MAX6651EEE datasheet(14/25 Pages) MAXIM

14 / 25 page

Fan-Speed Regulators and Monitors

with SMBus/I2C-Compatible Interface

Maxim Integrated

MAX6650/MAX6651

impedance of the tachometer input (150k

Ω max) or the

feedback input (150k

Ω max).

Compensation Capacitor

A compensation capacitor is needed from the fan’s low

side to ground to stabilize the analog control loop.

Typically, this capacitor should be 10µF, but depend-

ing on the type of fan being used, a value between 1µF

and 100µF may be required. The proper value has

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

Figure 7. Using the MAX6651 to Control Parallel Fans

MAX6651

VCC

SCL

10k

SDA

GPIO0

OUT

ADD

GND

TACH0

TACH1

TACH2

VCC

3V TO 5.5V

VFAN 5V OR 12V

CCOMP

SMBus/I2C

INTERFACE

GPIO1

FAN

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

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