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FSCTxxA-UH5

®

September 2003 - Ed: 2A

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Built in thermal sensor

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Brushless DC fan speed control

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Linear control and regulation of the fan speed

according to the temperature

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Green OFF mode operation with automatic

turn- ON in case of overtemperature

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Voltage limitation above fan stall voltage

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No external NTC required

FEATURES

FAN SPEED CONTROLER

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COMPUTER AND SERVER POWER SUPPLY

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TELECOM

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LOW COST VENTILATION (consumer)

APPLICATIONS

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Low cost integrated fan speed control

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High integration: only one or two external com-

ponents

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Reduced acoustic noise because of the linear

control of the external transistor

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Power supply noise rejection

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Good thermal coupling with heater

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Reduced ON/OFF oscillation because of large

hysteresis

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High current capability (base current higher

than 10 mA)

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Higher accuracy than discrete circuit with NTC

BENEFITS

IPPAK

1

5

GND

OUT

BIAS

F

ON/OFF

LOW

ON/OFF

HIGH

C=

100nF

OUT

HYST.-EN

HYSTERESIS

COMPARATOR

DRIVER

ON

REGULATION

VOLTAGE

REF.

TEMP.

SENSOR

ON

FSCTxxA-UH5

Fig. 1: Block Diagram.

A.S.D.

TM

Application Specific Discretes

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.

The mode OFF 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.

GENERAL DESCRIPTION