LM99 datasheet(18/20 Pages) NSC

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3.0 Application Hints

The LM99 can be applied easily in the same way as other

integrated-circuit temperature sensors, and its remote diode

sensing capability allows it to be used in new ways as well.

It can be soldered to a printed circuit board, and because the

path of best thermal conductivity is between the die and the

pins, its temperature will effectively be that of the printed

circuit board lands and traces soldered to the LM99’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

LM99 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 LM99’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 LM99’s temperature. The LM99 has been

optimized to measure the NVIDIA® GeForce™FX family

thermal diode. Remember that a discrete diode’s tempera-

ture will be affected, and often dominated, by the tempera-

ture of its leads.

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

−23 joules/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 LM99 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. 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 tempera-

ture 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.

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 LM99 is calibrated for the non-ideality of

the NVIDIA GeForceFX family thermal diode. When a tem-

perature 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.

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). See Offset Register

table below.

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.

Offset Register Settings for Specific Devices

Processor Family

Offset Register Settings

∆T, ˚C

NVIDIA GeForceFX Graphics Processor

default

Intel® Pentium® 4 Processor

+2.625

0000 0010

1010 0000

Intel Pentium 3 Processor

+2.375

0000 0010

0110 0000

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