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LM317A: Current Limit

Y.Ottey
Mastermind 6145 points
Part Number: LM317A

Dear all

I would like to ask a question about current limit.

The current limit is set as shown in the table above. Please tell us why the difference between TO-220, TO-252, and SOT-223 appears.

I thought that it was different due to the problem of heat capacity and checked the specifications in the table below, but when I look at this,

I can not understand why the Current Limit in SOT-223 is larger than that in TO-252 because RθJA value is almost the same between SOT-223 and TO-252 .

Best Regards,

Y.Ottey

  • 0
    Mastermind 6145 points

    Dear all,

    When checking the data sheet again, the value of ψ in the table below is smaller for SOT223 than for TO-252. I think this is the reason.

    Is there anything else you can think of as a reason?

    Regards,

    Y.Ottey

  • 0 in reply to Y.Ottey
    TI__Mastermind 37015 points

    Hi Y.Ottey,

    This is an interesting question.
    I believe the answer also lies with the ψ parameters. 
    The junction to board characterization parameter is much better for the SOT-223 than the TO-252 package.
    Most of the heat is dissipated through the board, so a lower thermal impedance from the junction to the board will allow much larger current through the device.

    Please review the following app note if you have further questions.

    http://www.ti.com/lit/an/spra953c/spra953c.pdf

    Thanks,

    - Stephen