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© Andigilog, Inc. 2006
www.andigilog.com
December 2006 - 70A05012
aSC7531A / aSC7531B
It is recommended that the following guidelines be used to
minimize noise and achieve highest accuracy:
1.
Place a 0.1µF bypass capacitor to digital ground as
close as possible to the power pin of the aSC7531.
2.
Match the trace routing of the D+ and D- leads and
use a 1.0nF filter capacitor close to the aSC7531. Use
ground runs along side the pair to minimize differential
coupling as in Figure 20.
3.
Place the aSC7531B as close to the CPU or GPU
remote diode leads as possible to minimize noise and
series resistance.
4.
Avoid running diode connections close to or in parallel
with high-speed busses, staying at least 2cm away.
5.
Avoid running diode connections close to on-board
switching power supply inductors.
6.
PC board leakage should be minimized by maintaining
minimum trace spacing and covering traces over their
full length with solder mask.
Thermal Considerations
The temperature of the aSC7531 will be close to that of the
PC board on which it is mounted. Conduction through the
leads is the primary path for heat flow. The reported local
sensor is very close to the circuit board temperature and
typically between the board and ambient.
In order to measure PC board temperature in an area of
interest, such as the area around the CPU where voltage
regulator components generate significant heat, a remote
diode-connected transistor should be used. A surface-mount
SOT-23 or SOT-223 is recommended. The small size is
advantageous in minimizing response time because of its low
thermal mass, but at the same time it has low surface area
and a high thermal resistance to ambient air. A compromise
must be achieved between minimizing thermal mass and
increasing the surface area to lower the junction-to-ambient
thermal resistance.
In order to sense temperature of air-flows near board-
mounted heat sources, such as memory modules, the sensor
should be mounted above the PC board. A TO-92 packaged
transistor is recommended.
The power consumption of the aSC7531 is relatively low and
should have little self-heating effect on the local sensor
reading. At the highest measurement rate the dissipation is
less than 2mW, resulting in only a few tenths of a degree
rise.
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