Datasheet
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T =
q x 'V
BE
K
x k x ln
I
F2
I
F1
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-1
LM95233
www.ti.com
SNIS145E –AUGUST 2006–REVISED MARCH 2013
The LM95233’s TruTherm BJT beta compensation technology allows accurate sensing of integrated thermal
diodes, such as those found on most processors. With TruTherm BJT beta compensation technology turned off,
the LM95233 can measure a diode-connected transistor such as the MMBT3904 or the thermal diode found in an
AMD processor.
The LM95233 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 any of the
four remote inputs can be optimized for 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
• (2)
• 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 (2)
In the active region, the -1 term is negligible and may be eliminated, yielding the following equation
(3)
In Equation 3, 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:
(4)
Solving Equation 4 for temperature yields:
(5)
Equation 5 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 20 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 5 is an approximation.
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