LM89 datasheet(17/20 Pages) NSC

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3.0 Applications Hints (Continued)

optimized to measure the remote thermal diode of a 0.13

micron Pentium 4 or a Mobile Pentium 4 Processor-M pro-

cessor. A discrete diode can also be used to sense the

temperature of external objects or ambient air. Remember

that a discrete diode’s temperature will be affected, and often

dominated, by the temperature of its leads.

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.

An LM89 with a diode-connected 2N3904 approximates the

temperature reading of the LM89 with a Pentium 4 micropro-

cessor less 1˚C. T

2N3904 =TP4 − 1˚C

3.1 DIODE NON-IDEALITY

3.1.1 Diode Non-Ideality Factor Effect on Accuracy

When a transistor is connected as a diode, the following

relationship holds for variables V

BE, T and If:

where:

•

q = 1.6x10

−19 Coulombs (the electron charge),

•

T = Absolute Temperature in Kelvin

•

k = 1.38x10

−23joules/K (Boltzmann’s constant),

•

η is the non-ideality factor of the process the diode is

manufactured on,

•

S = Saturation Current and is process dependent,

•

f= Forward Current through the base emitter junction

•

BE = Base Emitter Voltage drop

In the active region, the -1 term is negligible and may be

eliminated, yielding the following equation

In the above equation,

η and I

S 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 I

S term. Solving for the forward voltage

difference yields the relationship:

The voltage seen by the LM89 also includes the I

FRS voltage

drop of the series resistance. The non-ideality factor,

η,is

the only other parameter not accounted for and depends on

the diode that is used for measurement. Since

∆V

BE is

proportional to both

η and T, the variations in η cannot be

distinguished from variations in temperature. Since the non-

ideality 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 non-ideality variation of

±0.1%. The resulting accuracy of the temperature sensor at

room temperature will be:

ACC = ± 1˚C+(±0.1% of 298 ˚K) = ±1.4 ˚C

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, non-ideality

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

3.1.2 Compensating for Diode Non-Ideality

In order to compensate for the errors introduced by non-

ideality, the temperature sensor is calibrated for a particular

processor. National Semiconductor temperature sensors are

always calibrated to the typical non-ideality of a given pro-

cessor type. The LM89 is calibrated for the non-ideality of a

0.13 micron, Mobile Pentium 4, 1.0021. When a temperature

sensor calibrated for a particular processor type is used with

a different processor type or a given processor type has a

non-ideality that strays from the typical, errors are intro-

duced.

Temperature errors associated with non-ideality may be re-

duced in a specific temperature range of concern through

use of the offset registers (11h and 12h).

Please send an email to hardware.monitor.team@nsc.com

requesting further information on our recommended setting

of the offset register for different processor types.

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