TMP421 PNP transistor selsetion

Hi Patrick,

We normally recommended a specific transistor like 2N3904 (NPN) or 2N3906 (PNP) stated in TMP421 datasheet on page 24 because we did a full characterization for both transistors.

I compared the transistor attached LMBT3906LT1 to MMBT3906, and the beta, VBE(sat), VCE(sat) are similar; therefore, that shouldn’t be a problem using LMBT3906LT1; however, LMBT2907 wouldn’t be a good choice since we have never characterized this transistor before.

If customer wants to use LMBT2907, they need to characterize the transistor with TMP421 in an enclosed chamber (oilbath) to find the non-ideality factor. The n-factor and I(sat) are depended on the process for each particular transistor. Once the n-factor is found, it can be programmed in the register; the curvature can be adjusted and the offset can be compensated.

Hope this helps!

Aaron