FAIL Output

The FAIL output is an active-low, open-drain alarm.

Three fan-failure modes are possible (see the Fan-

Failure Detection section).

OFF

Drive OFF low to turn off power to the fan. If OFF is tied

high or floating, the MAX6684 is enabled.

Applications Information

Fan Compatibility

This device can be used with fans that require operat-

ing voltages up to 24V and supply currents up to

250mA. See the Fan-Failure Detection section regard-

ing fan-current waveform issues.

Figures 6 and 7 show two ways to increase the current

capability of the MAX6684. In Figure 6, a parallel exter-

nal resistance between SENSE and PGND is used to

increase current capability. This method eliminates the

fan-control functionality normally associated with the

MAX6684 OFF pin. Select the external resistor, R1,

such that approximately 100mA flows across the inter-

Ω.

Figure 7 also shows how to use an external current-

boost PNP bipolar transistor to increase the current

capability of the MAX6684. This method preserves the

fan-control functionality of the OFF pin. A 6

allows approximately 100mA of the fan current to flow

through the MAX6684.

The MAX6684 is not compatible with fans designed for

use with external PWM fan controllers.

Fan-Specific Concerns

Because fan-current waveforms can vary substantially

from one given fan make or model to another, validate

the performance of the MAX6684 with the intended fan.

It is possible to encounter fans where the MAX6684 is

limited to detecting locked-rotor conditions only,

because of the nature of the fan-current waveform. In

cases where fan-speed detection does not seem to be

working properly (although locked-rotor detection is tak-

ing place), adding a 100µF capacitor across the fan

may solve the problem.

When the MAX6684 is used with fans that include

locked-rotor protection, the FAIL output is active when

the rotor locks, and toggles each time the locked-rotor

protection built into the fan attempts a restart, over a

timeframe typically measured in seconds. Toggling

should be considered an indication of fan failure; con-

versely, a fan is functioning properly only when FAIL is

constantly inactive.

Capacitor Selection

to FC+. The capacitor blocks the DC component of the

signal, allowing the MAX6684 to monitor the AC current

consumption of the fan. See the Minimum Speed and

Locked-Rotor Detection section for more information.

Power Supply and Bypassing

The effects of noise can be minimized by placing a 1µF

ceramic bypass capacitor close to the device’s supply pin.

Fan-Failure Detector with Integrated

Power Switch

6

_______________________________________________________________________________________

Figure 6. Increased Current Capability Using External

Resistance

MAX6684

OFF

SENSE

PGND

GND

FC+

10k

Ω

R1

FC-

3.3V

FAIL

12V

Figure 7. Increased Current Capability Using PNP Transistor

MAX6684

OFF

SENSE

PGND

GND

FC+

10k

Ω

FC-

3.3V

FAIL

12V