MAX6651EEE datasheet(19/25 Pages) MAXIM

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Fan-Speed Regulators and Monitors

with SMBus/I2C-Compatible Interface

Maxim Integrated

MAX6650/MAX6651

For parallel fan control while monitoring only a single

fan, select the MAX6650.

Synchronized Fan Control (MAX6651 Only)

In systems with multiple fans, an audible beat frequency

can sometimes be detected due to fan speed mismatch.

This happens in systems where fans are connected in

parallel or in systems with a MAX6650 controlling each

fan. In parallel fan systems, speed mismatches occur

because no two fans are identical. Slight mechanical

variations or loading differences can result in enough of

a speed mismatch to cause an audible beat.

Even in systems where there is a MAX6650/MAX6651

for each fan, there can still be speed mismatches. This

is primarily due to the oscillator tolerance. The

MAX6650/MAX6651 oscillator tolerance is specified to

be ±10%. In the worst case, this could result in a 20%

(one 10% high, one 10% low) speed mismatch.

The solution is to use a single MAX6651 for each fan,

and configure the parts to use a shared clock. The

shared clock can either be an external system clock or

one of the MAX6651’s internal clocks. If an external

clock is used, its frequency can range from approxi-

mately 50kHz to 500kHz.

For synchronized fan control, select the MAX6651.

Combination

In more complex systems, a combination of some or all

of the above control types may be needed.

Choosing a Fan

Once the topology is chosen, the next step is to choose

a fan. See the appropriate section.

Enter a zero in bit 3 of the configuration register for a

5V fan and 1 for a 12V fan.

Configuring this bit also adjusts the tachometer input

threshold voltage. This optimizes operation of the

MAX6650/MAX6651 for the operating voltage of the fan

being used.

Setting the Mode of Operation

The MAX6650/MAX6651 have four modes of operation

as determined by bits 5 and 6 of the configuration reg-

ister: full on, full off, open loop, and closed loop.

Full-On

The full-on mode applies the maximum available volt-

age across the fan, guaranteeing maximum cooling.

Full-on mode can be entered through software or hard-

MAX6651

VCC

SCL

10k

SDA

GPIO0

OUT

ADD

GND

TACH0

VCC

3V TO 5.5V

VFAN

5V OR 12V

CCOMP

μF

SMBus/I2C

INTERFACE

GPIO1

GPIO2

FULL ON

ALERT

FAN

2.2k

VID

HOT-SWAP SECTION

Figure 13. Hot-Swap Application

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