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MAX6684ESA+T

P1-P3P4-P6P7-P7
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, V
FAN
, and highpass filtered by
the capacitor, C
F
, 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
V
CC
SENSE
GND PGND
FAIL
V
FAN
70mV
C
F
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 I
FAN
is
less than 35mA
P-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 C
F
.
Because of the complexity of fan-current waveforms,
the value of C
F
has 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
t
FD
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) (mA
P-P
)
MAX6684
OFF
SENSE
APPROXIMATE FAILURE FREQUENCIES:
V
CC
FUNCTION
GENERATOR*
*35mA
P-P
SINE-WAVE AC COMPONENT
50mA TO 300mA DC COMPONENT
PGND
GND
FC+
C
F
= 0.033µF < 25Hz
C
F
= 0.01µF < 86Hz
C
F
= 0.003µF < 250Hz
10kΩ
FC-
3.3V
FAIL
C
F
Figure 5. Test Circuit Demonstrates Failure Frequency as a
Function of the Value of C
F
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
P1-P3P4-P6P7-P7

MAX6684ESA+T

Mfr. #:
Buy MAX6684ESA+T
Manufacturer:
Maxim Integrated
Description:
Motor / Motion / Ignition Controllers & Drivers Fan-Failure Detector
Lifecycle:
New from this manufacturer.
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