ASC7511 Datasheet, PDF (15/19 Pages) List of Unclassifed Manufacturers – LOW- OLTAGE 2-WIRE DIGITAL TEMPERATURE SENSOR

aSC7511
ideality variation and series resistance. However, the
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.
2N3906
2N3904
D+
aSC7511
D-
D+
aSC7511
D-
Use ground runs along side the pair to minimize
differential coupling as in Figure 14.
3. Place the aSC7511 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.
Figure 10. Discrete Remote Diode Connection
Board Layout Considerations
The distance between the remote sensor and the
aSC7511 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 15. 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.
10 mils
10 mils
GND
D+
D-
GND
Thermal Considerations
The temperature of the aSC7511 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 ambient air temperature, 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.
The power consumption of the aSC7511 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.
Application Circuit
Figure 11. Recommended Remote Diode Circuit
Board Interconnect
Noise filtering is accomplished by using a bypass
capacitor placed as close as possible to the aSC7511 D+
and D- pins. A 2.2nF ceramic capacitor is recommended,
but up to 3.3nF may be used. Additional filtering takes
place within the aSC7511.
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
aSC7511.
The aSC7511 may be used to turn a fan on and off in
response to the internal or remote sensor. The active-low
THERM pin offers a programmable turn-on temperature
and the THERM hysteresis setting will turn the fan off.
An SMBus host is used to provide the settings
for THERM and THERM hysteresis. The fan will come on
when the THERM limit is reached and will turn off when it
falls below THERM temperature by the amount set into
the THERM hysteresis register. Figure 16 provides a
circuit diagram.
2. Match the trace routing of the D+ and D- leads and
use a 2.2nF filter capacitor close to the aSC7511.
© Andigilog, Inc. 2006
- 15 -
www.andigilog.com
August 2006 - 70A04010