Rainbow Electronics MAX6661 - Manual (Page 18)

MAX6661
The 83.4Hz value is out of the 33Hz to 66Hz decre-
ment/increment range.
Set bits in the FTD register to divide the signal down
within the 33Hz to 66Hz range. Bits 1, 0 = 10 (divide
by 2: P = 2). The result is 83.4Hz/2 = 41.7Hz.
4) Set the FS register to yield approximately 42Hz:
42Hz = 8415Hz / FS (value)
FS (value)
≈ 200
FS register = 11001000
5) In current-sense feedback, a pulse is generated
whenever there is a step increase in fan current. The
frequency of pulses is then not only determined by
the fan rpms and the number of poles, but also by
the update rate at which the fan driver forces an
increase in voltage across the fan.
The maximum pulse frequency is then given by:
f C Hz = f TACH ✕ P / (P-1)
Where f = (rpm/60) ✕ poles and P is the value in
FTD.
The value required for the fan FS register is:
FS = 8415Hz / (f TACH / (P-1))
The fan speed limit in FTPL should be set to:
f L = 8415Hz / (N ✕ f TACH )
A value of P = 1 cannot be used in current-sense
mode.
Fan Selection
For closed-loop operation and fan monitoring, the
MAX6661 requires 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
connection to the MAX6661. For a fan with an open-
drain/collector output, a pullup resistor of typically 5k
Ω
must be connected between TACH IN and V FAN . 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 revolu-
tion are the most common). Table 12 lists the represen-
tative fan manufacturers and the model 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 MAX6661 is also lowering the supply volt-
age for the electronic commutation and tachometer
electronics. If the voltage supplied to the fan is lowered
too far, the internal electronics may no longer function
properly. 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
rule of thumb, 12V fans can be expected to experience
problems somewhere around 1/4 and 1/2 their rated
speed.
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
18 ______________________________________________________________________________________
MANUFACTURER FAN MODEL OPTION
Comair Roton
All DC brushless models can be
ordered with optional tachometer
output.
EBM-Papst
Tachometer output optional on
some models.
JMC Tachometer output optional.
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 12. Fan Manufacturers
Chip Information
TRANSISTOR COUNT: 6479
PROCESS: BiCMOS