This should help us evaluate the design better.

Finally, are you using the EVM for this device, or are you using your own board? If you are using your own board, are you able to share these files for review too?

Thank you,

Joshua Austria

Hello Joshua,

I apologize for the misunderstanding. There is no voltage drop at the 3.3V output; there is only a plateau period before the voltage reaches 3.3V.

I don't want this plateau period. Do you know how to eliminate it? Would eliminating the plateau period be effective if a capacitor is not connected to the SS pin?

Thanks,

Xiangling

Hi Xiangling, 

Thank you for the clarification. 

I am assuming that you are configured for a two-phase single output interleaved operation for this device. 

Did you attempt to disconnect any downstream components from the output and test the waveform in this state. It almost looks like something is pulling up the output voltage on initial startup before soft start initiates. 

You can also try floating the SS pins and see if this eliminates your plateau, but you may be subjecting the device to an in-rush from the high output cap. 

Please when you get a chance, fill out the calculator as well. This will help us determine if the rest of the circuit is valid as well for your given application conditions. 

Thank you,

Joshua Austria

Hello Joshua,

1. We leave the SS open and still have a step voltage. 

2.We still try to disconnect load. Will reply to you if have any result

3.I filled the calculate sheet, please check. 

LM(2)5143-Q1 quickstart design tool - revB2.xlsx

Hi Dandan,

Can you please share the waveform when SS was left open?

Thank you,

Joshua Austria

Please see picture below, and if we leave SS open, the following bucks which are powered by LM25143 will not startup successfully. Maybe the time sequence is disturbed by SS open.

Hi Dandan,

That waveform looks odd. Do you have an EVM for this device on hand? Looking at the datasheet startup curves, there does not look to be this issue on VOUT startup. 

Would you be able to conduct an ABA swap between the two? This would help us in determining if its an application or a device level issue.

Thank you,

Joshua Austria