MAX6651EEE datasheet(20/25 Pages) MAXIM

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Fan-Speed Regulators and Monitors

with SMBus/I2C-Compatible Interface

Maxim Integrated

MAX6650/MAX6651

ware control. To enter full-on mode through hardware,

see the

Setting Up the GPIOs section. Note that a hard-

ware full-on overrides all other modes.

Configure the MAX6650/MAX6651 to run in software

full-on mode by entering 00 into bits 5 and 4 of the con-

figuration register.

Full-Off

The full-off mode removes all the voltage across the

fan, causing the fan to stop. Because the MAX6650/

MAX6651 work by controlling the voltage on the low

side of the fan, either 5V or 12V will be on both leads.

Enter full-off mode by entering 01 into bits 5 and 4 of

the configuration register.

Open Loop

In open-loop mode, the MAX6650/MAX6651 do not

actually regulate the fan speed. Speed regulation

requires an external µC. Although open-loop mode

allows maximum flexibility, it also requires the most

software/processor overhead.

In open-loop mode, the MAX6650/MAX6651 act as an

SMB/I2C-controlled voltage regulator. The µC adjusts

the voltage across the fan by writing an 8-bit value to

the DAC register. This gives the µC direct control of the

voltage across the fan. Speed regulation is accom-

plished by periodically reading the tachometer regis-

ter(s) and adjusting the DAC register appropriately. The

DAC value controls the voltage across the fan accord-

ing to the following equation:

VFAN = VFAN_SUPPLY - [((R2) / R1) + 1] x VREF x KDAC /

256

where VFAN = the voltage across the fan, VFAN_SUPPLY

= the supply voltage for the fan (5V or 12V), R2 = 90k

(typ), R1 = 10k

Ω (typ), VREF = 1.5V (typ), and KDAC =

the value in the DAC register.

Note several important things in this equation. First, the

voltage across the fan moves in the opposite direction

of the DAC value. In other words, low DAC values cor-

respond to higher voltages across the fan and therefore

higher speeds. Second, DAC values greater than 180

will result in 0V across a 12V fan. Similarly, DAC values

greater than 76 will produce 0V across a 5V fan. This

limits the useful range of the DAC from 0 to 180 for 12V

fans and 0 to 76 for 5V fans.

Remember that device tolerances can cause the output

voltage value to vary significantly from unit to unit and

over temperature. However, because this voltage is

within a closed speed-control loop, such errors are cor-

rected by the loop.

Below is a possible strategy for controlling the fan

under open-loop mode:

1) On power-up, put the device in open-loop mode with

a DAC value of 00 (full speed).

2) Allow the fan speed to settle.

3) Read the TACH register to determine the speed.

4) Gradually increase the DAC register value (in steps

of 1 or 2) until the desired speed is obtained.

In open-loop mode, any one of the four tachometer regis-

ters (MAX6651) can be used to measure and regulate the

fan’s speed. This is especially useful in parallel fan sys-

tems where up to four fans will be controlled as one unit.

Care must be taken with this mode to prevent instabili-

ty, which can be caused by trying to update the fan

speed too often or in increments that are too large.

Instability can result in the fan speeding up and slowing

down repeatedly. Determining the proper update rate,

as shown in the following steps, depends largely on the

fan’s mechanical time constant and the system’s loop

gain (DAC step sizes):

1) Enter open-loop mode by setting bits 5 and 4 of the

control register to 11.

2) Determine the speed of the fan(s) by reading the

TACH register(s).

3) Increase or decrease the DAC register to decrease

or increase the voltage across the fan, thereby

adjusting its speed.

Closed Loop

In closed-loop mode, the SMBus/I2C master (usually a

µC) writes a desired fan speed to the MAX6650/

MAX6651, and the device automatically adjusts the

voltage across the fan to maintain this speed. This

operation mode requires less software/processor over-

head than the open-loop mode. Once the desired

speed has been written, the MAX6650/MAX6651 con-

trol the fan’s speed independently, with no intervention

required from the master. If desired, the MAX6650/

MAX6651 can be configured to generate an interrupt if

it is unable to regulate the fan’s speed at the desired

value (see

Setting Up Alarms). The MAX6650/MAX6651

can regulate only the speed of the fan connected to the

TACH0 input. Fans connected in parallel to the TACH0

fan will tend to run at similar speeds (assuming similar

fans). When going from full-off to closed-loop-mode, it

is recommended following this sequence:

1) Full-off mode

2) Full-on mode (with sufficient pause to initiate

movement)

3) Closed-loop mode

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