MAX6643LBFAEE+【PDF 6/17 页】IC CTLR PWM FAN-SPEED 16QSOP

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There are two options for the behavior of the PWM out-

puts at power-up. Option 1 (minimum duty cycle = 0):

at power-up, the PWM duty cycle is zero. Option 2

(minimum duty cycle = the start duty cycle): at power-

up, there is a startup delay, after which the duty cycle

goes to 100% for the spin-up period. After the startup

delay and spin-up, the duty cycle drops to its minimum

value. The minimum duty cycle is in the 0% to 50%

range (see the Selector Guide).

To control fan speed based on temperature, T

HIGH

set to the temperature beyond which the fan should spin

at 100%. T

LOW

is set to the temperature below which

the duty cycle can be reduced to its minimum value.

After power-up and spin-up (if applicable), the duty

cycle reduces to its minimum value (either 0% or the

start duty cycle). For option 1 (minimum duty cycle = 0),

if the measured temperature remains below T

HIGH

, the

duty cycle remains at zero (see Figure 1). If the temper-

ature increases above T

HIGH

, the duty cycle goes to

100% for the spin-up period, and then goes to the start

duty cycle (for example, 40%). If the measured temper-

ature remains above T

HIGH

when temperature is next

measured (4s later), the duty cycle begins to increase,

incrementing by 1.5% every 4s until the fan is spinning

fast enough to reduce the temperature below T

HIGH

For option 2 (minimum duty cycle = start duty cycle), if

the measured temperature remains below T

HIGH

, the

duty cycle does not increase and the fan continues to

run at a slow speed. If the temperature increases

above T

HIGH

, the duty cycle begins to increase, incre-

menting by 1.5% every 4s until the fan is spinning fast

enough to reduce the temperature below T

HIGH

(see

Figure 2). In both cases, if only a small amount of extra

cooling is necessary to reduce the temperature below

Automatic PWM Fan-Speed Controllers with

Overtemperature Output

6 _______________________________________________________________________________________

Table 2. Setting T

LOW

(MAX6643 and MAX6644)

STARTUP

TIME

HIGH

SPIN-UP

LOW

Figure 1. Temperature-Controlled Duty-Cycle Change with

Minimum Duty Cycle 30%

STARTUP

MAX664_B HAS 30% PWM_OUT DUTY CYCLE DURING STARTUP.

TIME

HIGH

LOW

SPIN-UP

Figure 2. Temperature-Controlled Duty-Cycle Change with

Minimum Duty Cycle 30%

TL2

TL1

LOW

(癈)

L SUFFIX

High-Z

High-Z = High impedance.

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