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WEBENCH® Tools/LM3481: LM3481

Jeff Groves
Prodigy 60 points
Part Number: LM3481

Tool/software: WEBENCH® Design Tools

Please could you explain the parameter "Switch Current Limit (Typ) (A)",  value 10A for the LM3481.

I can understand the external components FET, Inductor, Diode or caps having a switching current limit but the controller chip?

  • 0
    TI__Mastermind 21720 points
    Hi Jeff,

    The limit is just a rough number of what the maximum switch current might be in an application. That being said you are correct. The external components can be change to tailor the current limit as needed.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 60 points

    Hi Garret,

    Thanks for your reply.

    Back in September we did a webench design based on the LM3481.

    4-6V input 12V output at 5A.

    The output capacitors get hotter than expected, 60 degC at 3A continuous output.

    Unfortunately cannot submit the design to webench because of the maximum switch current parameter of the LM3481.

    I have attached a pdf of the original design.

    Have you any suggestions?

    Thanks,

    Martin

    webench_design_1584118_20_927153386.pdf

  • 0 in reply to Garrett Roecker
    Prodigy 60 points
    Hi Garret,
    Thanks for your reply.
    Back in September we did a webench design based on the LM3481.
    4-6V input 12V output at 5A.
    The output capacitors get hotter than expected, 60 degC at 3A continuous output.
    Unfortunately cannot submit the design to webench because of the maximum switch current parameter of the LM3481.
    I have attached a pdf of the original design.
    Have you any suggestions?
    Thanks,
    Jeff

    webench_design_1584118_20_927153386.pdf
  • 0 in reply to Jeff Groves
    TI__Mastermind 21720 points
    Hi Jeff,

    So only the output capacitor was getting hot? I would suggest checking to make sure that the RMS current rating is large enough for the electrolytic capacitors. It's also good to add some ceramic capacitors to the output capacitor bank. The ceramic capacitors have a low ESR value and should help with the heating issue.

    Thanks,
    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 60 points
    Hi Garrett,

    We are using 2 off Panasonic 20SVPF560M caps on the output.
    Each has a rating of 5.4A RMS ripple current.
    The webench design tool said 8.309A RMS ripple current.

    We have added 4 off 0.1uF X7R ceramic caps on the output close to the diode and the aluminium caps get to about 54 deg C after 15mins, 4V in, 12V out at 3A.

    54 deg C seems hot?

    Can you suggest a way forward?

    Thanks,

    Jeff
  • 0 in reply to Jeff Groves
    Prodigy 60 points
    Hi Garrett,

    Is the ripple current quoted in the webench design report for Cout combined or for each physical capacitor?
    We assumed that as two capacitors were specified to deal with 8.3A of ripple current with some margin, 4.15A each with a rating of 5.4A RMS

    Thanks again,

    Jeff
  • 0 in reply to Jeff Groves
    TI__Mastermind 21720 points

    Hey Jeff,

    The RMS current quoted by WEBENCH is the total for all the capacitors. However it is important to remember that RMS current is only one parameter in selecting the proper output capacitors. These include ripple current specifications, ESR across frequency layout and so on. Adding ceramic capacitors lows the ESR across frequency and are capable of absorbing large amount of ripple current. This will help to reduce the ripple current that the electorylic capacitors see and help keep them cool. Layout can also make a difference. If there is not a lot of copper around the capacitor it will not be able to dissipate heat easily.

    thanks,

    Garrett