Fan driving circuit

A driving circuit for a fan, comprising:

• a digital-analog converting circuit configured for converting a Pulse-Width Modulation (PWM) signal from a controlling integrated circuit (IC) into an analog signal;
• a voltage sampling circuit coupled to the digital-analog converting circuit and configured for receiving the analog signal and outputting a sampled analog signal;
• an amplifier circuit having an input terminal coupled to the voltage sampling circuit for receiving the sampled analog signal, and an output terminal for outputting an amplified analog signal;
• and
• a controlling circuit comprising an isolation diode, a transistor, and a first resistor, an anode of the isolation diode coupled to the output terminal of the amplifier circuit, a cathode of the isolation diode coupled to a base of the transistor, the base of the transistor coupled to a power supply via the first resistor, a collector of the transistor coupled to the power supply, an emitter of the transistor coupled to the fan such that when the duty cycle of the PWM signal becomes too low and unable to drive the amplifier circuit, voltage from the power supply is capable of keeping the transistor on to thereby allow the power supply keeping the fan rotating at a minimum speed.