LM86 datasheet(17/21 Pages) NSC

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3.0 Application Hints (Continued)

pins, its temperature will effectively be that of the printed

circuit board lands and traces soldered to the LM86’s pins.

This presumes that the ambient air temperature is almost the

same as the surface temperature of the printed circuit board;

if the air temperature is much higher or lower than the

surface temperature, the actual temperature of the of the

LM86 die will be at an intermediate temperature between the

surface and air temperatures. Again, the primary thermal

conduction path is through the leads, so the circuit board

temperature will contribute to the die temperature much

more strongly than will the air temperature.

To measure temperature external to the LM86’s die, use a

remote diode. This diode can be located on the die of a

target IC, allowing measurement of the IC’s temperature,

independent of the LM86’s temperature. The LM86 has been

optimized to measure the remote diode of a Pentium III

processor as shown in Figure 11. 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.

A diode connected 2N3904 approximates the junction avail-

able on a Pentium III microprocessor for temperature mea-

surement. Therefore, the LM86 can sense the temperature

of this diode effectively.

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 ration

(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 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 III Intel specifies a ±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 manu-

facture the diode has a non-ideality variation of ±1%. The

resulting accuracy of the temperature sensor at room tem-

perature will be:

ACC = ± 1˚C+(±1% of 298 ˚K) = ±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.

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

variety of processors.

Processor Family

η, non-ideality

min

typ

max

Pentium II

1.0065

1.0173

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

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 LM86 is calibrated for the non-ideality of a

10130315

Mobile Pentium III or 3904 Temperature vs LM86

Temperature Reading

FIGURE 11.

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