MAX6640AEE Datasheet

IC型号：
MAX6640AEE
PDF描述：
2-Channel Temperature Monitor with Dual Automatic PWM Fan-Sp...
文件大小：
722.10KB
PDF页数：
22页
芯片厂家：
MAXIM

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2-Channel Temperature Monitor with Dual

Automatic PWM Fan-Speed Controller

FAIL,

the fan with a tachometer fault goes to full speed for

2s in an attempt to restart the fan and then returns to the

original duty-cycle settings. Reading the status register

clears the

FANFAIL

status bits and the output. The

MAX6640 measures the fan speed again after 2s. The

MAX6640 asserts

FANFAIL

if it detects the fan fault again.

In RPM mode (either automatic or manual), the

MAX6640 checks for fan failure only when the duty

cycle reaches 100%. It asserts

FANFAIL

when the

tachometer count is greater than twice the target

tachometer count. In manual RPM mode, registers 22h

and 23h store the target tachometer count value. In

automatic RPM mode, these registers store the maxi-

mum tachometer count.

14h) to zero. In the manual RPM control mode, the

MAX6640 adjusts the duty cycle and measures the fan

speed. Enter the target tachometer count in register

22h for fan 1 and register 23h for fan 2. The MAX6640

compares the target tachometer count with the mea-

sured tachometer count and adjusts the duty cycle so

that the fan speed gradually approaches the target

tachometer count.

The first time manual RPM control mode is entered, the

initial PWM duty cycle is determined by the target

tachometer count:

Initial duty cycle

255

鈭?/div>

t arg etTACH

MAX6640

Fan-Speed Control

The MAX6640 adjusts fan speed by controlling the duty

cycle of a PWM signal. This PWM signal then either

modulates the DC brushless fan鈥檚 power supply or dri-

ves a speed-control input on a fan that is equipped with

one. There are three speed-control modes:

PWM,

which the PWM duty cycle is directly programmed over

the SMBus;

manual RPM,

in which the desired

tachometer count is programmed into a register and

the MAX6640 adjusts its duty cycle to achieve the

desired tachometer count; and

automatic RPM,

which the tachometer count is adjusted based on a

programmed temperature profile.

The MAX6640 divides each PWM cycle into 120 time

slots. Registers 26h and 27h contain the current values

of the duty cycles for PWM1 and PWM2, expressed as

the effective time slot length. For example, the PWM1

output duty cycle is 25% when register 26h reads 1Eh

(30/120).

PWM Control Mode

Enter PWM mode by setting bit 7 of the fan 1 or 2 con-

figuration 1 register (10h and 14h) to 1. In PWM control

mode, the MAX6640 generates PWM signals whose

duty cycles are specified by writing the desired values

to fan duty-cycle registers 26h and 27h. When a new

duty-cycle value is written into one of the fan duty-cycle

registers, the duty cycle changes to the new value at a

rate determined by the rate-of-change bits [6:4] in the

fan 1 or 2 configuration 1 register. The rate-of-change

of the duty cycle ranges from 000 (immediately

changes to the new programmed value) to 111

(changes by 1/120 every 4s). See Table 4 and the

Fan

1 and 2 Configuration 1 (10h and 14h)

section.

Manual RPM Control Mode

Enter manual RPM control mode by setting bits 2, 3,

and 7 of the fan 1 or 2 configuration 1 register (10h and

where targetTACH is the value of the target tachometer

count in the target tach count register (22h or 23h).

If the initial duty cycle value is over 120, the duty cycle

is 100%. If spin-up is enabled (bit 7 in registers 13h

and 17h) and the fan is not already spinning, the duty

cycle first goes to 100% and then goes to the initial

duty-cycle value. Every 2s, the MAX6640 counts the

fan鈥檚 period by counting the number of pulses stored in

registers 24h and 25h. If the count is different from the

target count, the duty cycle is adjusted.

If a nonzero rate-of-change is selected, the duty cycle

changes at the specified rate until the tachometer count

is within

卤5

of the target. Then the MAX6640 gets into a

locked state and updates the duty cycle every 2s.

Automatic RPM Control Mode

In the automatic RPM control mode, the MAX6640 mea-

sures temperature, sets a target tachometer count

based on the measured temperature, and then adjusts

the duty cycle so the fan spins at the desired speed.

Enter this mode by setting bit 7 of the fan 1 or 2 config-

uration 1 register (10h and 14h) to zero and selecting

the temperature channel that controls the fan speed

using bits 2 and 3 of the configuration register.

In both the RPM modes (automatic and manual), the

MAX6640 implements a low limit for the tachometer

counts. This limits the maximum speed of the fan by

ensuring that the fan鈥檚 tachometer count does not go

lower than the tachometer count specified by bits 5

through 0 of register 24h for fan 1 and register 25h for

fan 2. Typical values for the minimum tachometer count

are 30h to 60h. Set the value to correspond to the full-

rated RPM of the fan. See Figure 4.

Figure 5 shows how the MAX6640 calculates the target

tachometer value based on the measured temperature.

At T

MIN

, the fan spins at a minimum speed value corre-

sponding to the maximum tachometer count value

_______________________________________________________________________________________

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