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SNIS128C – AUGUST 2002 – REVISED MARCH 2013
APPLICATIONS HINTS
The LM89 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 circuit board lands and traces soldered to the LM89'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 LM89 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 contribute to the die temperature much more strongly
than will the air temperature.
To measure temperature external to the LM89'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 LM89's temperature. The LM89
has been optimized to measure the remote thermal diode of a 0.13 micron Pentium 4 or a Mobile Pentium 4
Processor-M processor. 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 a 2N3904 transistor
base emitter junction be used with the collector tied to the base.
An LM89 with a diode-connected 2N3904 approximates the temperature reading of the LM89 with a Pentium 4
microprocessor less 1°C.
T 2N3904 = T P4 ? 1°C
DIODE NONIDEALITY
Diode Nonideality Factor Effect on Accuracy
When a transistor is connected as a diode, the following relationship holds for variables V BE , T and I f :
V be
I F = I S e
where
V t = k q T
where
K V t
-1
(1)
?
?
?
?
?
?
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 nonideality 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
(2)
In the active region, the -1 term is negligible and may be eliminated, yielding the following equation
V be
I F = I S e
K V t
(3)
In the above equation, 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 (N) and measuring the resulting voltage difference, it is
possible to eliminate the I S term. Solving for the forward voltage difference yields the relationship:
V be = K k q T ln (N)
(4)
22
Product Folder Links: LM89
Copyright ? 2002–2013, Texas Instruments Incorporated
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