LM95235CIMM National Semiconductor, LM95235CIMM Datasheet - Page 22

• Manufacturer Part Number: LM95235CIMM
• Description: IC,TEMPERATURE SENSOR,TSSOP,8PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM95235CIMM

• Rohs Compliant: NO

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 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
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 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
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:
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 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
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 2.79Ω
BE
T
T
ACC
is proportional to both η and T, the variations in η
T
ACC
ACC
=
= + 1.0˚C + (+4.06% of 353 K) = +15.3 ˚C
= - 1.0˚C + (−0.89% of 353 K) = −4.1 ˚C
±
0.75˚C + (
±
1.0˚C at a temperature of 80˚C (353 Kelvin)
±
0.39% of 353 K) =
±
0.39% for the Pentium 4
(Continued)
FIGURE 7. Thermal Diode Current Paths
±
2.16 ˚C
F
R
S
22
to 6.24Ω or
ture error due to series resistance (T
simply:
Solving Equation (6) for R
additional error due to the spread in the series resistance of
±
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
Pentium III CPUID
67h
Pentium III CPUID
68h/PGA370Socket/
Celeron
Pentium 4, 423 pin 0.9933
Pentium 4, 478 pin 0.9933
Processor Family
Intel Processor on
65 nm process
1.07˚C. The spread in error cannot be canceled out, as it
±
1.73Ω. The equation to calculate the tempera-
1.0057
20174943
min
Transistor Equation η
0.997
min
1
PCB
Diode Equation η
equal to
non-ideality
non-ideality
1.0065
1.0045
1.0045
1.008
1.001
typ
typ
ER
±
) for the LM95235 is
1.73Ω results in the
1.0125
1.0125
1.0368
1.0368
1.005
max
max
D
,
T
,
Series
Series
4.52
R,Ω
R,Ω
(6)