LM99CIMMX NSC [National Semiconductor], LM99CIMMX Datasheet - Page 18

• Manufacturer Part Number: LM99CIMMX
• Manufacturer: NSC [National Semiconductor]

www.national.com
3.0 Application Hints
The LM99 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 LM99’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 of the
LM99 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 LM99’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 LM99’s temperature. The LM99 has been
optimized to measure the NVIDIA
thermal diode. Remember that a discrete diode’s tempera-
ture will be affected, and often dominated, by the tempera-
ture of its leads.
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
where:
In the active region, the -1 term is negligible and may be
eliminated, yielding the following equation
• q = 1.6x10
• T = Absolute Temperature in Kelvin
• k = 1.38x10
• η is the non-ideality factor of the process the diode is
• I
• I
• V
manufactured on,
S
f
BE
Processor Family
NVIDIA GeForceFX Graphics Processor
Intel
Intel Pentium 3 Processor
= Forward Current through the base-emitter junction
= Saturation Current and is process dependent,
= Base-Emitter Voltage drop
®
Pentium
−19
−23
Coulombs (the electron charge),
joules/K (Boltzmann’s constant),
®
4 Processor
BE
, T and I
®
GeForce
Offset Register Settings for Specific Devices
f
:
™
FX family
+2.625
+2.375
default
∆T, ˚C
18
In the above equation, η and I
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
difference yields the relationship:
The voltage seen by the LM99 also includes the I
drop of the series resistance. The non-ideality factor, η, is
the only other parameter not accounted for and depends on
the diode that is used for measurement. Since ∆V
proportional to both η and T, the variations in η cannot be
distinguished from variations in temperature. Since the non-
ideality factor is not controlled by the temperature sensor, it
will directly add to the inaccuracy of the sensor. As an
example, assume a temperature sensor has an accuracy
specification of
process used to manufacture the diode has a non-ideality
variation of
ture sensor at room temperature will be:
The additional inaccuracy in the temperature measurement
caused by η, can be eliminated if each temperature sensor is
calibrated with the remote diode that it will be paired with.
3.1.2 Compensating for Diode Non-Ideality
In order to compensate for the errors introduced by non-
ideality, the temperature sensor is calibrated for a particular
processor. National Semiconductor temperature sensors are
always calibrated to the typical non-ideality of a given pro-
cessor type. The LM99 is calibrated for the non-ideality of
the NVIDIA GeForceFX family thermal diode. When a tem-
perature sensor calibrated for a particular processor type is
used with a different processor type or a given processor
type has a non-ideality that strays from the typical, errors are
introduced.
Temperature errors associated with non-ideality may be re-
duced in a specific temperature range of concern through
use of the offset registers (11h and 12h). See Offset Register
table below.
Please send an email to hardware.monitor.team
requesting further information on our recommended setting
of the offset register for different processor types.
Register 11h
Offset Register Settings
T
0000 0010
0000 0010
ACC
default
±
=
0.1%. The resulting accuracy of the tempera-
±
±
1˚C + (
1˚C at room temperature of 25 ˚C and the
S
term. Solving for the forward voltage
±
0.1% of 298 ˚K) =
S
Register 12h
are dependant upon the
1010 0000
0110 0000
default
±
1.4 ˚C
F
R
@
S
nsc.com
voltage
BE
is