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Fan Control Methods: Two methods of controlling the speed of
the fans are settable via the I2C Bus: temperature sensor
(thermistor) input or I2C bus input. Following is a description
of each control method:
 1) Temperature Based Fan Control: The speed of all fans is
proportional to the temperature sensor connected to header J12,
or the temperature of the board mounted sensor. Chart 2.0
shows fan voltage as a function of sensed temperature. I2C bus
register 0xF9 is used to set the control temperature, TC (The
control temperature is the temperature at which full supply
voltage is applied to the fans.). Temperature sensor status and
temperature readings are available on the I2C
bus.
2) I2C Bus Based Fan Control: When an I2C speed command
is given, the Fusion automatically switches to I2C control mode
for fan control via the I2C bus. The speed of all fans is mapped
to the command sent through the I2C on header J12. See the
Operation Section for a full definition of the I2C protocol.
Temperature Sensors: When temperature sensor based fan
control is specified, the user can choose the remotely mounted
sensor or the board mounted sensor.
Selectable Control Temperature and Slope: When
temperature sensor based fan control is specified, the user can
set the control temperature and choose a 5°C or 10°C slope via
the I2C bus.
Selectable Off Temperature: When temperature sensor based
fan control is specified, the user can select an off temperature,
which will turn off the regulator when the temperature drops
below the set point.
Selectable Alarm Temperature: When temperature sensor
based fan control is specified, the user can set the alarm
temperature. When the temperature exceeds the alarm
temperature, an alarm is triggered and full supply voltage is
applied to the fans.
On Board Power Supply: An isolated 3.3 VDC is provided on
board to power the I2C bus and control signal interfaces.
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Active Current Limiting: The load regulating circuitry is
configured with an active current limit, which limits to current to
10A with 48 VDC applied. The current is delayed and instant
off to prevent arcing of contacts during live insertion. The
inrush circuit has a turn-on voltage of 34 VDC and a turn-off
voltage of 30 VDC. The regulator is equipped with a soft start
circuit to limit start-up currents. Chart 1.0 shows a typical
insertion current with a 48 VDC supply.
