Datasheet

x
PCB
R
¸
¹
·
:
62.0
=
ER
T
C
º
¨
©
§
MMBT3904
LM95214
100 pF
ASIC
I
C
I
R
I
E
= I
F
100 pF
7
6
D1+
D2+
5
D-
I
F
I
R
LM95214
SNIS146A MARCH 2007REVISED MARCH 2013
www.ti.com
Figure 21. Thermal Diode Current Paths
TruTherm technology can be found in the LM95234 four channel remote diode sensor that is pin and register
compatible with the LM95214. The LM95214 does not support this technology.
Calculating Total System Accuracy
The voltage seen by the LM95214 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 ΔV
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 for Intel processor on 65 nm 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
temperature 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:
T
ACC
= + 1.0°C + (+4.06% of 353 K) = +15.3 °C
and
T
ACC
= - 1.0°C + (0.89% of 353 K) = 4.1 °C
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
MMBT3904 transistor, this is specified at 0 typical. The LM95214 accommodates the typical series resistance
of a circuit with the offset register compensation. The error that is not accounted for is the spread of the thermal
diodes series resistance. If a circuit has a series resistance spread that is 2.79 to 6.24 or 4.515 ±1.73, the
4.515 can be cancelled out with the offset register setting. The ±1.73 spread cannot be cancelled out. The
equation to calculate the temperature error due to series resistance (T
ER
) for the LM95214 is simply:
(6)
Solving Equation 6 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 bulk of the error caused by the 4.515 ohms will cause a positive offset in the
temperature reading of 2.79°C wich can be cancelled out by setting the offset register to - 2.75°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 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 LM95214.
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