Datasheet

MMBT3904
LM96163
100 pF
PROCESSOR
I
R
I
F
I
C
I
R
I
E
= I
F
LM96163
100 pF
2
2
3
3
D+
D+
D-
D-
=
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
·
©
§
¹
·
©
§
q
kT
I
F2
'V
BE
= K x
x ln
I
F1
©
§
¹
·
e
«
«
¬
ª
»
»
¼
º
I
F
= I
S
x
V
BE
K x V
t
LM96163
www.ti.com SNAS433D –JUNE 2008–REVISED MAY 2013
• 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
In the active region, the -1 term is negligible and may be eliminated, yielding the following equation
(6)
In Equation 6, 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:
(7)
Solving Equation 7 for temperature yields:
(8)
Equation 8 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 15 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 8 is an approximation.
TruTherm BJT beta compensation technology uses the transistor equation, Equation 9, which is a more accurate
representation of the topology of the thermal diode found in an FPGA or processor.
(9)
TruTherm should only be enabled when measuring the temperature of a transistor integrated as shown in the
processor of Figure 15, because Equation 9 only applies to this topology.
Figure 15. Thermal Diode Current Paths
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