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© Andigilog, Inc. 2006
www.andigilog.com
October 2006 - 70A06010
aSC7621
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 aSC7621 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 aSC7621 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.
Evaluation Board
The Andigilog SMBus EVB provides a platform for
evaluation of the operational characteristics of the
aSC7511, aSC7512 and aSC7621. The board features a
graphical user interface (GUI) to control and monitor all
activities and readings of these parts. The provided
software will run on a Windows XP™-based desktop or
laptop PC with a USB port.
In addition to being a self-contained fan speed control
demonstration, it may be connected into an operating
PC’s fan and CPU diode to evaluate various settings
under real operating conditions without the need to adjust
BIOS code. After optimization, the settings may be
programmed into the system.
Features:
•
Interactive GUI for setting limits and operational
configuration
•
aSC7512 and aSC7621 Automatic Fan Control
•
Powered and operated from the USB port
•
Support for reading or writing to any register
•
User-defined, time-stamped logging of any
registers, saved in spreadsheet-compatible
format
•
Graphical readouts:
•
Temperature and alarms
•
Fan RPM
•
Automatic fan control state
•
Voltage
•
Selectable on-board 2N3904 or wired remote
diode
•
Headers for 2-, 3- and 4-wire fans with PWM for
aSC7512
•
Headers for 3 4-wire fans for aSC7621
•
Saving of register setting configurations
•
LED indicators of pin alarm states
•
Optional use of external 12V fan power for
higher current fans
•
Optional connection to off-board SMBus clients
Application Diagrams
The aSC7621 may be easily adapted to 2-, 3- or 4-wire
fans for precise, wider-range fan speed control when
compared to variable DC drive. Up to four fans may be
controlled. Fans 1 and 2 are independent. Fan 3 is
independent and may be tied to fan 4 for speed control.
Separate tachometer readings may be reported for all
four.
Application diagram in Figure 16 shows connections to
four 4-wire fans. External FETs may be added to the
PWM output to drive 3-wire fans.
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