Rainbow Electronics MAX6651 - Manual (Page 15)

MAX6650/MAX6651
Fan-Speed Regulators and Monitors
with SMBus/I 2 C-Compatible Interface
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Full-On Mode
By occasionally (over a period of days or weeks) turning
the fan on full and measuring the resultant speed, a
failing fan can be detected by a trend of decreasing
speeds at a given power-supply voltage. Power-up is a
convenient time to measure the maximum fan speed.
Open-Loop Mode
The fan’s condition can also be monitored using open-
loop mode. By characterizing the fan while it is new,
fan failure can be determined by writing a predeter-
mined value to the DAC and measuring the resultant
fan speed. A decrease over time of the resultant speed
may be an indication of future fan failure.
Closed-Loop Mode
The MAX6650 allows the system to read the DAC value
used to regulate the fan speed. For a given speed, a
significant change in the required DAC value may indi-
cate future fan problems.
Monitoring More than 4 Fans
Use the MAX6651 to monitor up to four fans at a time
(Figure 7). For systems requiring more than four fans,
Figure 8 shows an application using an analog multi-
plexer (mux) to monitor 11 fans. GPIO2, GPIO3, and
GPIO4 are connected to the mux’s address pins. By
writing the appropriate value to the GPIO pins, the
desired tachometer gets selected and counted by the
TACH3 input. Because the TACH inputs are double-
buffered, and only sampled every other time slot, it is
important to wait at least 4 times the tachometer count
time before reading the register after changing the mux
address. In the extreme case, a total of 25 fans can be
monitored using three multiplexers connected to
TACH1, TACH2, and TACH3. Do not connect TACH0 to
a mux if the MAX6651 is under closed-loop mode.
N+1 Fan Application
As shown in Figure 9 , if any MAX6650 cannot maintain
speed regulation, all other fans will automatically be
turned on full. This can be useful in high-reliability sys-
tems where any single fan failure should not cause
downtime. The system should be designed so that the
number of fans used is one more than are actually
needed. This way, there is sufficient cooling even if a
fan fails. With all fans operating correctly, it is unneces-
sary to run the fans at their maximum speed. Reducing
fan speed can reduce noise and increase the life of the
fans. However, once a fan fails, it is important that the
remaining fans spin at their maximum speed.
In Figure 9, all the GPIO0s are configured as ALERT
outputs, and all the GPIO1s are configured as
FULL ON inputs. If any MAX6650 generates an ALERT
(indicating failure), the remaining MAX6650s will auto-
matically turn their fans on full.
MAX6651
TACH0
TACH1
TACH2
TACH3
GPIO4
GPIO3
GPIO2
MAX4051
V CC
3V OR 5.5V
COM
ADDA
ADDB
ADDC
INH
GND V-
NO0
NO1
NO2
NO3
NO4
NO5
NO6
FAN TACH 4
FAN TACH 9
FAN TACH 5
FAN TACH 6
FAN TACH 7
FAN TACH 8
FAN TACH 1
FAN TACH 2
FAN TACH 3
FAN TACH 10
NO7 FAN TACH 11
TO FAN VOLTAGE
5V OR 12V
Figure 8. Monitoring Multiple Fans