Datasheet
MMBT3904
LM95235
100 pF
PROCESSOR
I
R
I
F
I
C
I
R
I
E
= I
F
LM95235
100 pF
2
2
3
3
D+
D+
D-
D-
=
T
q
x '
BE
V
x
k
K
x
ln
¸
¸
¹
·
¨
¨
©
§
I
2C
I
1C
¸
¸
¹
·
¨
¨
©
§
T =
q x 'V
BE
K
x k x ln
I
F2
I
F1
¹
·
©
§
x
¹
·
©
§
x=
ln
q
kT
K
'V
BE
I
F2
I
F1
LM95235
LM95235-Q1
www.ti.com
SNIS142F –APRIL 2006–REVISED MARCH 2013
(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 18 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 18. Thermal Diode Current Paths
TruTherm should only be enabled when measuring the temperature of a transistor integrated as shown in the
processor of Figure 18, because Equation 5 only applies to this topology.
Calculating Total System Accuracy
The voltage seen by the LM95235 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 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 (6)
and
T
ACC
= - 1.0°C + (-0.89% of 353 K) = -4.1°C (7)
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