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LM95235 Datasheet(PDF) 21 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM95235
Description  Precision Remote Diode Temperature Sensor with SMBus Interface and TruTherm Technology
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM95235 Datasheet(HTML) 21 Page - National Semiconductor (TI)

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3.0 Applications Hints
The LM95235 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 be-
cause the path of best thermal conductivity is between the
die and the pins, its temperature will effectively be that of the
printed circuit board lands and traces soldered to the
LM95235’s pins. This presumes that the ambient air tem-
perature 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 tempera-
ture of the LM95235 die will be at an intermediate tempera-
ture between the surface and air temperatures. Again, the
primary thermal conduction path is through the leads, so the
circuit board temperature will contribute to the die tempera-
ture much more strongly than will the air temperature.
To measure temperature external to the LM95235’s die, use
a remote diode. This diode can be located on the die of a
target IC, allowing measurement of the IC’s temperature,
independent of the LM95235’s temperature. A discrete diode
can also be used to sense the temperature of external
objects or ambient air. Remember that a discrete diode’s
temperature will be affected, and often dominated, by the
temperature of its leads. Most silicon diodes do not lend
themselves well to this application. It is recommended that
an MMBT3904 transistor base-emitter junction be used with
the collector tied to the base.
The LM95235’s TruTherm technology allows accurate sens-
ing of integrated thermal diodes, such as those found on
most processors. With TruTherm technology turned off, the
LM95235 can measure a diode-connected transistor such as
the MMBT3904 or the thermal diode found in an AMD pro-
cessor.
The LM95235 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 either pair of remote
inputs can be assigned to be either 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
relationship holds for variables V
BE, T and IF:
(1)
where:
q = 1.6x10
−19 Coulombs (the electron charge),
T = Absolute Temperature in Kelvin
k = 1.38x10
−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
(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 /IF1) and
measuring the resulting voltage difference, it is possible to
eliminate the I
S term. Solving for the forward voltage differ-
ence 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 tran-
sistor with its collector tied to GND as shown in Figure 7 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 equation, Equation
(5), which is a more accurate representation of the topology
of the thermal diode found in an FPGA or processor.
(5)
www.national.com
21


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