MAX6641AUB92+【PDF 13/17 页】IC TEMP MONITOR SMBUS 10UMAX

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SMBus-Compatible Temperature Monitor with

Automatic PWM Fan-Speed Controller

______________________________________________________________________________________ 13

The frequency-select register controls the frequency of

the PWM signal. When the PWM signal modulates the

power supply of the fan, a low PWM frequency (usually

33Hz) should be used to ensure the circuitry of the

brushless DC motor has enough time to operate. When

driving a fan with a PWM-to-DC circuit, as in Figure 6,

the highest available frequency (35kHz) should be

used to minimize the size of the filter capacitors. When

using a fan with a PWM control input, the frequency

should normally be high as well, although some fans

have PWM inputs that accept low-frequency drive.

The duty cycle of the PWM can be controlled in two ways:

1) Manual PWM control by setting the duty cycle of

the fan directly through the fan-target duty-cycle

2) Automatic PWM control by setting the duty cycle

based on temperature.

Manual PWM Duty-Cycle Control

Setting bits D5 and D4 to zero in the fan-configuration

mode, the duty cycle written to the fan-target duty-

cycle register controls the PWMOUT duty cycle. The

value is clipped to a maximum of 240, which corre-

sponds to a 100% duty cycle. Any value above that is

limited to the maximum duty cycle. In manual control

mode, the value of the maximum duty-cycle register is

ignored and does not affect the duty cycle.

Automatic PWM Duty-Cycle Control

In the automatic control mode, the duty cycle is con-

trolled by the local or remote temperature, according to

the settings in the control registers. Below the value of

the fan-start temperature threshold (set by registers 03h

and 04h), the duty cycle is equal to the fan-start duty

cycle. Above the fan-start temperature, the duty cycle

increases by one duty-cycle step each time the tempera-

ture increases by one temperature step. Below the fan-

start temperature, the duty cycle is either 0% or it is

equal to the fan-start duty cycle, depending on the value

of bit D3 in the configuration byte register. See Figure 8.

The target duty cycle is calculated based on the follow-

ing formula:

For temperature > fan-start temperature:

where:

DC = DutyCycle

FSDC = FanStartDutyCycle

T = Temperature

FST = FanStartTemperature

DCSS = DutyCycleStepSize

TS = TempStep

Duty cycle is recalculated after each temperature con-

version if temperature is increasing. If the temperature

begins to decrease, the duty cycle is not recalculated

until the temperature drops by 5癈 from the last peak

temperature. The duty cycle remains the same until the

temperature drops 5癈 from the last peak temperature

or the temperature rises above the last peak tempera-

ture. For example, if temperature goes up to +85癈 and

starts decreasing, duty cycle is not recalculated until

the temperature reaches +80癈 or the temperature

rises above +85癈. If temperature decreases further,

the duty cycle is not updated until it reaches +75癈.

For temperature < fan-start temperature and bit D3 of

the configuration byte register = 0:

DutyCycle = 0

For temperature < fan-start temperature and bit D3 of

the configuration byte register = 1:

Dutycycle = FanStartDutyCycle

Once the temperature crosses the fan-start tempera-

ture threshold, the temperature has to drop below the

fan-start temperature threshold minus the hysteresis

before the duty cycle returns to either 0% or fan-start

duty cycle. The value of the hysteresis is set by D7 of

the fan-configuration register.

DC FSDC T FST

DCSS

?/DIV>

(

)

FAN START

DUTY CYCLE

TEMPERATURE

DUTY CYCLE

BIT D3 = 1

DUTY CYCLE

STEP SIZE

FAN START

TEMPERATURE

TEMP

STEP

BIT D3 = 0

Figure 8. Automatic PWM Duty Control

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