Temp Measurement using Temp Diode in MC33978

[tomtalkington]

The MC33978 / 34978 has a temp diode for measuring junction temperature that can be output on the AMUX.  The data sheet gives a coefficient of 8mV per degree C.  That is the only information that I can find on how to use this feature.  Is this merely a matter of applying a linear equation?  If so what is the slope intercept value?

If anyone can supply some guidance here I would greatly appreciate it.

Regards,

Tom

[TomasVaverka · Accepted solution]

Hi Tom,

I have asked our device group to comment on this, here is the answer.

The temperature sensor is 4  series diodes.

At room temp,

Vbe = ~ 0.65V (one diode only)

4*Vbe = ~ 2.6V

TC of Vbe is ~ -2mV/degC

TC of the temp sensor = 4*TC(Vbe) =  ~ -8mV/degC

Die temp is approximated by the equation of a line y = mx +b  where

y is the voltage as a function of temperature

m is -8mV/degC

x = temp

b is the voltage @ 0 degC

Y(0) = -8mV/degC x 0 degC + b  => b = 2.6V + (0-25C)(-8mV/degC) = 2.8V

So the equation is:

Vtemp = (-8mV/degC)(Temp) +2.8V

More practical is to solve for Temp = (Vtemp-2.8)/(-8mV)

I hope it helps.

Regards,

Tomas