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TMP423: use with 2N3904 and 2N3906

hui zhang
Intellectual 280 points
Part Number: TMP423

Hi,expect.

When i use TMP423（DXP1 connect with 2N3904 NPN and DXP2 connect with 2N3906 PNP)，and put them in a constant environment such as 25℃,the temp reading from Remote Temp1 and Remote Remp2 will be about 2℃ different。

How can i avoid this 2℃ difference? and which one is more accurate when measure the real temperature?

  • 0
    TI__Mastermind 35835 points
    Hi Hui,

    There can be significant variation in N-Factor from one transistor vendor to another. This is why TI cannot guarantee accuracy for a specific transistor. We guarantee accuracy for a transistor with an ideal N-Factor of 1.008.

    You will need to use another temperature reference to adjust the N-Factor register (0x21-23) in TMP423. You can read more about the N-Factor correction register on page 16 of the datasheet.

    Thanks,
    Ren
  • 0 in reply to Ren Schackmann
    Intellectual 280 points

    Hi,expect.

    Does it means, if i use another temperature reference, for example @25℃, then adjust the N-Factor register(0x21-23) to make CH1 with 2N3904 and CH2 with 2N3906 to shown 25℃, I will get accurate value during the full temperature range(-40℃ to 85℃) , right ?

    I can understand if N-Factor and △VBE is the only error source. But different transistor vendor have different VBE value, does VBE have no influence on temperature reading, and how can it canceled by N-Factor register(0x21-0x23)?

    Thanks!  

  • 0 in reply to hui zhang
    TI__Mastermind 35835 points
    Hi Hui,

    Yes, this is the intention.

    N-Factor is the last parameter in the equation which isn't solved by using multiple currents. The temp sensor automatically alternates currents and solves for temperature. N has an over-temperature effect, so the only way to be sure you're only tuning the device-dependent N is to check it at different temperatures. However, viewing it as an offset at one temperature is adequate for many customers. en.wikipedia.org/.../Silicon_bandgap_temperature_sensor

    Thanks,
    Ren
  • 0 in reply to Ren Schackmann
    Intellectual 280 points

    Hi,expect.

    Another question, if customer use a diode such as 1N4148 but not BJT 2N3904 & 2N3906, Is this method (adjust N-Factor at one temperature) also effective?

    Thanks.