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MAX6684ESAMAXIM ?N/a25avaiFan-Failure Detector with Integrated Power Switch


MAX6684ESA ,Fan-Failure Detector with Integrated Power SwitchELECTRICAL CHARACTERISTICS(V = 3.0 to 5.5V, OFF = V , T = -40°C to +85°C, unless otherwise noted. T ..
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MAX6684ESA
Fan-Failure Detector with Integrated Power Switch
General Description
The MAX6684 is an integrated fan-failure detector that
detects when a fan exhibits excessive underspeed or a
locked rotor. This device is especially well suited for criti-
cal systems where no fan control, or simple on/off control
is desired. The MAX6684 detects fan failure by evaluat-
ing fluctuations in current at the low side of the fan; no
tachometer signal is necessary. The output of the device,
FAIL, is an active-low, open-drain alarm. The MAX6684
can also be used to switch the fan on or off, based on
the state of a logic-level input, OFF.This device can be
used with fans rated at up to 24V and 250mA. The
MAX6684 is available in an 8-pin SO package, and is
specified for operation from -40°C to +85°C.
Applications

Desktop PCs
Notebooks
Networking Equipment
Telecommunications
Industrial Applications
Features
Dedicated Fan-Failure DetectorWorks with Ordinary 2-Wire FansNo Fan Tachometer Output RequiredNo Software Development RequiredNo Analog Circuit Design RequiredLogic-Level Fan Driver ControlWorks with Fans Rated Up to 24V/250mA
MAX6684
Fan-Failure Detector with Integrated
Power Switch
Pin Configuration
Ordering Information
Typical Operating Circuit

19-2306; Rev 0; 1/02
MAX6684
Fan-Failure Detector with Integrated
Power Switch
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS

(VCC= 3.0 to 5.5V, OFF= VCC, TA= -40°C to +85°C, unless otherwise noted. Typical values are at VCC= 3.3V, TA= +25°C.) (Note 1)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Note 2:
The MAX6684 is guaranteed to register a fault when the fan current fluctuates less than the minimum; it is guaranteed not to
register a fault when the fan current is above the maximum.
Voltages Referenced to GND, Unless Otherwise Noted
VCC........................................................................-0.3V to +6.0V
FC+, FC-.....................................................-0.3V to (VCC+ 0.3V)
OFF, FAIL..............................................................-0.3V to +6.0V
PGND....................................................................-0.3V to +0.3V
SENSE to PGND..................................................-0.3V to +28.0V
SENSE Current................................................................1400mA
Continuous Power Dissipation (TA= +70°C)
8-Pin SO (derate 5.9mW/°C above +70°C)..................470mW
Operating Temperature Range...........................-40°C to +85°C
Storage Temperature Range.............................-65°C to +150°C
Junction Temperature......................................................+150°C
Soldering Temperature (vapor phase, 60s).....................+215°C
Soldering Temperature (infrared, 15s).............................+220°C
MAX6684
Fan-Failure Detector with Integrated
Power Switch
RDSON vs. TEMPERATURE

MAX6684 toc01
TEMPERATURE (°C)
DSON 3510-15
SUPPLY CURRENT vs. TEMPERATURE
MAX6684 toc02
TEMPERATURE (°C)
SUPPLY CURRENT (3510-15
OVERCURRENT OPERATION
MAX6684 toc03
20ms/div
ISENSE
500mA/div
VFAIL
5V/div
VSENSE
2V/div
Typical Operating Characteristics

(VCC= 3.3V, TA= +25°C, unless otherwise noted.)
Pin Description
MAX6684
Detailed Description

The MAX6684 detects fan failure in brushless DC fans.
This device is especially well suited for critical systems
where no fan control is desired. No software is neces-
sary to control the MAX6684.
Fan-Failure Detection

Fan failure is determined based on the fan current
observed at SENSE. The current observed at SENSE is
converted to a voltage, VFAN, and highpass filtered by
the capacitor, CF, from FC+ to FC- (Figure 1).
Fan-Failure Detector with Integrated
Power Switch

Figure 1. MAX6684 Functional Diagram
Figure 2. MAX6684 State Diagram
Undercurrent (AC Component)
Fan failure is signaled if the AC component of IFANis
less than 35mAP-Pand remains out of specification for
at least 1s (Figure 2). The fan remains powered during
undercurrent failures.
Minimum Speed and
Locked-Rotor Detection

The MAX6684 asserts FAILif the fan-current fluctuation
frequency is below 25Hz, which corresponds to a fan
speed of approximately 700rpm. The fan remains pow-
ered during a locked rotor or an under-speed failure
condition (Figures 3 and 4).
The MAX6684 can be designed to detect fan failure
below intended speeds by varying the value of CF.
Because of the complexity of fan-current waveforms,
the value of CFhas to be arrived at empirically and
must be verified by bench testing. The guidelines of
Figure 5are only appropriate for the test signals used
and do not represent all possible fan waveforms. They
are to illustrate the ability of the MAX6684 to discrimi-
nate failure due to low fan speed. As a rule, failure typi-
cally occurs when the amplitude measured at pin 4 of
the MAX6684 drops below 70mV.
Overcurrent Protection

If an overcurrent condition begins and continues for 2ms,
fan failure is signaled for 60ms. During this 60ms period,
the power to the fan is turned off. If the part does not enter
thermal shutdown and the overcurrent condition contin-
ues, power to the fan is turned on every 62ms for 2ms
(see Overcurrent Operation in Typical OperatingChar-
acteristics). Once the overcurrent condition is removed,
the fan is powered continuously. A 0.1µF capacitor
between SENSE and PGND prevents the internal DMOS
switch from being damaged by back EMF current.
Thermal Shutdown

A die temperature in excess of +160°C initiates thermal
shutdown. In thermal shutdown, the MAX6684 shuts off
the fan and the FAILoutput asserts. While in thermal
shutdown, the MAX6684 monitors the die temperature.
Once the die has cooled to below +145°C, the MAX6684
exits thermal shutdown and power is returned to the fan.
A thermal shutdown fault condition has precedence over
all other failure modes. While the MAX6684 die is over
temperature, power is not cycled to the fan, as occurs
during overcurrent failure.
MAX6684
Fan-Failure Detector with Integrated
Power Switch
MAX6684
FAILOutput

The FAILoutput is an active-low, open-drain alarm.
Three fan-failure modes are possible (see the Fan-
Failure Detectionsection).
OFF

Drive OFFlow to turn off power to the fan. If OFFis 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 Detectionsection regard-
ing fan-current waveform issues.
Figures 6 and 7show two ways to increase the current
capability of the MAX6684. InFigure 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 OFFpin. Select the external resistor, R1,
such that approximately 100mA flows across the inter-
nal RDSONof the MAX6684, which is typically 1Ω.
Figure 7also 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 OFFpin. A 6ΩRBOOST
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 FAILoutput 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 FAILis
constantly inactive.
Capacitor Selection

A ceramic or mylar capacitor, CF, 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 Detectionsection 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
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