LM90

www.ti.com

SNIS126A – MAY 2004 – REVISED MARCH 2013

Figure 15. Mobile Pentium III or 3904 Temperature vs LM90 Temperature Reading

Most silicon diodes do not lend themselves well to this application. It is recommended that a 2N3904 transistor

base emitter junction be used with the collector tied to the base.

A diode connected 2N3904 approximates the junction available on a Pentium III microprocessor for temperature

measurement. Therefore, the LM90 can sense the temperature of this diode effectively.

DIODE NON-IDEALITY

Diode Non-Ideality Factor Effect on Accuracy

When a transistor is connected as a diode, the following relationship holds for variables VBE, T and If:

where

•

(2)

•

q = 1.6×10

−19 Coulombs (the electron charge),

•

T = Absolute Temperature in Kelvin

•

k = 1.38×10

−23joules/K (Boltzmann's constant),

•

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

•

IS = Saturation Current and is process dependent,

•

If= Forward Current through the base emitter junction

•

VBE = Base Emitter Voltage drop

(2)

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

(3)

In the above equation,

η and IS are dependant upon the process that was used in the fabrication of the particular

diode. By forcing two currents with a very controlled ration (N) and measuring the resulting voltage difference, it

is possible to eliminate the IS term. Solving for the forward voltage difference yields the relationship:

(4)

Copyright © 2004–2013, Texas Instruments Incorporated

Submit Documentation Feedback

21

Product Folder Links: LM90