LM89CIM NSC [National Semiconductor], LM89CIM Datasheet - Page 17

• Manufacturer Part Number: LM89CIM
• Manufacturer: NSC [National Semiconductor]

3.0 Applications Hints
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
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:
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 LM89 also includes the I
drop of the series resistance. The non-ideality factor, η, is
the only other parameter not accounted for and depends on
• q = 1.6x10
• T = Absolute Temperature in Kelvin
• k = 1.38x10
• η is the non-ideality factor of the process the diode is
• I
• I
• V
manufactured on,
S
f
= Forward Current through the base emitter junction
BE
= Saturation Current and is process dependent,
= Base Emitter Voltage drop
−19
−23
Coulombs (the electron charge),
S
2N3904
joules/K (Boltzmann’s constant),
term. Solving for the forward voltage
= T
P4
− 1˚C
S
BE
are dependant upon the
, T and I
(Continued)
f
:
F
R
S
voltage
17
the diode that is used for measurement. Since ∆V
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
assume a temperature sensor has an accuracy specification
of
to manufacture the diode has a non-ideality variation of
room temperature will be:
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 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
requesting further information on our recommended setting
of the offset register for different processor types.
±
±
Processor Family
Pentium III CPUID 67h
Pentium III CPUID
68h/PGA370Socket/Celeron
Pentium 4, 423 pin
Pentium 4, 478 pin
0.13 micron, Pentium 4
MMBT3904
AMD Athlon MP model 6
0.1% variation in η from part to part. As an example,
0.1%. The resulting accuracy of the temperature sensor at
±
1˚C at room temperature of 25 ˚C and the process used
T
ACC
=
±
1˚C + (
±
0.1% of 298 ˚K) =
1.0057
0.9933
0.9933
1.0011
1.002
min
1
η, non-ideality
1.0065
1.0045
1.0045
1.0021
1.008
1.003
1.008
typ
±
1.4 ˚C
www.national.com
@
1.0125
1.0125
1.0368
1.0368
1.0030
nsc.com
1.016
max
BE
is