Datasheet

T
ER
= R
PCB
x 0.62°C/ :
LM95231
www.ti.com SNIS139E –FEBRUARY 2005–REVISED MARCH 2013
(9)
Solving Equation 9 for R
PCB
equal to +0.264Ω and −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 9 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/ 1.0057 1.008 1.0125
Celeron
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 1.0083 1.011 1.023 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
AMD Athlon 64 1.008 1.008 1.096
AMD Opteron 1.008 1.008 1.096
AMD Sempron 1.00261 0.93 Ω
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. Texas Instruments temperature sensors are always calibrated to the typical non-ideality and
series resistance of a given processor type. The LM95231 is calibrated for two non-ideality factors and series
resistance values thus supporting the MMBT3904 transistor and the Pentium 4 processor on 90nm process
without the requirement for additional trims. For most accurate measurements TruTherm mode should be turned
on when measuring the Pentium 4 processor on the 90nm process to minimize the error introduced by the false
non-ideality spread (see Diode Non-Ideality Factor Effect on Accuracy). When a temperature sensor calibrated
for a particular processor type is used with a different processor type, additional errors are introduced.
Temperature errors associated with non-ideality of different processor types may be reduced in a specific
temperature range of concern through use of software calibration. Typical Non-ideality specification differences
cause a gain variation of the transfer function, therefore the center of the temperature range of interest should be
the target temperature for calibration purposes. The following equation can be used to calculate the temperature
correction factor (T
CF
) required to compensate for a target non-ideality differing from that supported by the
LM95231.
T
CF
= [(η
S
−η
Processor
) ÷ η
S
] × (T
CR
+ 273 K) (10)
where
• η
S
= LM95231 non-ideality for accuracy specification
• η
T
= target thermal diode typical non-ideality
• T
CR
= center of the temperature range of interest in °C
The correction factor of Equation 10 should be directly added to the temperature reading produced by the
LM95231. For example when using the LM95231, with the 3904 mode selected, to measure a AMD Athlon
processor, with a typical non-ideality of 1.008, for a temperature range of 60 °C to 100 °C the correction factor
would calculate to:
T
CF
=[(1.003−1.008)÷1.003]×(80+273) =−1.75°C (11)
Therefore, 1.75°C should be subtracted from the temperature readings of the LM95231 to compensate for the
differing typical non-ideality target.
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