Datasheet

MMBT3904
LM95233
100 pF
PROCESSOR
I
C
I
R
I
E
= I
F
100 pF
7
6
D1+
D2+
5
D-
I
F
I
R
=
T
q
x '
BE
V
x
k
K x
ln
¸
¸
¹
·
¨
¨
©
§
I
2C
I
1C
LM95233
SNIS145E –AUGUST 2006–REVISED MARCH 2013 www.ti.com
TruTherm BJT beta compensation technology uses the transistor equation, Equation 6, which is a more accurate
representation of the topology of the thermal diode found in an FPGA or processor.
(6)
Figure 20. Thermal Diode Current Paths
TruTherm BJT beta compensation should only be enabled when measuring the temperature of a transistor
integrated as shown in the processor of Figure 20, because Equation 6 only applies to this topology.
Calculating Total System Accuracy
The voltage seen by the LM95233 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 5, 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 (7)
and
T
ACC
= - 1.0°C + (−0.89% of 353 K) = −4.1 °C (8)
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 ±0.39% for the
Intel processor on 90 nm process. The resulting accuracy when using TruTherm technology improves to:
T
ACC
= ±0.75°C + (±0.39% of 353 K) = ± 2.16 °C (9)
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