Rotating speed adjustment circuit and related control system for a heat dissipation fan

A fan control system for a heat dissipation fan comprising:

• a temperature-based rotating speed control circuit for sensing a temperature of the heat dissipation fan to generate a first temperature control signal;
• a pulse width modulation (PWM) rotating speed control circuit for receiving a system control signal to generate a second temperature control signal;
• an AND gate coupled to the temperature-based rotating speed control circuit and the PWM rotating speed control circuit, for performing an AND operation on the first temperature control signal and the second temperature control signal, so as to generate a first control signal;
• a rotating speed adjustment circuit comprising:
• a first node;
• a second node;
• a reception end coupled to the AND gate for receiving the first control signal;
• a first resistor coupled between a voltage source and the first node;
• a second resistor coupled between the first node and the second node;
• a third resistor coupled between the second node and a ground end;
• a capacitor coupled between the first node and the ground end;
• a transistor comprising a first end coupled to the reception end, a second end coupled to the second node, and a third end coupled to the ground end, for controlling a signal connection from the second end to the third end according to a signal of the first end;
• a first oscillator for generating a first oscillating signal;
• and
• a first comparator comprising a first input end coupled to the first node, a second input end coupled to the first oscillator and an output end, for comparing signals of the first node and the first oscillating signal, so as to output a second control signal via the output end;
• and
• a logic circuit coupled to the output end of the first comparator and the heat dissipation fan, for driving the heat dissipation fan according to the second control signal.