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SNIS143E – AUGUST 2006 – REVISED MARCH 2013
I C1
T=
' V BE x q
I C2
K x k x ln
(5)
65nm/90nm
PROCESSOR
I E = I F
I C I R
100 pF
100 pF
1
2
3
4
D1+
D1-
D2+
D2-
Q1
I F
LM95241
MMBT3904
I R
Figure 12. Thermal Diode Current Paths
TruTherm should only be enabled when measuring the temperature of a transistor integrated as shown in the
processor of Figure 12 , because Equation 5 only applies to this topology.
Calculating Total System Accuracy
The voltage seen by the LM95241 also includes the I F R S voltage 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 Δ 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 temperature sensor, it will directly add to the
inaccuracy of the sensor. For the Pentium D processor on 65nm process, Intel specifies a +4.06%/ ? 0.89%
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 ±1.25°C at a
temperature of 65 °C (338 Kelvin) and the processor diode has a non-ideality variation of +4.06%/ ? 0.89%. The
resulting system accuracy of the processor temperature being sensed will be:
and
T ACC = + 1.25°C + (+4.06% of 338 K) = +14.97 °C
T ACC = - 1.25°C + ( ? 0.89% of 338 K) = ? 4.26 °C
(6)
(7)
TrueTherm technology uses the transistor equation, Equation 5 , resulting in a non-ideality spread that truly
reflects the process variation which is very small. The transistor equation non-ideality spread is ±0.4% for the
Pentium D processor on 65nm process. The resulting accuracy when using TruTherm technology improves to:
T ACC = ±1.25°C + (±0.4% of 338 K) = ± 2.60 °C
(8)
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 D
processor on 65 nm process, this is specified at 4.52 ? typical. The LM95241 accommodates the typical series
resistance of the Pentium D processor on 65nm process. The error that is not accounted for is the spread of the
Pentium's series resistance, that is 2.79 ? to 6.24 ? or ±1.73 ? . The equation to calculate the temperature error
due to series resistance (T ER ) for the LM95241 is simply:
T ER = R PCB x 0.62°C/ :
(9)
Solving Equation 9 for R PCB equal to ±1.73 ? 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 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 LM95241.
Processor Family
Transistor Equation n D , non-ideality
Series R
min
typ
max
Intel processor on 65nm process
0.997
1.001
1.005
4.52 ?
20
Product Folder Links: LM95241
Copyright ? 2006–2013, Texas Instruments Incorporated
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