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Lock-up of LM92 temp sensors

We use the LM92 to report the temperature to a PIC microcontroller via I2C once per second. Sometimes, in some environments, the LM92 will go crazy and report a very high temperature, and lock in that state. For example, after working correctly reporting temperatures around 37 degrees C every second for 2 weeks, it will report a temperature such as 216.9 degrees C and then lock in that state, reporting 216.9 for all subsequent requests.  When the device is powered off and back on, normal operation resumes.

Is this a known phenomenon with spikes of too much electrical noise? We'd like to find a robust solution. Yes, the sensor has a 0.1uF cap on its 5vdc power supply right next to it. We are using the LM92 in a non-standard way (via a wire to it's own PCB), and if it would be possible for a LM92 expert to call me by telephone to discuss this, I would be happy to update this forum with the results of the discussion, but I think trying to describe everything else and answer all of your questions will be time consuming and laborious in the forum and we can make a lot more progress in a 5 minute phone call. I can be reached at 310-441-3767 after noon, Pacific time.

I think this is an important issue because it can cause critical device failures.

Thanks,

Andrew

25 Replies

  • In reply to JohnO47112:

    Hi Andy and John,

    Thank you so much for the great suggestion. I apologize for any inconvenience this may have caused you, and I really appreciate the troubles you have gone through to troubleshoot our product.


    I have brought this issue to the attention of my manager and to the designer manager about a month ago.  They suggested that I duplicate this test to find the root cause. At this moment, I’m currently designing a schematic for the LM92 evaluation board (we don’t have any more board in stock).  However, it will take a couple of months to finish the board.  After we have boards, then I will investigate this issue further and will inform you of my findings.  If you guys have a board that you can spare, then please send it to me so I can take a look at this issue sooner.  If not, then I can give you my report in a couple of months when I am done. Once again, thank you for your patience and your help to find the root cause of this issue.

    Regards,

    Aaron Heng

  • In reply to Aaron Heng:

    Aaron,

    Do not hesitate to contact me for further information on my experiences with the LM92 if you want to.

    I just have one handcrafted prototype system/board at present, so unfortunately cannot send you any useful hardware.

    Kind Regards,

    John O'Connor

  • In reply to JohnO47112:

    Update: I implemented a pure software I2C port in the PIC 18F4620 and was able to successfully read the ambient temperature every 2s. Then I breathed on the LM92 and it behaved as when used with the hardware I2C in the 18F4620 - reading climbed up a few degrees then reading went up to maximum number, all 1's, and stayed there.

    I think this helps a little in that it exonerates the PIC substantially, maybe completely, I reckon.

    John O'C

  • In reply to JohnO47112:

    Thanks lot to all of you for your posts. They helped me a lot. I was thinking to use this temperature sensor into a new project for natural gas application, where reliable operation is paramount. Now I changed my mind completely, of NOT ever using it. I will stay with a very reliable operation using a NTC resistor, LOG101 and ADC12 of MSP430. It is more expensive, but NO FAILURES.

  • In reply to Nicolae Miron:

    You have made the right decision.

  • In reply to JohnO47112:

    Aaron, are you still monitoring this thread? Or someone else at TI?  I'm still interested in an update on the LM92, it's lock up problem, and if a revision or replacement is scheduled.  As you can tell from the recent post, you would have a larger market for the part if you made it robust.

    Best

    Andrew

  • In reply to JohnO47112:

    Is there any action on TI on this, after more than 1 year?

    Can we return our LM92s?

  • In reply to Lu Chiu:

    Hi Lu,

    We are sorry you are having issues with the LM92.  We have a formal process for handling customer returns. I'm sorry but I cannot help you directly. Please contact the distributor you purchased your LM92s from for returning the parts.

    I would also like to say that we working diligently on new temp sensors. I'm not at liberty at the moment to disclose exactly what we are working on - all I can say is stay tuned. (Hopefully you will!)

    You are replying to a post that was started some time ago, so I am assuming that you are also having issues with lock up. If you would like further help to debug the issue on E2E, please describe your problem in more detail. Scope photos of the communication before the lock up and a schematic would help us understand your problem further. Not all problems have the same root cause, I always learn something new…

     Take care,

  • In reply to Emmy Denton:

    Hi Emmy,

    I have a problem with writing Thyst, Tcrit, and Tlow of LM92. I've tried to writing values to the three register, but default value can be read. I2C data scope showed right writing bit stream.

    I need your help.

    Best,

  • In reply to Shinwoo Kang:

    Hello Shinwoo Kang,

    The only thing I can think of immediately to help you is:  Check out the pullup resistors on the SCL and SDA lines of the I2C bus to the LM92. Some found that reducing these from 10k to 2k2 improved the behaviour of the LM92.

    I never got as far as programming Thyst, Tcrit and Tlow. I had problems with temperature register locking up

    and reading too high.

    Regards,

    John O'Connor