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LM95231BIMM Datasheet(PDF) 18 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM95231BIMM
Description  Precision Dual Remote Diode Temperature Sensor with SMBus Interface and TruTherm Technology
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM95231BIMM Datasheet(HTML) 18 Page - National Semiconductor (TI)

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3.0 Applications Hints (Continued)
TruTherm should only be enabled when measuring the tem-
perature of a transistor integrated as shown in the processor
of Figure 3, because Equation (5) only applies to this topol-
ogy.
3.1.2 Calculating Total System Accuracy
The voltage seen by the LM95231 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 de-
pends 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 tempera-
ture sensor, it will directly add to the inaccuracy of the
sensor. For the Pentium 4 processor on 90nm process, Intel
specifies a +1.19%/−0.27% variation in
η from part to part
when the processor diode is measured by a circuit that
assumes diode equation, Equation (4), as true. As an ex-
ample, assume a temperature sensor has an accuracy
specification of ±0.75˚C at a temperature of 65 ˚C (338
Kelvin) and the processor diode has a non-ideality variation
of +1.19%/−0.27%. The resulting system accuracy of the
processor temperature being sensed will be:
T
ACC = ± 0.75˚C + (+1.19% of 338 K) = +4.76 ˚C
and
T
ACC = ± 0.75˚C + (−0.27% of 338 K) = −1.65 ˚C
TrueTherm technology uses the transistor equation, Equa-
tion (5), resulting in a non-ideality spread that truly reflects
the process variation which is very small. The transistor
equation non-ideality spread is ±0.1% for the Pentium 4
processor on 90nm process. The resulting accuracy when
using TruTherm technology improves to:
T
ACC = ±0.75˚C + (±0.1% of 338 K) = ± 1.08 ˚C
The next error term to be discussed is that due to the series
resistance of the thermal diode and printed circuit board
traces. The thermal diode series resistance is specified on
most processor data sheets. For the Pentium 4 processor on
90 nm process, this is specified at 3.33
Ω typical. The
LM95231 accommodates the typical series resistance of the
Pentium 4 processor on 90 nm process. The error that is not
accounted for is the spread of the Pentium’s series resis-
tance, that is 3.242
Ω to 3.594Ω or +0.264Ω to −0.088Ω. The
equation to calculate the temperature error due to series
resistance (T
ER) for the LM95231 is simply:
(6)
Solving Equation (6) for R
PCB
equal to +0.264
Ω and
−0.088
Ω results in the additional error due to the spread in
the series resistance of +0.16˚C to −0.05˚C. The spread in
error cannot be canceled out, as it would require measuring
each individual thermal diode device. This is quite difficult
and impractical in a large volume production environment.
Equation (6) can also be used to calculate the additional
error caused by series resistance on the printed circuit
board. Since the variation of the PCB series resistance is
minimal, the bulk of the error term is always positive and can
simply be cancelled out by subtracting it from the output
readings of the LM95231.
Processor Family
Diode Equation
η
D,
non-ideality
Series
R
min
typ
max
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
Pentium 4 on 90 nm
process
1.0083
1.011
1.023
3.33
Pentium M Processor
(Centrino)
1.00151 1.00220 1.00289 3.06
MMBT3904
1.003
AMD Athlon MP model
6
1.002
1.008
1.016
AMD Athlon 64
1.008
1.008
1.096
AMD Opteron
1.008
1.008
1.096
AMD Sempron
1.00261
0.93
3.1.3 Compensating for Different 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 and series resis-
tance of a given processor type. The LM95231 is calibrated
for two non-ideality factors and series resistance values thus
20120243
FIGURE 3. Thermal Diode Current Paths
www.national.com
18


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