Rainbow Electronics MAX6651 - Manual (Page 14)

MAX6650/MAX6651
Fan-Speed Regulators and Monitors
with SMBus/I
2 C-Compatible Interface
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been selected when no ringing is present on the volt-
age at the fan’s low side.
Fan Selection
For closed-loop operation and fan monitoring, the
MAX6650/MAX6651 require fans with tachometer outputs.
A tachometer output is typically specified as an option on
many fan models from a variety of manufacturers. Verify
the nature of the tachometer output (open collector, totem-
pole) and the resultant levels, and configure the connec-
tion to the MAX6650/MAX6651 accordingly. Note how
many pulses per revolution are generated by the
tachometer output (this varies from model to model and
among manufacturers, though two pulses per revolution is
the most common).
Table 10 lists the representative fan manufacturers and
the models they make available with tachometer outputs.
Low-Speed Operation
Brushless DC fans increase reliability by replacing
mechanical commutation with electronic commutation. By
lowering the voltage across the fan to reduce its speed,
the MAX6650/MAX6651 are also lowering the supply volt-
age for the electronic commutation and tachometer elec-
tronics. If the voltage supplied to the fan is lowered too
far, the internal electronics may no longer function prop-
erly. Some of the following symptoms are possible:
• The fan may stop spinning.
• The tachometer output may stop generating a signal.
• The tachometer output may generate more than two
pulses per revolution.
The problems that occur, and the supply voltages at
which they occur, depend on which fan is used. As a
very rough rule of thumb, 12V fans can be expected to
experience problems somewhere around 1/4 to 1/2
their rated speed.
Predicting Future Fan Failure
In systems that require maximum reliability, such as
servers and network equipment, it can be advantageous
to predict fan failure before it actually happens, to alert
the system operator before the fan fails, minimizing down
time. The MAX6650 allows the user to monitor the fan’s
condition through the following modes.
Figure 7. Using the MAX6651 to Control Parallel Fans
MAX6651
V CC
SCL
10k
10k
10k
SDA
GPIO0 OUT
ADD
GND
FB
TACH0
TACH1
TACH2
V CC
3V TO 5.5V
V FAN 5V OR 12V
C COMP
I 2 C/SMBus
INTERFACE
GPIO1
FAN
0
FAN
1
FAN
2
FULL ON
ALERT
MANUFACTURER FAN MODEL OPTION
Comair Rotron
All DC brushless models can be
ordered with optional tachometer
output.
EBM-Papst
Tachometer output optional on some
models.
NMB
All DC brushless models can be
ordered with optional tachometer
output.
Panasonic
Panaflo and flat unidirectional
miniature fans can be ordered with
tachometer output.
Sunon
Tachometer output optional on some
models.
Table 10. Fan Manufacturers