LM95235EVAL/NOPB National Semiconductor, LM95235EVAL/NOPB Datasheet - Page 23

• Manufacturer Part Number: LM95235EVAL/NOPB
• Description: BOARD EVALUATION LM95235
• Manufacturer: National Semiconductor
• Series: PowerWise®, TruTherm®r

Specifications of LM95235EVAL/NOPB

• Sensor Type: Temperature
• Sensing Range: -40°C ~ 125°C
• Interface: SMBus (2-Wire/I²C)
• Sensitivity: ±1°C
• Voltage - Supply: 3 V ~ 3.6 V
• Embedded: Yes, MCU, 8-Bit
• Utilized Ic / Part: LM95235
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM95235EVAL

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 topology.
3.1.2 Calculating Total System Accuracy
The voltage seen by the LM95235 also includes the I
age drop of the series resistance. The non-ideality factor, η,
is the only other parameter not accounted for and depends
on the diode that is used for measurement. Since ΔV
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 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 example, assume a temperature
sensor has an accuracy specification of ±1.0°C at a temper-
ature 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 processor temperature being sensed will be:
and
TrueTherm technology uses the transistor equation, Equation
5, resulting in a non-ideality spread that truly reflects the pro-
cess 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:
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 4.52Ω typical. The LM95235 ac-
commodates the typical series resistance of Intel Processor
on 65 nm process. The error that is not accounted for is the
spread of the processor's series resistance, that is 2.79Ω to
6.24Ω or ±1.73Ω. The equation to calculate the temperature
error due to series resistance (T
T
T
ACC
T
ACC
ACC
= ±0.75°C + (±0.39% of 353 K) = ± 2.16 °C
= + 1.0°C + (+4.06% of 353 K) = +15.3 °C
= - 1.0°C + (−0.89% of 353 K) = −4.1 °C
ER
) for the LM95235 is simply:
FIGURE 7. Thermal Diode Current Paths
F
R
S
BE
volt-
is
23
Solving Equation 6 for R
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 can-
celled out by subtracting it from the output readings of the
LM95235.
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 -
3.06 GHz
Pentium 4 on 90 nm
process
Intel Processor on
65 nm process
Processor Family
Intel Processor on
65 nm process
1.0057
0.9933
0.9933
1.0011
1.0083
20174943
1.000
Diode Equation η
min
Transistor Equation η
0.997
PCB
min
1
equal to ±1.73Ω results in the
non-ideality
ideality
1.0065
1.0045
1.0045
1.0021
1.008
1.011
1.009
1.001
typ
typ
D
, non-
1.0125
1.0125
1.0368
1.0368
1.0030
1.005
1.023
1.050
max
max
T
www.national.com
,
Series
Series
4.52
R,Ω
3.64
3.33
4.52
R,Ω
(6)