TPS40422: Temperature Sense Transistor Selection Guide

Allan Fan

Part Number: TPS40422

Hi Expert,

Customer would like to choose temperature sense transistor 2nd source of MMBT3904 which we used on the EVM. Could you please share what parameters should we consider when select the transistor?

Thanks.

Allan

over 4 years ago

+1 Matt Schurmann over 4 years ago

TI__Genius 11095 points

The TPS40422 uses a Delta-VBE temperature sensing method, and this device MMBT3904 is diode connected. The key metric you want to look for is the "ideality factor" of the temp sensing diode you use, and this device assumes it is appx 1.008.

MMBT3904 should be easy to find, there is also 2N3904, etc... these are commodity NPNs

0 cheng poli over 4 years ago

Prodigy 20 points

Hi Expert,

there is no ideality factor data from transistor vendor and what other parameter could I use to select the transistor ?

0 Allan Fan over 4 years ago in reply to cheng poli

TI__Expert 7090 points

Hi Poli,

Could you please check/request the transistor vendor for n-factor? The "ideality factor" is also known as "n-factor". There is an app note SBOA173A introducing the sensor design and error sources for your information.

Regards,

Allan

0 Matt Schurmann over 4 years ago in reply to Allan Fan

TI__Genius 11095 points

The ideality factor is a derived quantity. It is basically n in the equation here.

https://en.wikipedia.org/wiki/Shockley_diode_equation

Are you having issues to source a 2N3904 NPN for this application? I pasted below a list of similar part numbers (from a Microchip app note found online), which would also be applicable.

0 cheng poli over 4 years ago

Prodigy 20 points

Hi Expert,

Thanks a lot. and could I know what is the n-factor value will be ok in this app

(1.008+/- ?% will be ok?)

Thanks

0 Matt Schurmann over 4 years ago in reply to cheng poli

TI__Genius 11095 points

Hello,

This device assumes n=1.008. We will see a gain error in the temperature proportional to the difference. As long as we keep the n close to this value, the reported temperature can be within a few degrees of the measured value.

Approximately:

T_reported = (n_actual / 1.008)*T_actual

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TPS40422: Temperature Sense Transistor Selection Guide