Datasheet

x '

T =

q x 'V

x k x ln

LM95245

www.ti.com

SNIS148G –OCTOBER 2007–REVISED MARCH 2013

• q = 1.6×10

−19

Coulombs (the electron charge),

• T = Absolute Temperature in Kelvin

• k = 1.38×10

−23

joules/K (Boltzmann's constant),

• η is the non-ideality factor of the process the diode is manufactured on,

• I

= Saturation Current and is process dependent,

• I

= Forward Current through the base-emitter junction

• V

= Base-Emitter Voltage drop (1)

In the active region, the -1 term is negligible and may be eliminated, yielding the following equation

(2)

In Equation 2, η and I

are dependant upon the process that was used in the fabrication of the particular diode.

By forcing two currents with a very controlled ratio(I

/ I

) and measuring the resulting voltage difference, it is

possible to eliminate the I

term. Solving for the forward voltage difference yields the relationship:

(3)

Solving Equation 3 for temperature yields:

(4)

Equation 4 holds true when a diode connected transistor such as the MMBT3904 is used. When this “diode”

equation is applied to an integrated diode such as a processor transistor with its collector tied to GND as shown

in Figure 19 it will yield a wide non-ideality spread. This wide non-ideality spread is not due to true process

variation but due to the fact that Equation 4 is an approximation.

TruTherm technology uses the transistor (BJT) equation, Equation 5, which is a more accurate representation of

the topology of the thermal diode found in an FPGA or processor.

(5)

TruTherm should only be enabled when measuring the temperature of a transistor integrated as shown in the

processor of Figure 19, because Equation 5 only applies to this topology.

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