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 Detector
♦Works with Ordinary 2-Wire Fans
♦No Fan Tachometer Output Required
♦No Software Development Required
♦No Analog Circuit Design Required
♦Logic-Level Fan Driver Control
♦Works with Fans Rated Up to 24V/250mA
MAX6684
Fan-Failure Detector with Integrated
Power Switch
________________________________________________________________ Maxim Integrated Products 1
VCC
FC+FC-
1
2
8
7
PGND
OFFFAIL
GND
SENSE
SO
TOP VIEW
3
4
6
5
MAX6684
Pin Configuration
Ordering Information
MAX6684
OFF
SENSE
VCC
12V
PGND
GND
FC+
10kΩ
FC-
3.3V
1µF
0.1µF
0.1µF
FAIL
Typical Operating Circuit
19-2306; Rev 0; 1/02
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
PART TEMP RANGE PIN-PACKAGE
MAX6684ESA -40°C to +85°C 8 SO
MAX6684
Fan-Failure Detector with Integrated
Power Switch
2 _______________________________________________________________________________________
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 1: Specifications to -40°C are guaranteed by design and not production tested.
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
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Voltage VCC 3.0 5.5 V
Supply Current ICC IFAN = 300mA 3.4 mA
VCC Shutdown Supply Current ISHDN OFF = GND 10 µA
SENSE-to-PGND Output
Low Voltage IFAN = 300mA 0.3 0.66 V
SENSE-to-PGND Output
On-Resistance RDSON 1 2.2 Ω
SENSE Leakage Current VSENSE = 26V 1 10 µA
V FAIL Output Low Voltage I FAIL = 3mA 0.8 V
V FAIL Output Leakage Current V FAIL = 5.5V 0.1 1 µA
Average SENSE (Fan) Current 50 300 mA
SENSE Current Shutdown VCC = 3V 600 1200 mA
Thermal Shutdown of SENSE 15°C hysteresis 160 °C
Fan-Current Fluctuation
Frequency No fault detected 25 400 Hz
V FAIL Output Delay After Fault tFD 0.3 1 2.0 s
Minimum Fan-Current Fluctuation
Level (Note 2) No fault detected 15 35 60 mAP-P
OFF Input High Voltage VIH 0.7 x
VCC V
OFF Input Low Voltage VIL 0.3 x
VCC V
OFF Input Current -10 0 1 µA
MAX6684
Fan-Failure Detector with Integrated
Power Switch
_______________________________________________________________________________________ 3
RDSON vs. TEMPERATURE
MAX6684 toc01
TEMPERATURE (°C)
RDSON (Ω)
603510-15
0.4
0.8
1.2
1.6
2.0
0
-40 85
VCC = 3V
ISENSE = 300mA
VCC = 5.5V
SUPPLY CURRENT vs. TEMPERATURE
MAX6684 toc02
TEMPERATURE (°C)
SUPPLY CURRENT (µA)
603510-15
225
250
275
300
200
-40 85
VCC = 3V
NO LOAD
VCC = 5.5V
OVERCURRENT OPERATION
MAX6684 toc03
20ms/div
0
0A
ISENSE
500mA/div
VFAIL
5V/div
VSENSE
2V/div
0
Typical Operating Characteristics
(VCC = 3.3V, TA= +25°C, unless otherwise noted.)
Pin Description
PIN NAME FUNCTION
1 SENSE Positive Current-Sensing Terminal. Connect SENSE to low side of fan.
2FAIL Active-Low, Open-Drain Fan-Failure Output
3 GND Ground
4 FC- Connect to 0.1µF capacitor for most locked-rotor detection applications. To detect minimum speed,
select CF according to Minimum Speed and Locked-Rotor Detection.
5 FC+ Connect to 0.1µF capacitor for most locked-rotor detection applications. To detect minimum speed,
select CF according to Minimum Speed and Locked-Rotor Detection.
6V
CC Supply Voltage Input. Bypass VCC to GND with a 1µF capacitor.
7OFF Acti ve- Low Fan- C ontr ol Inp ut. D r i ve O FF hi g h or l eave fl oati ng to tur n fan on. D r i ve O FF l ow to tur n fan off.
8 PGND Power Ground. Connect to GND.
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
4 _______________________________________________________________________________________
FC- BLANK
NOISE SQ
R
CURRENT-SENSE AND
CURRENT-LIMITING
CIRCUIT
OFF
FC+
LATCH
COMP
8.2kHz 8Hz
OSCILLATOR FAULT-
DETECTION
DELAY
VCC
SENSE
GND PGND
FAIL
VFAN
70mV
CF
MAX6684
Figure 1. MAX6684 Functional Diagram
NORMAL
FAN OPERATION
UNDERCURRENT*
UNDERCURRENT*
NORMAL
CURRENT
OVERCURRENT NO THERMAL
SHUTDOWN
THERMAL
SHUTDOWN
OVERCURRENT
OVERCURRENT
OVERCURRENT
OVERCURRENT
*INVALID COMMUTATION CURRENT
NOTE: A THERMAL SHUTDOWN CONDITION OVERIDES ALL OTHER CONDITIONS,
IMMEDIATELY SHUTTING THE FAN OFF AND SIGNALING FAIL.
THERMAL SHUTDOWN THERMAL SHUTDOWN
NORMAL
CURRENT
FAN ON,
FAIL HIGH
2ms DELAY
FAN ON,
FAIL LOW
FAN OFF,
FAIL LOW
2ms DELAY
1s DELAY 60ms DELAY
NORMAL
CURRENT
Figure 2. MAX6684 State Diagram
Undercurrent (AC Component)
Fan failure is signaled if the AC component of IFAN is
less than 35mAP-P and 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 FAIL if 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 5 are 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 Operating Char-
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 FAIL output 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
_______________________________________________________________________________________ 5
Figure 3. MAX6684 Commutation Fault Timing Diagram
FAIL
100Hz
tFD
ƒ
Figure 4. Current Fluctuation and Commutation Frequency
Diagram
FAIL HIGH
FAIL LOW
0
10
20
30
40
50
60
70
80
0 100 200 300 400
CURRENT FLUCTUATION
vs. COMMUTATION FREQUENCY
CURRENT COMMUTATION FREQUENCY (Hz)
CURRENT FLUCTUATION
(AC COMPONENT) (mAP-P)
MAX6684
OFF
SENSE
APPROXIMATE FAILURE FREQUENCIES:
VCC
FUNCTION
GENERATOR*
*35mAP-P SINE-WAVE AC COMPONENT
50mA TO 300mA DC COMPONENT
PGND
GND
FC+
CF = 0.033µF < 25Hz
CF = 0.01µF < 86Hz
CF = 0.003µF < 250Hz
10kΩ
FC-
3.3V
FAIL
CF
Figure 5. Test Circuit Demonstrates Failure Frequency as a
Function of the Value of CF
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 RDSON 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 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 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, 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 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
VCC
PGNDGND
FC+
10kΩ
R1
FC-
3.3V
FAIL
12V
CF
Figure 7. Increased Current Capability Using PNP Transistor
MAX6684
OFF
SENSE
VCC
PGNDGND
FC+
10kΩ
RBOOST
FC-
3.3V
FAIL
12V
CF
MAX6684
Fan-Failure Detector with Integrated
Power Switch
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________ 7
© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
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Chip Information
TRANSISTOR COUNT: 3993
PROCESS: BiCMOS
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