3.0 Applications Hints

The LM95221 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 be-

cause 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

LM95221’s pins. This presumes that the ambient air tem-

perature 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 tempera-

ture of the LM95221 die will be at an intermediate tempera-

ture 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 tempera-

ture much more strongly than will the air temperature.

To measure temperature external to the LM95221’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 LM95221’s temperature. The LM95221

has been optimized to measure the remote thermal diode

with a non-ideality of 1.008 and a series resistance of 2.7

Ω.

The thermal diode on the Pentium 4 processor on the 90 nm

process has a typical non-ideality of 1.011 and a typical

series resistance of 3.33

Ω. Therefore, when measuring this

thermal diode with the LM95221 a typical offset of +1.5˚C will

be observed. This offset can be compensated for easily by

subracting 1.5˚C from the LM95221’s readings. 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.

When measuring a diode-connected 2N3904, with an

LM95221, an offset of -3.25˚C will be observed. This offset

can simply be added to the LM95221’s reading: T

T

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

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 ratio (N)

and measuring the resulting voltage difference, it is possible

to eliminate the I

difference yields the relationship:

The voltage seen by the LM95221 also includes the I

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

η, is the only other parameter not accounted for and de-

pends on the diode that is used for measurement. Since

∆V

η and T, the variations in η

cannot be distinguished from variations in temperature.

Since the non-ideality factor is not controlled by the tempera-

ture sensor, it will directly add to the inaccuracy of the

sensor. For the Pentium 4 and Mobile Pentium Processor-M

η from part to part. As an

example, assume a temperature sensor has an accuracy

process used to manufacture the diode has a non-ideality

ture sensor at room temperature 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.

Processor Family

η, non-ideality

Series

R

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

Pentium 4 on 0.13

micron process,

2-3.06GHz

1.0011 1.0021 1.0030 3.64

Ω

Pentium 4 on 90 nm

process

1.011

3.33

Ω

Pentium M Processor

(Centrino)

1.00151 1.00220 1.00289 3.06

Ω

MMBT3904

1.003

AMD Athlon MP model

6

1.002

1.008

1.016

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