LM89 datasheet(25/37 Pages) TI

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Vbe = K

ln (N)

k T

IF = IS e

Vbe

KVt

Vt =

k T

IF = IS e

- 1

Vbe

KVt

LM89

www.ti.com

SNIS128D – AUGUST 2002 – REVISED JUNE 2014

Typical Application (continued)

where

(1)

where

•

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 nonideality 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 ratio (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)

The voltage seen by the LM89 also includes the IFRS voltage drop of the series resistance. The nonideality

factor,

η, is the only other parameter not accounted for and depends on the diode that is used for measurement.

Since

ΔVBE is proportional to both η and T, the variations in η cannot be distinguished from variations in

temperature. Since the nonideality factor is not controlled by the temperature sensor, it will directly add to the

inaccuracy of the sensor. For the Pentium 4 and Mobile Pentium Processor-M Intel specifies a ±0.1% variation in

η from part to part. As an example, assume a temperature sensor has an accuracy specification of ±1°C at room

temperature of 25 °C and the process used to manufacture the diode has a nonideality variation of ±0.1%. The

resulting accuracy of the temperature sensor at room temperature will be:

TACC = ± 1°C + (±0.1% of 298 °K) = ±1.4 °C

(5)

The additional inaccuracy in the temperature measurement caused by

η, can be eliminated if each temperature

sensor is calibrated with the remote diode that it will be paired with.

Processor Family

η, nonideality

MIN

TYP

MAX

Pentium III CPUID 67h

1.0065

1.0125

Pentium III CPUID 68h/PGA370Socket/Celeron

1.0057

1.008

1.0125

Pentium 4, 423 pin

0.9933

1.0045

1.0368

Pentium 4, 478 pin

0.9933

1.0045

1.0368

0.13 micron, Pentium 4

1.0011

1.0021

1.0030

MMBT3904

1.003

AMD Athlon MP model 6

1.002

1.008

1.016

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