The MAX6684 from Maxim (http://pdfserv.maxim-ic.com/ arpdf/MAX6684.pdf) is a device for monitoring fans. Available in an 8-pin small outline SMD package the IC is capable of detecting if the fan is jammed or if it is turning too slowly. Although the device itself requires a supply voltage in the range +3.3 V to +5 V, it can be connected to fans operating on up to +24 V and drawing currents of up to 250 mA. An internal sense resistor to ground (PGND) is used to detect the current pulses of the fan motor and the pulses are processed according to their shape and frequency. A power MOSFET, with an on resistance of around 1 Ω, is connected 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).
In the first case, the fan remains switched on, whereas in case of b)
the device will try to start the fan again every 62 ms. If the over-current condition persists, the MAX6684 will turn off again within 2 ms and wait another 60 ms. In case (c)
also, the fan remains switched on. The lower rotation rate threshold for this case is mainly determined by the coupling capacitor between FC+ and FC-.
The value should be determined experimentally if necessary. If the fan already has internal protection against jamming it will automatically switch itself off. Then the FAIL signal is only active during the period when the fan tries to restart itself. If the fan rotation rate threshold detector does not work reliably, a 100 μF electrolytic capacitor connected in parallel with the fan may help.
The OFF input can be used to turn the fan on and off.