Datasheet
MMBT3904
LM95234
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
¸
¸
¹
·
¨
¨
©
§
T =
q x 'V
BE
K
x k x ln
I
F2
I
F1
¹
·
©
§
x
¹
·
©
§
x='
ln
BE
I
I
q
kT
V
K
2F
1
F
F
I
=
I
x
K
t
V
BE
V
©
§
¹
·
S
x
e
«
«
¬
ª
»
»
¼
º
LM95234
www.ti.com
SNIS136D –AUGUST 2006–REVISED MARCH 2013
• 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:
(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 23 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 BJT beta compensation 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 23. Thermal Diode Current Paths
TruTherm should only be enabled when measuring the temperature of a transistor integrated as shown in the
processor of Figure 23, because Equation 5 only applies to this topology.
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