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Original question:

TMP709: TMP709 did not sink low for temperature higher than threshold temp

TMP709: TMP709 not working for temps under 60 C ?

Nico teWinkel
Prodigy 160 points
Part Number: TMP709
Other Parts Discussed in Thread: TMP392, TMP390,

Hello,

I am having the same issue as was asked about in this earlier thread:

https://e2e.ti.com/support/sensors-group/sensors/f/sensors-forum/1033587/tmp709-tmp709-did-not-sink-low-for-temperature-higher-than-threshold-temp

The OP was having trouble getting the TMP709 to switch for a 30 degree C setting.

In one of the answers, Ren points out:
"...TMP709 is not specified for temperature accuracy in the range you're using it. It is specified for 60 to 100C..."

And the OP switched to a TMP392 to solve the issue.

I'm confused, as the spec sheet clearly indicates, on page 8, that the TMP709 supports from 0 C to 125 C: "The TMP709 temperature threshold is programmable from 0°C to 125°C"
Page 6 of the spec sheet shows graphs from 0 to 120 for Rset values and Temperature errors.
Page 1 states the 60 to 100C mentioned by Ren, but from what I understand, that's for the +/- 3C accuracy.

What am I missing? I thought I had read the spec sheet correctly for my specific application.

I'm trying to keep a hummingbird feeder from freezing, so my target set point is about 10 or 15 C. I'm testing at 25 C and my workbench is around 18C, so the heat gun should make it step past the trip point. My circuit is pretty much exactly as the example on page 1 in the spec sheet (hyst to gnd, = 2 C), except instead of the 60 C Rset, I'm using Rset that aims at 25 C (I think it's about 80k). And I'm simply testing the value at OT with a DMM (no microprocessor). I switched the 470k resistor with 10k as is recommended later in the spec sheet, and I'm missing the 0.1 uF capacitor.

This is the spec sheet I'm looking at: https://www.ti.com/lit/ds/symlink/tmp709.pdf

Given that TI recommends using the "newer version [of TMP709] available" TMP390, and that this worked for the OP, I do plan to try those next, but I'm hoping someone can shed some light on where or whether I'm reading the specs wrong, or if the specs themselves are wrong or incomplete?

Thanks,
-Nico

  • +1
    TI__Mastermind 35895 points

    Hi Nico,

    If you're just building one bird feeder, you can tune your set resistor to get the result that you desire with TMP709. If you are going to mass produce bird feeders, you will see a lot of set point variation across TMP709s configured for such a low temperature. The spec in question is the +/-3C max in the EC Table. Since this +/-3C max only applies to the 60 to 100C range, this implies set point accuracy will be worse than 3C below 60C. 

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 160 points

    Hi Ren,

    Thanks for the quick reply, and for the extra clarification and advice Slight smile

    You've helped me see the "small print" I missed: the "brochure" says 0 to 125, but the errors are so bad outside of the 60-100 range that it's not reliably useable outside of that.

    It's just a hobby project for me - I was planning to build 2 feeder heaters, so I can mess around with it a bit to get a workable result. I'd like to eventually share the results for others to implement, so at some point I'll pick up a more suitable chip.

    Thanks,
    -Nico