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LM95235 Datasheet(PDF) 22 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM95235
Description  Precision 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)

LM95235 Datasheet(HTML) 22 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 7, because Equation (5) only applies to this topol-
ogy.
3.1.2 Calculating Total System Accuracy
The voltage seen by the LM95235 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 for Intel processor on 65nm process, Intel
specifies a +4.06%/−0.897% 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 ±1.0˚C at a temperature of 80˚C (353 Kelvin)
and the processor diode has a non-ideality variation of
+1.19%/−0.27%. The resulting system accuracy of the pro-
cessor temperature being sensed will be:
T
ACC = + 1.0˚C + (+4.06% of 353 K) = +15.3 ˚C
and
T
ACC = - 1.0˚C + (−0.89% of 353 K) = −4.1 ˚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 ±0.39% for the Pentium 4
processor on 90 nm process. The resulting accuracy when
using TruTherm technology improves to:
T
ACC = ±0.75˚C + (±0.39% of 353 K) = ± 2.16 ˚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 Intel processors in 65 nm
process, this is specified at 4.52
Ω typical. The LM95235
accommodates the typical series resistance of Intel Proces-
sor on 65 nm process. The error that is not accounted for is
the spread of the processor’s series resistance, that is 2.79
to 6.24
Ω or ±1.73Ω. The equation to calculate the tempera-
ture error due to series resistance (T
ER) for the LM95235 is
simply:
(6)
Solving Equation (6) for R
PCB equal to ±1.73
Ω results in the
additional error due to the spread in the series resistance of
±1.07˚C. The spread in error cannot be canceled out, as it
would require measuring each individual thermal diode de-
vice. 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 LM95235.
Processor Family
Transistor Equation
η
D,
non-ideality
Series
R,
min
typ
max
Intel Processor on
65 nm process
0.997
1.001
1.005
4.52
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
20174943
FIGURE 7. Thermal Diode Current Paths
www.national.com
22


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