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LM340: TO-220 Maximum Average Power Dissipation above 70C (LM7812CT/NOPB)

Stephen Bernhoeft
Prodigy 70 points
Part Number: LM340

Tool/software:

We are upgrading an old design which used LM7812CT in a high reliability application. I'd like to keep with this part because it satisfies our new temperature regime which is -40 to +100C. Also we have a long history of product operation over a narrower temperature range.

WRT "SNOSBT0L –FEBRUARY 2000–REVISED SEPTEMBER 2016", LM7812CT/NOPB is the only device meeting temperature requirements and this uses a TO-220 package. Figure 2 

 (the device is specified to operate across the range -40 to +125C). I don't know whether I can rely on simply extrapolating the curves?

  • 0
    Prodigy 70 points

    I suppose I can still rely on values calculated from thermal resistance even though the plot given stops at 70C.

  • 0 in reply to Stephen Bernhoeft
    Prodigy 70 points

    However, then I see there is "thermal metric" as distinct from thermal resistance. I'd like to use the values given as
    "For the TO-220 package (NDE), θJA is 54°C/W and θJC is 4°C/W."
    rather than delve into table 6.4 which has different values. They sum to about the same (56.1C/W).

  • 0 in reply to Stephen Bernhoeft
    Prodigy 70 points

    Also note (3) omits mention of heatsinking for the TO_220 package for some reason.

    For the TO-3 package (NDS), the junction-to-ambient thermal
    resistance (θJA) is 39°C/W. When using a heat sink, θJA is the sum of the 4°C/W junction-to-case thermal resistance (θJC) of the TO-3
    package and the case-to-ambient thermal resistance of the heat sink. For the TO-220 package (NDE), θJA is 54°C/W and θJC is 4°C/W.
    If SOT-223 is used, the junction-to-ambient thermal resistance is 174°C/W and can be reduced by a heat sink (see Applications Hints on
    heat sinking).If the DDPAK\TO-263 package is used, the thermal resistance can be reduced by increasing the PCB copper area

  • 0 in reply to Stephen Bernhoeft
    TI__Genius 14565 points

    Hello Stephen,

    Could you please clarify what question you have for our applications team to solve? 

    Here are the thermal specs for this device:

     

    Did you need to know the power dissipation for your specific application?

    If so, what is the current you are drawing in the application, and what is your voltage drop?

    Please let me know.

    Best,

    Hannah