Motor speed control circuit and control method thereof

A motor speed control circuit, comprising:

• a voltage-dividing module, comprising a first resistor unit and a second resistor unit, wherein a first terminal of said first resistor unit receives a supply voltage, a first terminal of said second resistor unit is coupled to a second terminal of said first resistor unit;
• a first analog-to-digital converter, coupled to a first terminal of said first resistor unit, for receiving said supply voltage, and converting said supply voltage into a digital supply voltage;
• a second analog-to-digital converter, coupled to a first terminal of said second resistor unit, for receiving a divided voltage generated by said voltage-dividing module, and converting said divided voltage into a digital divided voltage, wherein said divided voltage is associated with a resistance ratio between said first resistor unit and said second resistor unit;
• and
• an operation module, coupled to said first analog-to-digital converter and said second analog-to-digital converter, said operation module receiving said digital divided voltage and determining a motor speed curve according to said resistance ratio, said operation module generating a first pulse width modulation signal according to said motor speed curve and said digital supply voltage to drive a motor, wherein said motor speed curve is associated with said supply voltage and a rotational speed of said motor;
• wherein said operation module selects one from a plurality of motor speed curves according to said resistance ratio, and said operation module calculates said rotational speed according to an equation corresponding to said one from said plurality of motor speed curves, wherein said motor speed curves have different slopes associated with said resistance ratio;
• wherein said operation module is further coupled to a pulse width modulation signal generation unit, said pulse width modulation signal generation unit outputs an input pulse width modulation signal to said operation module, wherein a switching unit of said operation module switches said operation module to a pulse width driven (PW-driven) mode or a voltage-driven mode according to a logic level of said input pulse width modulation signal.