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TMP435: transistor selection guideline

Di Wang
Expert 1740 points
Part Number: TMP435

Hi,

I'm looking into TMP435 datasheet. Between GND Collector-Connected and Diode-Connected, which type is more accurate? 

I've to know the beta value of the transistor, and input into the register for beta compensation? Is there any other parameter of the transistor that I need to check?

Thanks.

  • 0
    TI__Expert 6475 points
    Hi Di,

    I'm looking into this.

    Kelvin
  • 0
    TI__Expert 6475 points
    Our device works with both transistor-connected and diode-connected configurations so no accuracy trade-offs there.

    From Figure 11:

    "Diode-connected configuration provides better settling time. Transistor-connected configuration provides better series resistance cancellation. NPN transistors must be diode-connected. PNP transistors can either be transistor or diode-connected.
    TI recommends this layout for the MMBT3906LP and MMBT3904LP devices."

    TMP435 has automatic beta compensation so you don't have to know the exact beta value. However, you will have to decide whether to turn the beta compensation on or not.

    Here are some notes on beta correction from the datasheet:

    Beta correction configuration set to '1000' and sensor is GND collector-connected (PNP collector to ground).
    Beta correction configuration set to '0111' or sensor is diode-connected (base shorted to collector).
    If beta correction is disabled ('0111'), then up to 1kΩ of series line resistance is cancelled; if beta correction is enabled ('1xxx'), up to
    300Ω is cancelled.

    If your transistor I-V characteristics are similar to the TI recommended MMBT3906LP or MMBT3904LP devices you will be fine. If not, you will have to find out the actual n-factor value and use the correction register to compensate for n-factor mismatch. N-factor corrects for the gain difference.

    -Kelvin
  • 0 in reply to Kelvin Le THS
    Expert 1740 points
    Thank you, Kevin.
    Could you explain what N-factor is? I didn't find the definition in MMBT3906LP datasheet.
  • 0 in reply to Di Wang
    TI__Expert 6475 points
    Di,

    N-factor is the ideality of the transistor.

    You can learn more about remote sensors here:

    www.ti.com/.../sboa173.pdf

    An ideal transistor has an N-factor of 1. Typically, our sensors are configured to 1.008 or 1.003.

    N-factor acts like a gain adjuster of the I-V curve.

    Kelvin