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Original question:

LMT70: thermal information

LMT70: LMT70 applying to a pipe surface

Timur Kharrasov
Prodigy 40 points
Part Number: LMT70
Other Parts Discussed in Thread: CC1310, LMT01

Hello, we are looking for a highly accurate (preferably as accurate as LMT70) and relatively cheap, SMD temperature sensor in order to apply it for the measurement of the cold water pipe surface temperature. Our initial idea was to use a SMD sensor that would have a contact with a pipe surface with its sensing side over a thermal pad whereas the other side would be soldered to the PCB that wouldn't have a direct contact with a pipe surface. Now when we discovered the LMT70 our understanding is that its recommended to sense the temperature over the PCB surface (through vias?) since the "balls" that are soldered to the PCB surface are more conductive than the other side of the LMT70. Did we understand that correct?

Given our application requirements, is LMT70 suitable for the pipe surface temperature measurement or could you please recommend any other sensor?

Thank you in advance.

Best,
Timur 

  • 0
    TI__Expert 6475 points
    Timur,

    Could you provide more specific requirements:

    What is the system accuracy do you need?
    What is the operating temperature range?
    What ADC do you have?
    Have you consider our digital temperature sensors?

    Here are a few app notes you might find useful for thermal considerations:

    www.ti.com/.../snia021a.pdf

    www.ti.com/.../snoa967.pdf

    -Kelvin
  • 0 in reply to Kelvin Le THS
    Prodigy 40 points
    Hi Kelvin,

    - System accuracy: 0.5 oC o better
    - Operating temperature range: -10 to +40 oC
    - 12 bit ADC in cc1310
    - Yes, we considered digital sensors. They are generally more expensive and power-hungry as analogue ones.

    Thank you.

    Timur
  • 0 in reply to Timur Kharrasov
    TI__Genius 11750 points
    Hi Timur,

    Can you specify your power consumption needs for the temperature sensors? We can use this information to verify if any digital sensors fit your application, or if we should recommend an analog sensor.

    Best Regards,
    Matt
  • 0 in reply to Matthew Sauceda
    Prodigy 40 points

    Hi Matthew.

    Our application is a battery powered sensor and we are attempting to aim for a 5-10 year battery life with 2 AA cells, we can go lower, but of course, maximizing this is better. We haven't calculated any strict power requirements.  

    We have a higher data rate (every 10 sec), but the hardware requirements are very similar to the cc1310 heat cost allocator example provided in resources. And as the sensors accuracy and matching is quite good, we decided to use the same one. 

    Isn't LMT70 a right choice for us? 

    Best,
    Timur 

  • 0 in reply to Timur Kharrasov
    TI__Expert 6475 points
    Timur,

    I did some digging on the CC1310 and it doesn't look like they have enough specifications on the voltage reference (pg 21). Your voltage reference tolerance error will probably be your biggest source of system error here. LMT70 won't be your bottleneck in this case to achieve 0.5C but it's the ADC+Vref. In addition, you will probably need to calibrate your ADC too. You might want to consider LMT01.

    -Kelvin
  • 0 in reply to Kelvin Le THS
    Prodigy 40 points

    Hi Kelvin, thanks for your reply. It seems like both of the sensors LMT70 and LMT01 would perform the same on the CC1310 chip. Isn't that correct? Replacing CC1310 at the moment does not look like an option due to time constraints. 

    Could you please tell us if the LMT70 is suitable for the application I described above, i.e. for the temperature measurement of the mains water pipe surface? And if the mounting of the sensor in that case should be done on the other side of pcb than the PCB side touching the pipe? Please have a look at my original post above for details. Thanks a lot.

    Best,
    Timur 

  • 0 in reply to Timur Kharrasov
    TI__Genius 11750 points
    Hi Timur,

    Kelvin is currently out of office, but I can forward this information to the correct channels and will get back to you once I receive the information.

    Best Regards,
    Matt
  • 0 in reply to Timur Kharrasov
    TI__Expert 6475 points

    Timur,

    The main challenge I was pointing to is the CC1310 ADC specs.  LMT70 + good ADC should be fine but the problem is the ADC that you have.  The LMT01 has a digital interface (pulse count) and doesn't depend on ADC specs.  That should give you a much better system accuracy.

    LMT70 + good ADC is fine but in the CC1310 the system accuracy won't be 0.5C.

    Please look at the documents I sent you earlier for mounting recommendation.  The rule-of-thumb is just imagine which thermal path has the least resistance and that is how you should approach layout.  The via has good conductivity but you will probably need a good metal plane for best conduction.  Again please refer to the documents I sent for a detailed description of thermal considerations.

    -Kelvin