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LM5117: LM5117 jitter issue from the DCM to CCM

user3521495
Prodigy 60 points
Part Number: LM5117

Dear Employee

I have a question with the LM5117 application design by WEBENCH. 

Operating Conditions:

Input 24V(20V~30V)

Output 2V 12A 

This media recorded the DCM change  to CCM wave,the yellow line shaow SW and purple is IL.

We find LM5117 is unstable within Iout=1.6A~1.9A ,when up to 2A~10A or <1.6A,LM5117 switching node is stable. 

Video 

DCM Wave 

CCM Wave

DCM to CCM Wave 

In addition,the drive signal of the FET have ring,we want to know the ring's good status.

HO DRI & CURRENT @ Iout=10A

LO DRI & CURRENT @Iout=10A

Please kindly help to provide suggestion

Thank you very much 

  • 0
    TI__Mastermind 21720 points

    Hello,

    Can you please provide a schematic and the layout? From the waveforms provided it looks like the might be some shoot through occurring which can be layout related.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 60 points

    schematic and layout is here thanks a lot !

    5342.schematic.pdf1122.layout.pdf

  • 0 in reply to user3521495
    Prodigy 60 points

    we found jitter issue on output ripple with current change 

    thanks a lot 

  • 0 in reply to user3521495
    TI__Mastermind 21720 points
    Hello,

    If there is jitter right around the DCM/CCM boundary there will be a difference in the output ripple as you show above. This jitter can be caused by noise. Please add a filter to the current sense signal. (R42=50Ohm, R48 = 50Ohm and C73 = 100pF should help).

    I'm having a hard time making it out for the the layout but the R48 should be directly connected to the currents sense resistor with a trace. This will help to reduce any noise that could be causing the jitter.

    -Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 60 points

    Hello 

    R42=50Ohm, R48 = 50Ohm and C73 = 100pF not have help

    Can you provide one Demo to contrast test for find the issue. 

    Thanks a lot

     

     

     

  • 0 in reply to user3521495
    TI__Mastermind 21720 points

    Hello,

    Please check the values that are in the schematic align with the WEBENCH design. The current sense resistor is about 10 times less than what it should be, schematic reads 750uOhm the calculated value is 7mOhm.

    Also, the quickstart calculator is a good reference to check that component values are calcualted correctly.

    Thanks,

    Garrett 

  • 0 in reply to Garrett Roecker
    Prodigy 60 points
    Hello,
    Sorry，30mOhm//10mOhm=7.5mOhm.
    All value in the schematic was checked with the WEBENCH design.
    Thanks a lot
  • 0 in reply to user3521495
    Prodigy 60 points
    Hello

    Can you provide one Demo board to contrast test for find the issue.

    Thanks a lot
  • 0 in reply to user3521495
    TI__Mastermind 21720 points

    Evaluation modules can be ordered at this link.

    -Garrett