Datasheet
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xK
t
V
BE
V
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¹
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x
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¼
º
I
F
I
S
q
kT
=
V
t
=
I
S
x
K
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V
BE
V
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-1
I
F
LM95235
LM95235-Q1
SNIS142F –APRIL 2006–REVISED MARCH 2013
www.ti.com
APPLICATIONS HINTS
The LM95235 can be applied easily in the same way as other integrated-circuit temperature sensors, and its
remote diode sensing capability allows it to be used in new ways as well. It can be soldered to a printed circuit
board, and because the path of best thermal conductivity is between the die and the pins, its temperature will
effectively be that of the printed circuit board lands and traces soldered to the LM95235's pins. This presumes
that the ambient air temperature is almost the same as the surface temperature of the printed circuit board; if the
air temperature is much higher or lower than the surface temperature, the actual temperature of the LM95235 die
will be at an intermediate temperature between the surface and air temperatures. Again, the primary thermal
conduction path is through the leads, so the circuit board temperature will contribute to the die temperature much
more strongly than will the air temperature.
To measure temperature external to the LM95235's die, use a remote diode. This diode can be located on the
die of a target IC, allowing measurement of the IC's temperature, independent of the LM95235's temperature. A
discrete diode can also be used to sense the temperature of external objects or ambient air. Remember that a
discrete diode's temperature will be affected, and often dominated, by the temperature of its leads. Most silicon
diodes do not lend themselves well to this application. It is recommended that an MMBT3904 transistor base-
emitter junction be used with the collector tied to the base.
The LM95235's TruTherm technology allows accurate sensing of integrated thermal diodes, such as those found
on most processors. With TruTherm technology turned off, the LM95235 can measure a diode-connected
transistor such as the MMBT3904 or the thermal diode found in an AMD processor.
The LM95235 has been optimized to measure the remote thermal diode integrated in a typical Intel processor on
65 nm or 90 nm process or an MMBT3904 transistor. Using the Remote Diode Model Select register either pair
of remote inputs can be assigned to be either a typical Intel processor on 65 nm or 90 nm process or an
MMBT3904.
DIODE NON-IDEALITY
Diode Non-Ideality Factor Effect on Accuracy
When a transistor is connected as a diode, the following relationship holds for variables V
BE
, T and I
F
:
where
•
• q = 1.6×10
-19
Coulombs (the electron charge),
• T = Absolute Temperature in Kelvin
• k = 1.38×10
-23
joules/K (Boltzmann's constant),
• η is the non-ideality factor of the process the diode is manufactured on,
• I
S
= Saturation Current and is process dependent,
• I
f
= Forward Current through the base-emitter junction
• V
BE
= Base-Emitter Voltage drop (1)
In the active region, the -1 term is negligible and may be eliminated, yielding the following equation
(2)
In Equation 2, η and I
S
are dependant upon the process that was used in the fabrication of the particular diode.
By forcing two currents with a very controlled ratio(I
F2
/ I
F1
) and measuring the resulting voltage difference, it is
possible to eliminate the I
S
term. Solving for the forward voltage difference yields the relationship:
24 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LM95235 LM95235-Q1