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.

A diode connected 2N3904 approximates the junction avail-

able on a Pentium III microprocessor for temperature mea-

surement. Therefore, the LM90 can sense the temperature

of this diode effectively.

4.1 DIODE NON-IDEALITY

4.1.1 Diode Non-Ideality Factor Effect on Accuracy

When a transistor is connected as a diode, the following

relationship holds for variables V

where:

•

q = 1.6x10

•

T = Absolute Temperature in Kelvin

•

k = 1.38x10

•

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

manufactured on,

•

I

•

I

•

V

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

eliminated, yielding the following equation

In the above equation,

η and I

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 I

voltage difference yields the relationship:

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

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

variation in

η from part to part. As an example, assume a

room temperature of 25 ˚C and the process used to manu-

resulting accuracy of the temperature sensor at room tem-

perature will be:

T

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.

The following table shows the variations in non-ideality for a

variety of processors.

Processor Family

η, non-ideality

min

typ

max

Pentium II

1

1.0065

1.0173

Pentium III CPUID 67h

1

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

MMBT3904

1.003

AMD Athlon MP model 6

1.002

1.008

1.016

4.1.2 Compensating for Diode Non-Ideality

In order to compensate for the errors introduced by

non-ideality, the temperature sensor is calibrated for a par-

ticular processor. National Semiconductor temperature sen-

sors are always calibrated to the typical non-ideality of a

given processor type. The LM90 is calibrated for the

non-ideality of a mobile Pentium III processor, 1.008. 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 introduced.

Figure 12 shows the minimum and

maximum errors introduced to a temperature sensor cali-

brated specifically to the typical value of the processor type

it is connected to. The errors in this figure are attributed only

to the variation in non-ideality from the typical value. In

Figure 13 is a plot of the errors that result from using a

temperature sensor calibrated for a Pentium II, the LM84,

with a typical Pentium 4 or AMD Athlon MP Model 6.

20033715

Mobile Pentium III or 3904 Temperature vs LM90

Temperature Reading

FIGURE 11.

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