LM95234_07 NSC [National Semiconductor], LM95234_07 Datasheet - Page 33

• Manufacturer Part Number: LM95234_07
• Description: Quad Remote Diode and Local Temperature Sensor with SMBus Interface and TruTherm Technology
• Manufacturer: NSC [National Semiconductor]

3.0 Applications Hints
The LM95234 can be applied easily in the same way as other
integrated-circuit temperature sensors, and its remote diode
sensing capability allows it to be used in new ways as well. It
can be soldered to a printed circuit board, and because the
path of best thermal conductivity is between the die and the
pins, its temperature will effectively be that of the printed cir-
cuit board lands and traces soldered to the LM95234's pins.
This presumes that the ambient air temperature is almost the
same as the surface temperature of the printed circuit board;
if the air temperature is much higher or lower than the surface
temperature, the actual temperature of the LM95234 die will
be at an intermediate temperature between the surface and
air temperatures. Again, the primary thermal conduction path
is through the leads, so the circuit board temperature will con-
tribute to the die temperature much more strongly than will the
air temperature.
To measure temperature external to the LM95234's die, in-
corporates remote diode sensing technology. This diode can
be located on the die of a target IC, allowing measurement of
the IC's temperature, independent of the LM95234's temper-
ature. A discrete diode can also be used to sense the tem-
perature of external objects or ambient air. Remember that a
discrete diode's temperature will be affected, and often dom-
inated, by the temperature of its leads. Most silicon diodes do
not lend themselves well to this application. It is recommend-
ed that an MMBT3904 transistor base emitter junction be
used with the collector tied to the base.
The LM95234’s TruTherm BJT beta compensation technolo-
gy allows accurate sensing of integrated thermal diodes, such
as those found on most processors. With TruTherm technol-
ogy turned off, the LM95234 can measure a diode-connected
transistor such as the MMBT3904 or the thermal diode found
in an AMD processor.
The LM95234 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 Re-
mote 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.
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:
BE
, T and I
F
:
(1)
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•
•
•
•
•
•
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
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 9 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 tran-
sistor equation, Equation 5, which is a more accurate repre-
sentation of the topology of the thermal diode found in an
FPGA or processor.
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
S
BE
= Forward Current through the base-emitter junction
= Saturation Current and is process dependent,
term. Solving for the forward voltage difference yields
= Base-Emitter Voltage drop
−19
−23
Coulombs (the electron charge),
joules/K (Boltzmann's constant),
S
are dependant upon the process that
F2
/ I
F1
) and measur-
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(2)
(3)
(4)
(5)