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  • TI Thinks Resolved

TMP461: transistor selection requirement?

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Replies: 3

Views: 117

Part Number: TMP461

Hi Sir,

Do we have any limitation of selecting transistor for TMP461?

Or just choose a general transistor of 2N3904(NPN) or 2N3906 (PNP) are fine.

My question is:

If general transistor of 2N3904(NPN) or 2N3906 (PNP) are fine, why we list the 4 criteria in as below?

Could you please help to verify attached transistor are all fine for TMP461?

Thanks

2N3904_4.pdf

PN2907A.PDF

  • Hi Anne,

    Many customers ask which transistor they can use, and we cannot test them all. We have provided insight into what parameters to look for in a transistor.  Ultimately, these parameters don't appear in transistor datasheets. Even the 2N3904 that we recommend does not disclose Vbe at a current below 1mA. The PN2907A seems to be an acceptable transistor, but will need to be evaluated for temperature accuracy the same as all others. 

    thanks,

    ren

  • In reply to Ren Schackmann:

    Hi Ren,

    If so, I am curious how TI to suggest the suitable BJT that described in the datasheet? and how we know we choose right BJT? or no matter what we choose, we can calibration it to get accuracy temperature information? many thanks.

  • In reply to Crystal Chen:

    Hi Crystal,

    The 3904 NPN and 3906 PNP discrete transistors are standards in the remote temperature sensing industry. They have been recommended for a very long time, and the companies selling them have made many package variations over the years. There are also many companies selling this device, so there should be a transistor for just about everyone and every application.

    Each one of these varieties of the standard 3904/3906 is slightly different, and some temperature error can be expected. The N Factor feature of our remote temperature sensors is intended to be adjusted to accommodate for manufacturer-to-manufacturer or model-to-model variation. N Factor refers to the non-ideality of the transistor, and is expected to normally be N=1.008. When we evaluate 3904/3906 transistors from different manufacturers, we often see various N from 1.000 up to 1.008. A transistor which empirically has an N=1.000 will read about 2.2°C colder when connected to a typical remote temperature sensor at its default setting of N=1.008. A specific model of transistor will also have a smaller amount of N variation from device-to-device. This is why every remote temperature sensor customer should evaluate their chosen transistor to determine that the system accuracy achieved meets their requirements.

    Transistors which do not meet our voltage requirements for forward voltage (VBE) will not function correctly as temperature sensors when paired with a remote temperature sensor. Some transistors, typically embedded ones, have very poor Beta which makes them inaccurate temperature sensors unless this issue is specifically addressed by the remote temperature sensor (see Beta Compensation feature.) Some transistors may have an N that is outside the range of our N Factor registers, which would prevent them from being calibrated. Any variety of 3904/3906 can be calibrated using N Factor adjustments.

    thanks,

    ren

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