Datasheet

Q1
MMBT3904
LM95231
1
2
3
4
D1+
D1-
D2+
D2-
100 pF
Pentium
®
4
PROCESSOR
100 pF
I
R
I
F
I
C
I
R
I
E
= I
F
T =
'V
BE
x q
I
C2
K x k x ln
I
C1
T =
'V
BE
x q
K x k x ln
I
F2
I
F1
'V
BE
= K x x ln
I
F2
I
F1
K x T
q
LM95231
SNIS139E –FEBRUARY 2005–REVISED MARCH 2013 www.ti.com
(3)
Solving Equation 3 for temperature yields:
(4)
Equation 4 holds true when a diode connected transistor such as the MMBT3904 is used. When this “diode”
equation is applied to an integrated diode such as a processor transistor with its collector tied to GND as shown
in Figure 11 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 equation, Equation 5, which is a more accurate representation of the
topology of the thermal diode found in an FPGA or processor.
(5)
Figure 11. Thermal Diode Current Paths
TruTherm should only be enabled when measuring the temperature of a transistor integrated as shown in the
processor of Figure 11, because Equation 5 only applies to this topology.
Calculating Total System Accuracy
The voltage seen by the LM95231 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 Pentium 4 processor on 90nm process, Intel specifies a +1.19%/−0.27%
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 ±0.75°C at a
temperature of 65 °C (338 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
= ± 0.75°C + (+1.19% of 338 K) = +4.76 °C (6)
and
T
ACC
= ± 0.75°C + (−0.27% of 338 K) = −1.65 °C (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 ±0.1% for the
Pentium 4 processor on 90nm process. The resulting accuracy when using TruTherm technology improves to:
T
ACC
= ±0.75°C + (±0.1% of 338 K) = ± 1.08 °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 4
processor on 90 nm process, this is specified at 3.33Ω typical. The LM95231 accommodates the typical series
resistance of the Pentium 4 processor on 90 nm process. The error that is not accounted for is the spread of the
Pentium's series resistance, that is 3.242Ω to 3.594Ω or +0.264Ω to −0.088Ω. The equation to calculate the
temperature error due to series resistance (T
ER
) for the LM95231 is simply:
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