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TMP235: Junction temp and temperature coefficient

Yue TANG
Expert 8485 points
Part Number: TMP235
Other Parts Discussed in Thread: TMP236

Hi,

My customer had two questions about TMP235:

1. According to TMP235 datasheet, DCK(S70) package has small size but higher RθJA, not sure if this will influence the temperature accuracy

? and does this mean small size  has lower accuracy ?

2.TMP235 has a 10mV/℃ temperature coefficient, but TMP236 is 19.5.  does this mean TMP236 is more suitable for ADC sampling due to a better recognition. means easy to be identify by ADC.

  • 0
    TI__Guru**** 244380 points
    Hello Yue,

    The RθJA is used to calculate the temperature of the die w.r.t the ambient for every uW of power dissipation. Generally when the power consumption of the device is low, it means that the temperature rise due to power dissipation will small that ambient temperature reading will not be affected beyond the resolution provided

    As for TMP235 v/s TMP236, yes it is correct that 19.5 mV/C would be much better resolved by a "not so good" ADC. However this will be at the expense of the working temperature range. For the TMP235 the range is -40 to 125C while for TMP236 it is -10 to 125C, besides other parameters like accuracy.
  • 0 in reply to Amit Ashara
    TI__Expert 8485 points
    Hello Amit
    Thanks for the reply. it's helpful.

    one more question: the max Iout could reach to 30mA according to the datasheet, will this affect the junction temperature? thanks.
  • 0 in reply to Yue TANG
    TI__Guru**** 244380 points
    Hello Yue,

    If due to any reason the max current sinked to the device is reached, of course there shall be heating in the device which shall cause a shift of the junction temperature.
  • 0 in reply to Amit Ashara
    TI__Expert 8485 points

    Amit,               

    Please see Datasheet: 6.1 Absolute Maximum Ratings-> Output current -> min -30mA, max +30mA .  What this 30mA stand for?   

    Under what condition will it reach 30mA?  When the Vout reach the max[for example 2v], what is the current? My customer had a bit confuse about this parameter. 

    if using this 30mA to calculate the heat, obviously, it will seriously affect the sensor.  thanks.  

  • 0 in reply to Yue TANG
    TI__Guru**** 244380 points
    Hello Yue,

    It depends on the load. If the load draws excessive current, it would mean a poor system design where the sink is loading the temp sensor when it should not. This should not be the case for most ADCs.
  • 0 in reply to Amit Ashara
    TI__Expert 8485 points
    Amit,
    Thanks. What is a typical current in a good design? can you recommend a ref design to show this? I can recommend a total solution to my customer.
  • 0 in reply to Yue TANG
    TI__Guru**** 244380 points
    Hello Yue,

    That is pure speculation at this point at it depends on the load circuit and the ADC pin current. You can have a look at the reference design for a typical circuit.

    www.ti.com/.../tida-010019
  • 0 in reply to Amit Ashara
    TI__Expert 8485 points
    Amit,
    Let's assume If the Iout current of TMP235 in a design reach to 500uA, Can I think this could cause junction temperature rise with following formula: 500*3.6*278 =0.45°? From the design you recommended, the current for TMP235 is approximately 500 μA. thanks.
  • 0 in reply to Yue TANG
    TI__Guru**** 244380 points
    Hello Yue,

    Yes, that would be a correct analysis. However this completely depends on what type of load is used. An input buffer with a low input current may be used if the temperature rise is an issue.