MAX6651EEE datasheet(15/25 Pages) MAXIM

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Fan-Speed Regulators and Monitors

with SMBus/I2C-Compatible Interface

Maxim Integrated

MAX6650/MAX6651

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.

Full-On Mode

By occasionally (over a period of days or weeks) turning

the fan on full and measuring the resultant speed, a

failing fan can be detected by a trend of decreasing

speeds at a given power-supply voltage. Power-up is a

convenient time to measure the maximum fan speed.

Open-Loop Mode

The fan’s condition can also be monitored using open-

loop mode. By characterizing the fan while it is new,

fan failure can be determined by writing a predeter-

mined value to the DAC and measuring the resultant

fan speed. A decrease over time of the resultant speed

may be an indication of future fan failure.

Closed-Loop Mode

The MAX6650 allows the system to read the DAC value

used to regulate the fan speed. For a given speed, a

significant change in the required DAC value may indi-

cate future fan problems.

Monitoring More than 4 Fans

Use the MAX6651 to monitor up to four fans at a time

(Figure 7). For systems requiring more than four fans,

Figure 8 shows an application using an analog multi-

plexer (mux) to monitor 11 fans. GPIO2, GPIO3, and

GPIO4 are connected to the mux’s address pins. By

writing the appropriate value to the GPIO pins, the

desired tachometer gets selected and counted by the

TACH3 input. Because the TACH inputs are double-

buffered, and only sampled every other time slot, it is

important to wait at least 4 times the tachometer count

time before reading the register after changing the mux

address. In the extreme case, a total of 25 fans can be

monitored using three multiplexers connected to

TACH1, TACH2, and TACH3. Do not connect TACH0 to

a mux if the MAX6651 is under closed-loop mode.

N + 1 Fan Application

As shown in Figure 9, if any MAX6650 cannot maintain

speed regulation, all other fans will automatically be

turned on full. This can be useful in high-reliability sys-

tems where any single fan failure should not cause

MAX6651

TACH0

TACH1

TACH2

TACH3

GPIO4

GPIO3

GPIO2

MAX4051

VCC

3V OR 5.5V

COM

ADDA

ADDB

ADDC

INH

GND

V-

NO0

NO1

NO2

NO3

NO4

NO5

NO6

FAN TACH 4

FAN TACH 9

FAN TACH 5

FAN TACH 6

FAN TACH 7

FAN TACH 8

FAN TACH 1

FAN TACH 2

FAN TACH 3

FAN TACH 10

NO7

FAN TACH 11

TO FAN VOLTAGE

5V OR 12V

Figure 8. Monitoring Multiple Fans

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