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LM76005-Q1: Tj calculation questions

Johnny Guo
Expert 4910 points
Part Number: LM76005-Q1

Hi team,

My cusotmer would like to get details on LM76005-Q1 Tj calculation,

Power supply spec:

  • VIN: 16~36V
  • VOUT: 14V/3.5A
  • TA max: 85C

From Webench report, below is calculation result, with ICThetaJA Effective=16.0degC/W.

WBDesign455.pdf

While in datasheet, R thetaJA is also listed, which is 29.6degC/W.

Questions:

1. Can you clarify which thermal resistance can be taken in Tj calculation? Since LM76005-Q1 has thermal pad, in my understanding, R junction to board should be taken, it is 9.1degC/W.

2. For this LM76005-Q1 application, can you provide a Tj calculation result at TA=85C?

Thanks.

Johnny

  • +1
    TI__Mastermind 22299 points

    Hi Johnny,

    See the figure 10-2. The R Junction-Ambient  will vary both with power dissipation and board area.

    The R junction-board can also be used, however often the board temperature also increases which is harder to calculate.

    The webench calculation is a good starting point.

    Once you know your board size you can use figure 10-2 for a closer R junction-ambient.

    Hope this helps,

    -Orlando

  • 0 in reply to Orlando Murray
    TI__Expert 4910 points

    Hi Orlando,

    For detail power dissipation calculation, will Webench result be accurate? Since IC power dissipation is also important for Tj.

    In my mind, if we know the board temperature, Tj calculation with R junction-board will be more accurate since the majority thermal will be dissipated through thermal pad.

    Thanks.

    Johnny

  • +1 in reply to Johnny Guo
    TI__Mastermind 22299 points

    Hi Johnny, 

    The WEBENCH IC power dissipation show be fairly close yes. I have not heard of any issues with it or early thermal shutdown.

    Yes you can use the board temperature with R junction board. 

    However note the board temperature will increase as a result of sinking heat. 

    See the EVM user guide and figure 6-5:

    https://www.ti.com/lit/ug/snvu694a/snvu694a.pdf 

    Even on the EVM the board temperature increased to 35C at the edges and was 45C around the IC. 

    Hope this helps,

    -Orlando

  • 0 in reply to Orlando Murray
    TI__Expert 4910 points

    Thank you Orlando.