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LM73 / abnormal temperature data

Guru 29690 points
Other Parts Discussed in Thread: LM73
Hi Team,
 
My customer uses LM73 for his system and some of them are observed abnormal temperature data in steady state.
(Not the case of power supply ramp-up as datasheet page.19.)
It seems there is noise on power-supply(3.3V) as attached.
 
Is it possible case to be read abnormal temperature data if there is power-supply noise?
For example, is it caused by the noise that falls below lower limit of operating rating(2.7V)?
 
Best Regards,
Yaita / Japan disty
  • Hi Kensuke,

    Let me look into this and get back to you.

    -Michael Wong

  • Hi Michael-san,
     
    Thank you for your support.
    I'm looking forward to your reply.
     
    Best Regards,
    Yaita
  • Hi Kensuke-san,

    Yes it is possible to read abnormal temperature data when there is power supply noise. The noise may be turning off the part when it falls below 2.7V.

    Make sure there are enough bypass capacitors to remove the power supply noise.

    -Michael Wong

  • Hi Michael-san,
     
    Thank you for your support.
    I understood the power supply noise below 2.7V may be turning off LM73.
     
    By the way, could you explain the detail of following considerations?
    My customer would like to know why abnormal temperature is read by slow power-supply ramp.
    -----------------------------------------
    (datasheet  Page.19)
    POWER SUPPLY RAMP-UP CONSIDERATIONS
    In systems where there is a large amount of capacitance on the VDD node, the LM73 power supply ramp-up
    time can become excessively long. Slow power-supply ramp times may result in abnormal temperature readings.
    A linear power-on-ramp of less than 0.7V/msec and an exponential ramp with an RC time constant of more than
    1.25 msec is categorized as a slow power-supply ramp.
    -----------------------------------------
    I believe this issue is caused by inernal POR(power on reset) can't be active in this case.
    Is my understanding correct?
     
    Best Regards,
    Yaita
  • Hi Kensuke-san,

    The architecture of the internal power-on-reset works best with a ramp of 0.7V/msec or faster. To avoid potential abnormal temperature reading due to slow ramp we recommend the power up sequence described in page 19 of the datasheet.

    -Michael Wong

  • Hi Michael-san,
     
    Thank you. I understood.
     
    I'd like to ask one more.
    Could you tell me the specific value of this abnormal temparature data?
    Is this random data?
     
    Best Regards,
    Yaita
  • Michael-san,
     
    My customer would like to know the value for considering countermeasure in his system(power supply noise issue).
    Could you please give me your comment?
     
    Best Regards,
    Yaita
  • Hi Kensuke-san,

    There is no specific value for the abnormal data. It is random and varies depending on the part and ramp rate.

    To answer your second question, a bypass capacitor is needed at the VDD pin of the device to reduce the power supply noise. The scope picture attached to the original question shows a 1.34V peak to peak noise, add a capacitor that reduces this noise to 100mV. I cannot provide an exact capacitor value as it entirely depends on the system.

    -Michael Wong

  • Hi Michael-san,
     
    Sorry for many times.
    I'd also like to confirm "abnormal data" mean.
     
    When slow power-supply ramp occurs, it may be uncorrect temparature reading even if it looks like true temparature data.
    (We can't judge correct or uncorrect data from the data itself.)
    Is my understanding correct?
     
    Best Regards,
    Yaita
  • Hi Kensuke-san,

    Yes, your understanding is correct.

    -Michael Wong

  • I had the same problem! Russell Bodine

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