Fan Monitoring with MAX6684
The MAX6684, designed by Maxim and detailed at http://pdfserv.maxim-ic.com/arpdf/MAX6684.pdf, serves as a dedicated device for Fan Monitor. Encased in an 8-pin small outline SMD package, this IC possesses the capability to identify potential issues such as fan jams or slow rotations. While the IC itself requires a supply voltage ranging from +3.3 V to +5 V, it can be seamlessly connected to fans operating at up to +24 V and drawing currents of up to 250 mA. Internally, a sense resistor to ground (PGND) is incorporated to detect the current pulses generated by the fan motor. These pulses are then processed based on their shape and frequency. To control these operations, a power MOSFET, featuring an on-resistance of approximately 1 Ω, is linked between SENSE and PGND.
The open drain FAIL signal goes low to indicate a failure condition when:
(a) the current drawn by the fan falls below 35 mApp (AC component);
(b)the current drawn by the fan rises above 600 mA;
(c) the fan rotation rate falls below about 700 revolutions per minute (25 Hz AC component);
(d)the die temperature of the chip rises above +160 °C (overtemperature protection).
Fan Control Scenarios with MAX6684
Continuous Fan Operation and Restart Attempts
In the first scenario (a), the fan remains continuously operational. Conversely, in scenario (b), the MAX6684 attempts to restart the fan every 62 ms if it encounters a problem. If the over-current situation persists, the MAX6684 will deactivate within 2 ms and then wait for another 60 ms before making a subsequent attempt.
Fan Operation with Rotation Rate Threshold
In scenario (c), the fan continues to operate. The lower threshold for rotation rate in this case is chiefly influenced by the coupling capacitor between FC+ and FC-. Its specific value needs to be determined experimentally if required. If the fan possesses built-in protection against jamming, it will shut off automatically, causing the FAIL signal to activate only during the fan’s self-restart attempts. If the fan’s rotation rate detection is unreliable, incorporating a 100 μF electrolytic capacitor in parallel with the fan could enhance the situation.
Control Options Using OFF Input
The OFF input provides the capability to toggle the fan’s operation, allowing for convenient on and off control.