LM88EVALA National Semiconductor, LM88EVALA Datasheet - Page 7

• Manufacturer Part Number: LM88EVALA
• Manufacturer: National Semiconductor

Specifications of LM88EVALA

• Lead Free Status / Rohs Status: Not Compliant

2.0 Application Hints
of the LM88’s temperature. The LM88 has been optimized to
measure the remote diode of a Pentium type processor as
shown in Figure 3 . 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 af-
fected, and often dominated, by the temperature of its leads.
As with any IC, the LM88 and accompanying wiring and
circuits must be kept insulated and dry, to avoid leakage and
corrosion. This is especially true if the circuit may operate at
cold
Printed-circuit coatings and varnishes such as Humiseal and
epoxy paints or dips are often used to ensure that moisture
cannot corrode the LM88 or its connections. Moisture may
also cause leakage on the diode wiring and therefore affect
the accuracy of the temperature set-points.
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.
A diode connected 2N3904 approximates the junction avail-
able on a Pentium III microprocessor for temperature mea-
surement. Therefore, the LM88 can sense the temperature
of this diode effectively.
2.3 EFFECTS OF THE DIODE NON-IDEALITY FACTOR
ON ACCURACY
The technique used in today’s remote temperature sensors
is to measure the change in V
points of a diode. For a bias current ratio of N:1, this differ-
ence is given as:
where:
The temperature sensor then measures V
to IT digital data. In this equation, k and q are well defined
universal constants, and N is a parameter controlled by the
FIGURE 3. Pentium or 3904 Temperature vs LM88
—
— q is the electron charge,
— k is the Boltzmann’s constant,
— N is the current ratio,
— T is the absolute temperature in ˚K.
manufactured on,
temperatures
is the non-ideality factor of the process the diode is
Temperature Set-point
where
BE
condensation
at two different operating
(Continued)
BE
10132615
and converts
can
occur.
7
temperature sensor. The only other parameter is , which
depends on the diode that is used for measurement. Since
cannot be distinguished from variations in temperature.
Since the non-ideality factor is not controlled by the tempera-
ture sensor, it will directly add to the inaccuracy of the
sensor. For the Pentium II, Intel specifies a
sensor has an accuracy specification of
temperature of 25 ˚C and the process used to manufacture
the diode has a non-ideality variation of
accuracy of the temperature sensor at room temperature will
be:
.
The additional inaccuracy in the temperature measurement
caused by
calibrated with the remote diode that it will be paired with.
2.4 PCB LAYOUT to MINIMIZE NOISE
In a noisy environment, such as a processor motherboard,
layout considerations are very critical. Noise induced on
traces running between the remote temperature diode sen-
sor and the LM88 can cause temperature conversion errors.
The following guidelines should be followed:
1. Place a 0.1 µF power supply bypass capacitor as close
2. The recommended 2.2nF diode bypass capacitor actu-
3. Ideally, the LM88 should be placed within 10cm of the
4. Diode traces should be surrounded by a GND guard ring
5. Avoid routing diode traces in close proximity to power
6. Avoid running diode traces close to or parallel to high
7. If it is necessary to cross high speed digital traces, the
V
from part to part. As an example, assume a temperature
BE
as possible to the V
capacitor as close as possible to the D+ and D− pins.
Make sure the traces to the two 2.2nF capacitor are
matched.
ally has a range of 200pF to 3.3nF. The average tem-
perature accuracy will not change over that capacitance
range. Increasing the capacitance will lower the corner
frequency where differential noise error will start to affect
the temperature reading thus producing a reading that is
more stable. Conversely, lowering the capacitance will
increase the corner frequency where differential noise
error starts to affect the temperature reading thus pro-
ducing a reading that is less stable.
remote diode pins with the traces being as straight, short
and identical as possible. Trace resistance of 1
cause as much as 1˚C of error. This error can be com-
pensated by using the Remote Temperature Offset Reg-
isters, since the value placed in these registers will
automatically be subtracted or added to the remote tem-
perature reading.
to either side, above and below if possible. This GND
guard should not go between the D+ and D− lines so
that in the event that noise does couple to the diode
lines, it would be coupled common mode and rejected-
.(See Figure 4 )
supply switching or filtering inductors.
speed digital and bus lines. Diode traces should be kept
at least 2cm apart from the high speed digital traces.
diode traces and the high speed digital traces should
cross at a 90 degree angle.
is proportional to both
T
ACC
can be eliminated if each temperature sensor is
=
±
3˚C + (
DD
±
pin and the recommended 2.2 nF
1% of 298 ˚K) =
and T, the variations in
±
1%. The resulting
±
±
1% variation in
3 ˚C at room
±
6˚C
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