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LM5116: Inquiry about output noise issue at low temperature

JH Shin
Guru 11090 points
Part Number: LM5116

Hello,

My customer encountered the issue of noise in the output of the LM5116 at low temperatures.

The output at room temperature is normal with no significant difference depending on the load as shown below.

4A load                                                                                               4.5A load                                                                                           5A load

   

The output at low temperatures generates unwanted ripple noise as shown below. Additionally, the size of ripple noise varies depending on the load.

4A load                                                                                              4.5A load                                                                                                5A load

   

Q1. Please review if there are any problems with the circuit they designed below.

       (They tried removing C62 to see if it was affected by an external signal, but the same symptoms occur)

6165.LM5116.pdf

Q2. Please advise on the cause of this issue and how to resolve it.

Thank you.

JH

  • 0
    TI__Guru 58851 points

    Hello JH 

    I think this issue is caused by either 1) sub-harmonic oscillation or 2) loop instability.

    1) Please reduce R39 ~ 30% and check if any improvement. 

    2) Please replace the loop compensation components with 0.1 x R37 and 10 x C67, and check if any improvement/change

    3) Please use Quick Start calculator https://www.ti.com/lit/zip/snvu051 and double check your component selections. 

    -EL  

  • 0 in reply to EricLee
    Guru 11090 points

    Hello Eric,

    Thanks for your reply.

    Below are the results of the customer's testing based on your suggestions.

    1. No change

    2. No improvement, but waveform changes

      1) 4A load

    Time div. 5.0uS                                                         Time div. 100uS

     

    2) 4.5A load

    Time div. 5.0uS                                                         Time div. 100uS

     

    3) 5A load

    Time div. 5.0uS                                                         Time div. 100uS

     

    Please provide further advice to resolve the issue.

    Thanks,

    JH

  • 0 in reply to JH Shin
    TI__Guru 58851 points

    Hello JH 

    There is an unexpectedly low duty-cycle switching (Please see the red circle below). Due to this low duty-cycle switching, the VOUT drops down.

    The waveform followed by the unexpected VOUT drop is caused by a natural operation of the converter. The converter is trying to regulated the VOUT after the unexpected VOUT drop. .    

    Please try to populate a small filter capacitor between the CS and the CSG pins. Please locate the filter capacitor close to the IC. 

    Also, please make it sure that the CSG pin is directly connected to the ground connection of the sense resistor. Please don't connect the CSG trace to other ground coppers except the ground connection of the sense resistor.  

    -EL

  • 0 in reply to EricLee
    TI__Guru 58851 points

    I am closing this thread since I have not heard from you for sometime.
    If you are still trying to resolve the issue, please feel free to open a NEW thread. Thanks
    - EL