Application Specific Discretes
FAN SPEED CONTROLER
s COMPUTER AND SERVER POWER SUPPLY
s LOW COST VENTILATION (consumer)
s Built in thermal sensor
s Brushless DC fan speed control
s Linear control and regulation of the fan speed
according to the temperature
s Green OFF mode operation with automatic
turn- ON in case of overtemperature
s Voltage limitation above fan stall voltage
s No external NTC required
s Low cost integrated fan speed control
s High integration: only one or two external com-
s Reduced acoustic noise because of the linear
control of the external transistor
s Power supply noise rejection
s Good thermal coupling with heater
s Reduced ON/OFF oscillation because of large
s High current capability (base current higher
than 10 mA)
s Higher accuracy than discrete circuit with NTC
Fig. 1: Block Diagram.
The FSCTxxA-UH5 is a high-integrated low cost fan speed controller suitable for PC desktops, notebooks
and server power supplies and also all kind of equipments where low cost ventilation system is needed. An
internal thermal sensor connected to the tab is used to regulate the fan speed. A continuous analog voltage
proportional to the tab temperature is produced at the OUT terminal and can control linearly an external
PNP transistor (Fan Transistor) connected in series with a Brushless DC Fan, in order to vary its speed.
The ON control terminal can be used to select two operation modes. If not connected (high impedance
state) ON mode is selected by default.
In Mode ON, the fan speed is regulated according to the tab temperature of the FSCT by adjusting the Vbe
voltage of the PNP fan Transistor. In low temperature conditions, the fan transistor voltage is limited to
about 5.5V (for a 12V fan) in order to guarantee a minimum voltage across the fan above its stalling voltage
and to keep it running (see figure 3).
This regulation is implemented with a high Fan supply voltage rejection.
www.DatTaShheeemt4oUd.ecoOmFF allows to operate in green Mode. The fan is stopped as long as the temperature remains
below the TON threshold. When the temperature rises above TON the fan is forced to ON and the speed is
regulated according to the temperature as in the same way as mode ON. When temperature falls below
TOFF, the fan is automatically turned OFF. The hysteresis has been dimensioned large enough to reduce
ON/OFF oscillations during the green mode operation.
September 2003 - Ed: 2A