User Manual

MAX6684
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-
nal R
DSON
of the MAX6684, which is typically 1Ω.
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 6Ω R
BOOST
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
A ceramic or mylar capacitor, C
F
, is required from FC-
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
V
CC
PGNDGND
FC+
10kΩ
R1
FC-
3.3V
FAIL
12V
C
F
Figure 7. Increased Current Capability Using PNP Transistor
MAX6684
OFF
SENSE
V
CC
PGNDGND
FC+
10kΩ
R
BOOST
FC-
3.3V
FAIL
12V
C
F