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SNIS143E – AUGUST 2006 – REVISED MARCH 2013
To measure temperature external to the LM95241'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 LM95241'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 a 2N3904 transistor base emitter
junction be used with the collector tied to the base.
The LM95241's TruTherm technology allows accurate sensing of integrated thermal diodes, such as those found
on sub-micron geometry processors. With TruTherm technology turned off, the LM95241 can measure a diode
connected transistor such as the 2N3904 or MMBT3904.
The LM95241 has been optimized to measure the remote thermal diode integrated in a Intel processor on 65nm
or 90nm process or an MMBT3904 transistor. Using the Remote Diode Model Select register either pair of
remote inputs can be assigned to measure either a Intel processor on 65nm or 90nm process or an MMBT3904
transistor.
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 :
V BE
I F = I S x e
where
K x V t
-1
V t = q
?
?
?
?
?
?
?
kT
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
V be
I F = I S e K V t
(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:
' V BE = K x x ln
KxT
q
I F2
I F1
(3)
Solving Equation 3 for temperature yields:
T=
' V BE x q
I F2
K x k x ln
I F1
(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 12 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.
Copyright ? 2006–2013, Texas Instruments Incorporated
Product Folder Links: LM95241
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