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19-2306; Rev 0; 1/02
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 critical systems where no fan control, or simple on/off control is desired. The MAX6684 detects fan failure by evaluating 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 -40C to +85C. o Dedicated Fan-Failure Detector o Works with Ordinary 2-Wire Fans o No Fan Tachometer Output Required o No Software Development Required o No Analog Circuit Design Required o Logic-Level Fan Driver Control o Works with Fans Rated Up to 24V/250mA
Features
MAX6684
Applications
Desktop PCs Notebooks Networking Equipment Telecommunications Industrial Applications
PART MAX6684ESA
Ordering Information
TEMP RANGE -40C to +85C PIN-PACKAGE 8 SO
Typical Operating Circuit
3.3V
Pin Configuration
TOP VIEW
12V VCC OFF 10k
SENSE 1 FAIL 2
8 7
PGND OFF VCC FC+
MAX6684
FC+ 1F 0.1F FCGND SENSE PGND 0.1F FAIL
MAX6684
GND 3 6 5 FC- 4
SO
________________________________________________________________ Maxim Integrated Products
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Fan-Failure Detector with Integrated Power Switch MAX6684
ABSOLUTE MAXIMUM RATINGS
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 = +70C) 8-Pin SO (derate 5.9mW/C above +70C)..................470mW Operating Temperature Range ...........................-40C to +85C Storage Temperature Range .............................-65C to +150C Junction Temperature ......................................................+150C Soldering Temperature (vapor phase, 60s).....................+215C Soldering Temperature (infrared, 15s).............................+220C
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.
ELECTRICAL CHARACTERISTICS
(VCC = 3.0 to 5.5V, OFF = VCC, TA = -40C to +85C, unless otherwise noted. Typical values are at VCC = 3.3V, TA = +25C.) (Note 1)
PARAMETER Supply Voltage Supply Current VCC Shutdown Supply Current SENSE-to-PGND Output Low Voltage SENSE-to-PGND Output On-Resistance SENSE Leakage Current V FAIL Output Low Voltage V FAIL Output Leakage Current Average SENSE (Fan) Current SENSE Current Shutdown Thermal Shutdown of SENSE Fan-Current Fluctuation Frequency V FAIL Output Delay After Fault Minimum Fan-Current Fluctuation Level (Note 2) OFF Input High Voltage OFF Input Low Voltage OFF Input Current VIH VIL -10 0 tFD No fault detected VCC = 3V 15C hysteresis No fault detected 25 0.3 15 0.7 x VCC 0.3 x VCC 1 1 35 RDSON VSENSE = 26V I FAIL = 3mA V FAIL = 5.5V 50 600 160 400 2.0 60 0.1 SYMBOL VCC ICC ISHDN IFAN = 300mA OFF = GND IFAN = 300mA 0.3 1 1 CONDITIONS MIN 3.0 TYP MAX 5.5 3.4 10 0.66 2.2 10 0.8 1 300 1200 UNITS V mA A V A V A mA mA C Hz s mAP-P V V A
Note 1: Specifications to -40C 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.
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Fan-Failure Detector with Integrated Power Switch
Typical Operating Characteristics
(VCC = 3.3V, TA = +25C, unless otherwise noted.)
MAX6684
RDSON vs. TEMPERATURE
MAX6684 toc01
SUPPLY CURRENT vs. TEMPERATURE
MAX6684 toc02
OVERCURRENT OPERATION
ISENSE 500mA/div 0A
MAX6684 toc03
2.0 VCC = 3V 1.6
300
RDSON ()
1.2
SUPPLY CURRENT (A)
275
VCC = 5.5V
250
0.8 VCC = 5.5V 0.4 ISENSE = 300mA 0 -40 -15 10 35 60 85 TEMPERATURE (C)
VFAIL 5V/div VSENSE 2V/div NO LOAD
0
225 VCC = 3V 200 -40 -15 10 35 60 85
0
20ms/div
TEMPERATURE (C)
Pin Description
PIN 1 2 3 4 5 6 7 8 NAME SENSE FAIL GND FCFC+ VCC OFF PGND FUNCTION Positive Current-Sensing Terminal. Connect SENSE to low side of fan. Active-Low, Open-Drain Fan-Failure Output Ground Connect to 0.1F capacitor for most locked-rotor detection applications. To detect minimum speed, select CF according to Minimum Speed and Locked-Rotor Detection. Connect to 0.1F capacitor for most locked-rotor detection applications. To detect minimum speed, select CF according to Minimum Speed and Locked-Rotor Detection. Supply Voltage Input. Bypass VCC to GND with a 1F capacitor. Active-Low Fan-Control Input. Drive OFF high or leave floating to turn fan on. Drive OFF low to turn fan off. Power Ground. Connect to GND.
