LM95231_06 NSC [National Semiconductor], LM95231_06 Datasheet - Page 18

• Manufacturer Part Number: LM95231_06
• Description: Precision Dual Remote Diode Temperature Sensor with SMBus Interface and TruTherm Technology
• Manufacturer: NSC [National Semiconductor]

www.national.com
3.0 Applications Hints
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
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
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
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:
and
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
processor on 90nm process. The resulting accuracy when
using TruTherm technology improves to:
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 3.33Ω 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 3.242Ω to 3.594Ω or +0.264Ω to −0.088Ω. The
equation to calculate the temperature error due to series
resistance (T
BE
T
T
T
is proportional to both η and T, the variations in η
ACC
ACC
ACC
=
=
=
±
±
±
ER
0.75˚C + (
0.75˚C + (+1.19% of 338 K) = +4.76 ˚C
0.75˚C + (−0.27% of 338 K) = −1.65 ˚C
) for the LM95231 is simply:
±
0.75˚C at a temperature of 65 ˚C (338
±
0.1% of 338 K) =
±
0.1% for the Pentium 4
(Continued)
FIGURE 3. Thermal Diode Current Paths
±
1.08 ˚C
F
R
(6)
S
18
Solving Equation (6) for R
−0.088Ω 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.
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
Processor Family
Pentium III CPUID 67h
Pentium III CPUID
68h/PGA370Socket/
Celeron
Pentium 4, 423 pin
Pentium 4, 478 pin
Pentium 4 on 0.13
micron process,
2-3.06GHz
Pentium 4 on 90 nm
process
Pentium M Processor
(Centrino)
MMBT3904
AMD Athlon MP model
6
AMD Athlon 64
AMD Opteron
AMD Sempron
20120243
1.00151 1.00220 1.00289 3.06 Ω
1.0057 1.008 1.0125
0.9933 1.0045 1.0368
0.9933 1.0045 1.0368
1.0011 1.0021 1.0030 3.64 Ω
1.0083
1.002
1.008
1.008
min
Diode Equation η
1
PCB
non-ideality
1.00261
1.0065 1.0125
equal to +0.264Ω and
1.003
1.008
1.008
1.008
1.011
typ
1.023 3.33 Ω
1.016
1.096
1.096
max
D
,
0.93 Ω
Series
R