MAX6661AEE+T【PDF 18/19 页】IC REG FAN SPEED 16-QSOP

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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) H 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:

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:

= 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&

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

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