Electrical Engineering - LM340 temp coefficient question

Author

LM340 temp coefficient question

Peter

2007-04-28, 8:25 pm

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?

Palindrome

2007-04-29, 3:25 am

Peter wrote:
> 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.

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.

>
>
>
> My questions:
>
> Is 0.8mV/c positive or negative from what... 25 degrees C?

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.

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

See the graph mentioned above. It depends on the value of the initial
temperature as well as the change in temperature.

>
> Also, how do I calcualte an accurate junction to case number?
>
No need, read the data sheet (more carefully, perhaps).

--
Sue

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