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LM339: Temperature sensitivity

Scott Taylor
Prodigy 120 points
Part Number: LM339

We are using the LM339 as an oscillator. When a Q-tip with cold spray is touched to the top of the LM339 the oscillator frequency changes. None of the other components are being cooled. The same thing happens when the entire circuit is placed in a thermal chamber and cooled. The colder the higher the frequency. Shouldn't the resistors and capacitors dominate?

  • 0
    TI__Guru 53536 points

    Hello Scott,

    How much frequency shift are you seeing? What is your measured frequencies?

    Temperature effects everything. Most likely the following are contributing:

     Change in output swing (output low decreasing)

     Change in Propagation Delay

     Change in offset voltage.

    The resistor values are fairly low, so bias current is probably not a major issue.

    You have over 5mA at the output when the output is "Low". If you look at figure 3 Output Saturation Voltage" graph, the output "low" will be about 0.125V, then drops to 0.1V at cold. That may be enough to shift the thresholds slightly.

  • 0 in reply to Paul Grohe
    Prodigy 230 points

    Paul,

    The 23C frequency is around 75kHz. It shifts up to about 80kHz when cold. The output transistor current and Vce changes were my thoughts but wondered if there was anything else. With the amount of hysteresis I would not expect input offset changes to have much impact. In any case, I did not see anything in the datasheet indicating input offset change versus temperature.

    Scott