lm95245eb National Semiconductor Corporation, lm95245eb Datasheet - Page 21

• Manufacturer Part Number: lm95245eb
• Description: Precision Remote Diode Digital Temperature Sensor With Trutherm? Bjt Beta Compensation Technology For 45nm Process
• Manufacturer: National Semiconductor Corporation

In the active region, the -1 term is negligible and may be elim-
inated, yielding the following equation
In Equation 2, η and I
was used in the fabrication of the particular diode. By forcing
two currents with a very controlled ratio(I
ing the resulting voltage difference, it is possible to eliminate
the I
the relationship:
Solving Equation 3 for temperature yields:
Equation 4 holds true when a diode connected transistor such
as the MMBT3904 is used. When this “diode” equation is ap-
plied to an integrated diode such as a processor transistor
with its collector tied to GND as shown in Figure 7 it will yield
a wide non-ideality spread. This wide non-ideality spread is
not due to true process variation but due to the fact that
Equation 4 is an approximation.
TruTherm technology uses the transistor (BJT) equation,
Equation 5, which is a more accurate representation of the
topology of the thermal diode found in an FPGA or processor.
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.
S
term. Solving for the forward voltage difference yields
S
are dependant upon the process that
F2
/ I
FIGURE 7. Thermal Diode Current Paths
F1
) and measur-
(2)
(3)
(4)
(5)
21
3.1.2 Calculating Total System Accuracy
The voltage seen by the LM95245 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. Intel thermal
diode specifications for processors on 45nm process were not
available at the publication of this document, therefore fol-
lowing examples will use the 65nm thermal diode specifica-
tions. 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 as-
sumes diode equation, Equation 4, as true. As an example,
assume a temperature sensor has an accuracy specification
of ±1.0°C at a temperature of 80°C (353 Kelvin) and the pro-
cessor 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
4, 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 LM95245 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
30015143
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) for the LM95245 is simply:
www.national.com
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