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Increasing temperature of LM317A

Takahiro Nishizawa
Genius 5575 points
Other Parts Discussed in Thread: LM317A

We have questions about LM317A.

We check the temperature increasing.

Vin=24V

Vout=3.3V

Iout=31.6mA

PD=(24-3.3) x 0.0316

     =0.65W

Increasing temperature = RΘjc x PD

                                      = 16.2 x 0.65

                                      = 10.53(℃)

But the increasing temperature is 28.7℃ when our customer takes measurement.

Formula and measurement result are different.

Please tell us the reason why hte formula and measurement result are different.

Is the formula correct?

Which part of 10.53℃(formula result) is the temperature?

Which parts of this IC should we measure when we want to check the increasing temperature?

Best regards,

Takahiro Nishizawa

  • 0
    TI__Mastermind 23361 points

    Hi Takahiro,

    The way of using RΘjc number to estimate the top temperature of an IC is the right way to use the number. Normally the thermal numbers on a LDO datasheet is using JEDEC standard. The actual layout of the board will actually make those numbers very different than a JEDEC boards. Having different copper size for heat dissipation and having different lay of the bouards both change the thermal nubers; having differnet number of Vias ont he copper may also change the numbers.

    Another thing that is worthing asking is how the customer measured the temperature of the top of the IC, did they use a thermal gun? Did they use a thermalcouple? Differnt measurement methods will also get a difference. 

    Normally the most important temperature of an IC is the junction temperature, as for each device, there is an absolute and recommended junction temperature; the specification of a LDO is also created using junction temperature.

    Regards,
    Jason Song