ASC7512 ETC2 [List of Unclassifed Manufacturers], ASC7512 Datasheet - Page 31

• Manufacturer Part Number: ASC7512
• Description: DIGITAL TEMPERATURE SENSOR WITH INTEGRATED FAN CONTROL
• Manufacturer: ETC2 [List of Unclassifed Manufacturers]

Series Resistance
Any external series resistance in the connections from
the aSC7512 to the CPU pins should be accounted for in
interpreting the results of a measurement.
The impact of series resistance on the measured
temperature is a result of measurement currents
developing offset voltages that add to the diode voltage.
This is relatively constant with temperature and may be
corrected with a fixed value in the offset register. To
determine the temperature impact of resistance is as
follows:
where:
For example, a total series resistance of 10Ω would give
an offset of +3.9°C.
Discrete Remote Diodes
When sensing temperatures other than the CPU or GPU
substrate, an NPN or PNP transistor may be used. Most
commonly used are the 2N3904 and 2N3906. These
have characteristics similar to the CPU substrate diode
with non-ideality around 1.0046. They are connected with
base to collector shorted as shown in Figure 12.
While it is important to minimize the distance to the
remote diode to reduce high-frequency noise pickup, they
may be located many feet away with proper shielding.
Shielded, twisted-pair cable is recommended, with the
shield connected only at the aSC7512 end as close as
possible to the ground pin of the device.
As with the CPU substrate diode, the temperature
reported will be subject to the same errors due to non-
ideality variation and series resistance. However, the
© Andigilog, Inc. 2006
Figure 12 Discrete Remote Diode Connection
T = scale of temperature vs. V
Δ
R = total series resistance of interconnect (both leads).
Δ
V
T
S
I
D
R
2N3906
2N3904
= difference in the temperature reading from actual.
= difference in the two diode current levels (90µA).
ΔT
or,
ΔT
R
R
= R
= R
S
S
× ΔI
×
230
D
90
μ
/ T
V / °C
μ
V
A
D +
D -
D +
D -
= R
BE
aSC7512
aSC7512
(230µV/°C).
S
× 0.391°C / Ω
www.andigilog.com
- 31 -
transistor’s die temperature is usually not the temperature
of interest and care must be taken to minimize the
thermal resistance and physical distance between that
temperature and the remote diode. The offset and
response time will need to be characterized by the user.
Board Layout Considerations
The distance between the remote sensor and the
aSC7512 should be minimized. All wiring should be
defended from high frequency noise sources and a
balanced differential layout maintained on D+ and D-.
Any noise, both common-mode and differential, induced
in the remote diode interconnect may result in an offset in
the temperature reported. Circuit board layout should
follow the recommendation of Figure 13. Basically, use
10-mil lines and spaces with grounds on each side of the
differential pair. Choose the ground plane closest to the
CPU when using the CPU’s remote diode.
Noise filtering is accomplished by using a bypass
capacitor placed as close as possible to the aSC7512 D+
and D- pins. A 1.0nF ceramic capacitor is recommended,
but up to 3.3nF may be used. Additional filtering takes
place within the aSC7512.
It is recommended that the following guidelines be used
to minimize noise and achieve highest accuracy:
1.
2.
3.
4.
5.
6.
10 mils
10 mils
Figure 13 Recommended Remote Diode
Place a 0.1µF bypass capacitor to digital ground
as close as possible to the power pin of the
aSC7512.
Match the trace routing of the D+ and D- leads and
use a 1.0nF filter capacitor close to the aSC7512.
Use ground runs along side the pair to minimize
differential coupling as in Figure 13.
Place the aSC7512 as close to the CPU or GPU
remote diode leads as possible to minimize noise
and series resistance.
Avoid running diode connections close to or in
parallel with high-speed busses, staying at least
2cm away.
Avoid running diode connections close to on-board
switching power supply inductors.
PC board leakage should be minimized by
maintaining minimum trace spacing and covering
traces over their full length with solder mask.
Circuit Board Interconnect
August 2006 - 70A05003
aSC7512
GND
D +
D -
GND