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. September 2003 - Ed: 2A