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LM340 temp coefficient question

Discussion in 'Electrical Engineering' started by Peter, Apr 29, 2007.

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  1. Peter

    Peter Guest

    I have a question about temperature coefficients.


    I'm using an LM340 (5 volt regulator) and don't understand how much heat it
    will disapate and how much it will change over temp.


    The datasheet specs 0.8mV/c but doesn't show a junction to case spec.



    My questions:

    Is 0.8mV/c positive or negative from what... 25 degrees C?

    So if change my temperature 10 degrees C, my output voltage will be 8mv
    higher (or lower) and from what external temp?

    Also, how do I calcualte an accurate junction to case number?
     
  2. Palindrome

    Palindrome Guest

    You perhaps missed the figures on the data sheet:

    http://www.ecst.csuchico.edu/ieee/pdfs/LM78XX.pdf

    Hint:
    The junction-to-case thermal resistance (θJC) for TO-3 and TO-220
    package is 4°C/W.
    The average temperature coefficient of output voltage is just that, an
    average slope. It is only at 0.8mV/c at one temperature (from the graph
    mentioned below, at about 60C).

    Output voltage falls with increasingly positive temperature and with
    increasingly negative temperature, either side of its design temperature
    of 25C. The graph of "Output Voltage (Normalized to 1V at TJ = 25°C)"
    shows the effect.

    See the graph mentioned above. It depends on the value of the initial
    temperature as well as the change in temperature.
    No need, read the data sheet (more carefully, perhaps).
     
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