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LM25143-Q1: LM25143-Q1 loop circuit issue and output capacitance

Part Number: LM25143-Q1
Other Parts Discussed in Thread: LM25143

Tool/software:

Hi team,

When designing the LM25143 external circuit, our customers have chosen very large output capacitance value, including two electrolytic capacitors, and the loop circuit is unstable, the dynamic response does not meet requirements. 

I adjusted the loop design on this basis, and when I tested with the loop analyzer, I found that when C53 (& Ccomp1) = 22nF, C47 (& Chf1) = 15pF R29 (& Rcomp1) = 20k, the loop is stable, and the dynamic response meets the requirements and the input / output current / voltage meets the requirements.

Could you please tell me that what is the risk for raising the capacitance value of C53 (& Ccomp1) to 22nF? And why the capacitance of the C53 (& Ccomp1) increases to 22nF can stabilize the loop circuit?

In addition, could you please tell me that what the specific effect of the large output capacitance on the loop circuit is and why?

Thank you!

  • Hello

    I assume that with the larger values of output capacitance you would need to adjust the compensation components.

    It appears that you have found a stable design.  You may want to further adjust your compensation to improve the phase margin.

    Thanks

  • Hi Shelly,

    You can use the LM(2)5143 quickstart calculator to assist with compensation design: https://www.ti.com/tool/LM5143DESIGN-CALC. Just ensure that the value for ceramic Cout is derated for voltage 

    Ccomp controls the compensator zero location, so a higher value increases the phase lead contribution at crossover. However, setting this cap too high results in a slow transient response settling time. We usually target a 25-50us Rcomp*Ccomp time constant for this reason.

    Regards,

    Tim