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TMP464: is transistor PMBT2369 suitable to use?

user4979989
Intellectual 2800 points
Part Number: TMP464

Hi sir,

Could you help to check if transistor PMBT2369 is suitable to use as remote temperature sensing BJT for TMP464?

Thank you.

PMBT2369.pdf

  • 0
    TI__Mastermind 27555 points

    Hi user4979989,

    This FAQ provides the important characteristics for choosing a transistor for remote temperature sensing: https://e2e.ti.com/support/sensors/f/1023/t/892887.

    The PMBT2369 is not a transistor we have tested with our part before, and that datasheet does not  provide enough over temperature information for me to say for certain whether or not this part meets all of those requirements. They may have to contact the transistor manufacturer to see if they can provide more information, or they can purchase and characterize this transistor themselves. 

    Best Regards,
    Brandon Fisher

  • 0 in reply to Brandon Fisher94
    Prodigy 10 points

    Hi Brandon Fisher,

    I'm currently finding some alternative BJTs for the temperature sensor TMP464. And I don't completely understand how to choose the BJT base on these criteria. Could I ask some questions?

    1. One of the criteria says "Base-emitter voltage is > 0.25 V at 7.5 uA". I guess the 7.5 uA is the base current, but the datasheet of BJT only shows the collector current. For example, I take the MMBT3904 datasheet which is shown in Fig. 3. So I guess you multiply 7.5 uA by h_FE which is around 100~200 then you get 0.75~1.5 mA. In this datasheet, it shows the base-emitter voltage is higher then 0.25 V at 0.75~1.5 mA. Is this statement right?

    2. This datasheet doesn't shows the base resistance. Where do I find it? I guess you can use the Fig. 3 again to get the base resistance by V_BE / I_B where I_B = I_C / h_FE. Is this statement right?

    3. I don't understand the meaning of "Tight control of V_BE indicated by small variations in h_FE (50 to 150)". Could you explain it? Maybe using an example?

    The datasheet: www.diodes.com/.../ds30036.pdf

    Thanks for your big help!

    BRs,

    Simon Heish

  • 0 in reply to Simon Hsieh1
    TI__Mastermind 27555 points

    Hi Simon,

    I should clarify that a transistor datasheet alone will not typically give you enough information to make all of those assessments. 

    Simon Hsieh1 said:
    1. One of the criteria says "Base-emitter voltage is > 0.25 V at 7.5 uA". I guess the 7.5 uA is the base current, but the datasheet of BJT only shows the collector current. For example, I take the MMBT3904 datasheet which is shown in Fig. 3. So I guess you multiply 7.5 uA by h_FE which is around 100~200 then you get 0.75~1.5 mA. In this datasheet, it shows the base-emitter voltage is higher then 0.25 V at 0.75~1.5 mA. Is this statement right?

    The collector current is actually the value we are referring to, the current values used by remote temperature sensors to take measurements are very small. It doesn't look like that datasheet goes down to single digit micro-amps for collector currents, but you could see from their chart that the base-emitter voltage is clearly lower than 0.95V when Ic is less than 120uA. The 125C line is also above 0.4V at 100uA, and proceeding downward at slightly less than 0.1/decade, so you could conclude from that it is above 0.25V at 6uA. In our case we have tested with several MMBT3904 style transistors to be sure that it works for our remote temperature sensors (including the TMP464).

    Simon Hsieh1 said:
    2. This datasheet doesn't shows the base resistance. Where do I find it? I guess you can use the Fig. 3 again to get the base resistance by V_BE / I_B where I_B = I_C / h_FE. Is this statement right?

    You're correct that this datasheet does not specify a base resistance. Base resistance typically varies a lot based on operating conditions, and can also vary based on manufacturer. Keep in mind that when calculating base resistance we want to know the value that our base emitter junction is above the saturation voltage. So you would actually calculate it something like this:

    Rb=hFE(Vb−Vbe(sat))/Ic

    Simon Hsieh1 said:
    3. I don't understand the meaning of "Tight control of V_BE indicated by small variations in h_FE (50 to 150)". Could you explain it? Maybe using an example?



    The most important part of that statement is just the small variation in hFE. These parameters are all related, and large variances in hFE mean larger variations from transistor to transistor, and make the quality of any corrections done using n-factor and offset registers worse. 

    We recommend the MMBT3904 transistor because we have tested our parts with these. Even among MMBT3904s there will be some variations between manufacturers that cause differences in characteristics such as n-factor.

    Best Regards,
    Brandon Fisher