Temperature compensated

Fan Control IC with Over-temperature Output Schematic Circuit Diagram

Proportional Fan Controller using MIC502 IC

A fan controller can be easily designed using the MIC502 from Micrel. This integrated circuit enables the fan to operate at lower speeds during low temperatures, reducing both noise and wear. The pulse width modulated output signal can control any fan through a driver transistor.

Fan Control IC with Over-temperature Output Schematic Circuit Diagram

Advantages of PWM Control

Utilizing PWM control offers the benefit of running the fan at much slower speeds compared to variable DC control methods. The MIC502 allows the connection of up to two NTC thermistors, providing flexibility in temperature control. Alternatively, the second control voltage can be obtained from the DAC output of a processor system.

Operating Range and Voltage Control

The MIC502 operates within a supply range of 4.5 V to 13.2 V (Vcc). Due to the drive mechanism via a transistor, the fan voltage can exceed the supply voltage, enabling the control of a 12 V fan from a 5 V controller. The control mechanism is independent of the supply voltage as it relies on the ratio of voltages at inputs VT1 and VT2, ensuring ‘ratiometric’ operation.

Precise Fan Speed Control

The control process is fine-tuned, where a voltage of 0.3 Vcc results in a 0% output duty cycle, stopping the fan completely. Conversely, applying 0.7 Vcc to inputs VT1 and/or VT2 triggers a 100% output duty cycle, causing the fan to operate at maximum speed. The input with the higher voltage (indicating the higher temperature) takes precedence in the control process.

Setting Fan Sleep Mode and Wake-Up Conditions

The VSLP input serves to establish the voltage threshold below which the fan enters sleep mode. To activate sleep mode, both inputs VT1 and VT2 must remain below this voltage level. Fan operation resumes if either VT1 or VT2 exceeds VSLP + 0.12 Vcc. When sleep mode is unnecessary, the VSLP input should be connected to the ground.

Control of PWM Signal Frequency

A capacitor connected to the CF pin determines the base frequency of the PWM signal. It is advisable to employ a 100 nF capacitor, resulting in a frequency of approximately 30 Hz. During power-up or when transitioning from sleep mode, an integrated start-up timer ensures the fan motor receives full voltage for a duration of 64/f (around 2 seconds at 30 Hz), ensuring a dependable start.

Over-Temperature Protection

Additionally, the device provides an open-collector over-temperature output, OTF (‘over-temperature failure’), which can be pulled up to the desired logic level using an external resistor. OTF switches to a low state if either VT1 or VT2 exceeds 0.75 Vcc, indicating an over-temperature condition.


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