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Fan-Failure Detector with Integrated Power Switch MAX6684
VCC
MAX6684
SENSE CURRENT-SENSE AND CURRENT-LIMITING CIRCUIT
VFAN
OSCILLATOR 8.2kHz 8Hz
OFF FC+ CF FC-
FAULTDETECTION DELAY
FAIL
70mV
COMP
NOISE BLANK
R S LATCH Q
GND
PGND
Figure 1. MAX6684 Functional Diagram
NORMAL FAN OPERATION
UNDERCURRENT* NORMAL CURRENT 1s DELAY
FAN ON, FAIL HIGH
60ms DELAY OVERCURRENT THERMAL SHUTDOWN
NORMAL UNDERCURRENT* CURRENT
NORMAL CURRENT
NO THERMAL SHUTDOWN OVERCURRENT
2ms DELAY
OVERCURRENT FAN ON, FAIL LOW FAN OFF, FAIL LOW OVERCURRENT 2ms DELAY THERMAL SHUTDOWN
THERMAL SHUTDOWN
OVERCURRENT
*INVALID COMMUTATION CURRENT NOTE: A THERMAL SHUTDOWN CONDITION OVERIDES ALL OTHER CONDITIONS, IMMEDIATELY SHUTTING THE FAN OFF AND SIGNALING FAIL.
Figure 2. MAX6684 State Diagram
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 necessary to control the MAX6684.
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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).
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Fan-Failure Detector with Integrated Power Switch MAX6684
100Hz
tFD
3.3V
VCC OFF
10k FUNCTION GENERATOR*
FAIL
MAX6684
FC+ CF FCSENSE PGND FAIL
Figure 3. MAX6684 Commutation Fault Timing Diagram
GND
CURRENT FLUCTUATION vs. COMMUTATION FREQUENCY
80 70 CURRENT FLUCTUATION (AC COMPONENT) (mAP-P) 60 50 40 30 20 10 0 0 100 200 300 400 CURRENT COMMUTATION FREQUENCY (Hz) FAIL LOW FAIL HIGH
*35mAP-P SINE-WAVE AC COMPONENT 50mA TO 300mA DC COMPONENT
APPROXIMATE FAILURE FREQUENCIES: CF = 0.033F < 25Hz CF = 0.01F < 86Hz CF = 0.003F < 250Hz
Figure 5. Test Circuit Demonstrates Failure Frequency as a Function of the Value of CF
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 discriminate failure due to low fan speed. As a rule, failure typically 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 continues, power to the fan is turned on every 62ms for 2ms (see Overcurrent Operation in Typical Operating Characteristics). Once the overcurrent condition is removed, the fan is powered continuously. A 0.1F 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 +160C 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 +145C, 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.
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Figure 4. Current Fluctuation and Commutation Frequency 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 powered 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 C F. Because of the complexity of fan-current waveforms, the value of CF has to be arrived at empirically and must be verified by bench testing. The guidelines of
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Fan-Failure Detector with Integrated Power Switch MAX6684
3.3V 12V VCC OFF 10k OFF VCC 10k 3.3V 12V
MAX6684
FC+ CF FCGND SENSE PGND R1 FAIL CF FCFC+
MAX6684
FAIL RBOOST SENSE PGND GND
Figure 6. Increased Current Capability Using External Resistance
Figure 7. Increased Current Capability Using PNP Transistor
FAIL Output
The FAIL output is an active-low, open-drain alarm. Three fan-failure modes are possible (see the FanFailure Detection section).
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 taking place), adding a 100F 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; conversely, a fan is functioning properly only when FAIL is constantly inactive.
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 operating voltages up to 24V and supply currents up to 250mA. See the Fan-Failure Detection section regarding fan-current waveform issues. Figures 6 and 7 show two ways to increase the current capability of the MAX6684. In Figure 6, a parallel external 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 internal RDSON of the MAX6684, which is typically 1. Figure 7 also shows how to use an external currentboost PNP bipolar transistor to increase the current capability of the MAX6684. This method preserves the fan-control functionality of the OFF pin. A 6 RBOOST 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.
Capacitor Selection
A ceramic or mylar capacitor, CF, is required from FCto 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 1F ceramic bypass capacitor close to the device's supply pin.
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Fan-Failure Detector with Integrated Power Switch
Chip Information
TRANSISTOR COUNT: 3993 PROCESS: BiCMOS
MAX6684
Package Information
9LUCSP, 3x3.EPS
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 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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