# temperature descranpancy between the junction and ambient

1. ## temperature descranpancy between the junction and ambient

Hi,
I always see many IC product assert they meet spec on ambient temperatutre from -40'C to 85'C. Then what junction temperature we should use in designing/simulating the cicruit ?In normal, what is the descranpancy between the junction and the ambient temperature?

Thanks

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2. ## Re: temperature descranpancy between the junction and ambien

One cause of elevated junction temperature is self heating.
Any dissipation in the part results in some increase in junction temperature over ambient.
For small signal parts, there may be a high thermal impedance to the surroundings.
If this is true, tiny dissipations can make a measureable temperature rise.

You can choose from these approaches for simulation temperatures, or pick another:
1) Choose a standard temperature, such as 25 C.
2) Calculate dissipation, determine thermal impedance and then calculate temperature rise.
3) Measure a junction voltage on the divice in a real circuit and calculate the temperature from the diode equation.
4) Guess

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