LM95245CIMMX-1 NSC [National Semiconductor], LM95245CIMMX-1 Datasheet - Page 21

• Manufacturer Part Number: LM95245CIMMX-1
• Manufacturer: NSC [National Semiconductor]

3.1 DIODE NON-IDEALITY
3.1.1 Diode Non-Ideality Factor Effect on Accuracy
When a transistor is connected as a diode, the following re-
lationship holds for variables V
where:
•
•
•
•
•
•
•
In the active region, the -1 term is negligible and may be elim-
inated, yielding the following equation
In Equation 2, η and I
was used in the fabrication of the particular diode. By forcing
q = 1.6×10
T = Absolute Temperature in Kelvin
k = 1.38×10
η is the non-ideality factor of the process the diode is
manufactured on,
I
I
V
S
f
BE
= Forward Current through the base-emitter junction
= Saturation Current and is process dependent,
= Base-Emitter Voltage drop
−19
−23
Coulombs (the electron charge),
joules/K (Boltzmann's constant),
S
are dependant upon the process that
BE
, T and I
F
:
FIGURE 8. Thermal Diode Current Paths
(1)
(2)
21
two currents with a very controlled ratio(I
ing the resulting voltage difference, it is possible to eliminate
the I
the relationship:
Solving Equation 3 for temperature yields:
Equation 4 holds true when a diode connected transistor such
as the MMBT3904 is used. When this “diode” equation is ap-
plied to an integrated diode such as a processor transistor
with its collector tied to GND as shown in Figure 8 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 (BJT) equation,
Equation 5, which is a more accurate representation of the
topology of the thermal diode found in an FPGA or processor.
TruTherm should only be enabled when measuring the tem-
perature of a transistor integrated as shown in the processor
of Figure 8, because Equation 5 only applies to this topology.
S
term. Solving for the forward voltage difference yields
30015143
F2
/ I
F1
) and measur-
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(3)
(4)
(5)