MAX6616AEG+T【PDF 9/20 页】IC TEMP MON FAN CNTRL 24QSOP

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brushless DC motor has enough time to operate. When

driving a fan with a PWM-to-DC circuit as shown in

Figure 5, 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 frequen-

cy normally should 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: setting the duty cycle of the fan

directly through the fan target duty-cycle registers

(0Bh and 0Ch).

2) Automatic PWM control: setting the duty cycle based

on temperature.

Manual PWM Duty-Cycle Control

Clearing the bits that select the temperature channels for

fan control (D5 and D4 for PWM1 and D3 and D2 for

PWM2) in the fan-configuration register (11h) enables

manual fan control. In this mode, the duty cycle written to

the fan target duty-cycle register directly controls the

corresponding fan. The value is clipped to a maximum of

240. Any value entered above that is changed to 240

automatically. In this control mode, the value in the maxi-

mum duty-cycle register is ignored and does not affect

the duty cycle used to control the fan.

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 fan-start

temperature, the duty cycle is either 0% or is equal to

the fan-start duty cycle, depending on the value of bit

D3 in the configuration byte register. Above the fan-

start temperature, the duty cycle increases by one

duty-cycle step each time the temperature increases by

one temperature step. The target duty cycle is calculat-

ed based on the following formula; for temperature >

FanStartTemperature:

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 temperature.

For example, if the 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 the temperature decreases further, the

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

For temperature < FanStartTemperature and D2 of

configuration register = 0:

DutyCycle = 0

For temperature < FanStartTemperature and D2 of

configuration register = 1:

DutyCycle = FanStartDutyCycle

Once the temperature crosses the fan-start temperature

threshold, the temperature has to drop below the fan-

start temperature threshold minus the hysteresis before

DC FSDC T FST

DCSS

?/DIV>

(

)

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

_______________________________________________________________________________________ 9

+3.3V

PWM

18k&

27k&

10k&

120k&

+3.3V

+12V

500k&

OUT

TO FAN

1礔

0.01礔

0.1礔

Figure 5. Driving a Fan with a PWM-to-DC Circuit

PWM

4.7k&

Figure 6. Controlling a PWM Input Fan with the MAX6615/

MAX6616s PWM Output (Typically, the 35kHz PWM

Frequency Is Used)

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