Datasheet

x
PCB
R
¸
¹
·
:
62.0
=
ER
T
C
º
¨
©
§
LM96163
SNAS433D –JUNE 2008–REVISED MAY 2013 www.ti.com
Calculating Total System Accuracy
The voltage seen by the LM96163 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 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 8, 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 +4.06%/−0.89%. 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 (10)
and
T
ACC
= - 1.0°C + (−0.89% of 353 K) = −4.1 °C (11)
TruTherm technology uses the transistor equation, Equation 8, resulting in a non-ideality spread that truly reflects
the process variation which is very small. The transistor equation non-ideality spread is ±0.39% for the 65nm
thermal diode. The resulting accuracy when using TruTherm technology improves to:
T
ACC
= ±0.75°C + (±0.39% of 353 K) = ± 2.16 °C (12)
Intel does not specify the diode model ideality and series resistance of the thermal diodes on 45nm so a similar
comparison cannot be calculated, but lab experiments have shown similar improvement. For the 45nm processor
the ideality spread as specified by Intel is -0.399% to +0.699%. The resulting spread in accuracy when using
TruTherm technology with the thermal diode on Intel processors with 45nm process is:
T
ACC
= -0.75°C + (-0.39% of 353 K) = -2.16 °C (13)
to
T
ACC
= +0.75°C + (+0.799% of 353 K) = +4.32 °C (14)
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 45 nm process, this is specified at 4.5Ω typical with a minimum of 3Ω and a maximum of 7Ω. The
LM96163 accommodates the typical series resistance of Intel Processor on 45 nm process. The error that is not
accounted for is the spread of the processor's series resistance. The equation used to calculate the temperature
error due to series resistance (T
ER
) for the LM96163 is simply:
(15)
Solving Equation 15 for R
PCB
equal to -1.5Ω to 2.5Ω results in the additional error due to the spread in this series
resistance of -0.93°C to +1.55°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 15 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 LM96163 using the Remote
Temperature Offset register.
Transistor Equation η
T
, non-ideality Series R,Ω
Processor Family
min typ max
Intel Processor on 45 nm process 0.997 1.001 1.008 4.5
Intel Processor on 65 nm process 0.997 1.001 1.005 4.52
40 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: LM96163