MAX6678AEP92+【PDF 14/19 页】IC TEMP MONITOR 2CH PWM 20-QSOP

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Sheet目录44

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Fan Configuration (11h)

The fan-configuration register controls the hysteresis

level, temperature step size, and whether the remote or

local diode controls the PWMOUT2 signal (see Table

2). Set bit D7 of the fan-configuration register to zero to

set the hysteresis value to 5癈. Set bit D7 to 1 to set the

hysteresis value to 10癈. Set bit D6 to zero to set the

fan-control temperature step size to 1癈. Set bit D6 to 1

to set the fan-control temperature step size to +2癈.

Bits D5 to D2 select which PWMOUT_ channel 1 or

channel 2 controls (see Table 2). If both are selected

for a given PWMOUT_, the highest PWM value is used.

If neither is selected, the fan is controlled by the value

written to the fan-target duty-cycle register. Also in this

mode, the value written to the target duty-cycle register

is not limited by the value in the maximum duty-cycle

240 is written. The POR state of the fan-configuration

Duty-Cycle Rate of Change (12h)

Bits D7, D6, and D5 (channel 1) and D4, D3, and D2

(channel 2) of the duty-cycle rate-of-change register set

the time between increments of the duty cycle. Each

increment is 2/240 of the duty cycle (see Table 4). This

allows the time from 33% to 100% duty cycle to be adjust-

ed from 5s to 320s. The rate-of-change control is always

active in manual mode. To make instant changes, set bits

D7, D6, and D5 (channel 1) or D4, D3, and D2 (channel

2) = 000. The POR state of the duty-cycle rate-of-change

Duty-Cycle Step Size (13h)

Bits D7D4 (channel 1) and bits D3D0 (channel 2) of the

duty-cycle step-size register change the size of the duty-

cycle change for each temperature step. The POR state

of the duty-cycle step size register is 55h (see Table 5).

PWM Frequency Select (14h)

Set bits D7, D6, and D5 (select A, B, and C) in the PWM

frequency-select register to control the PWMOUT frequen-

cy (see Table 6). The POR state of the PWM frequency-

select register is 40h, 33Hz. The lower frequencies are

usually used when driving the fans power-supply pin as

in the Typical Application Circuit, with 33Hz being the

most common choice. The 35kHz frequency setting is

used for controlling fans that have logic-level PWM input

pins for speed control. The minimum duty-cycle resolution

is decreased from 2/240 to 4/240 at the 35kHz frequen-

cy setting. For example, a result that would return a value

of 6/240 is truncated to 4/240.

2-Channel Temperature Monitor with Dual Automatic

PWM Fan-Speed Controller and Five GPIOs

14 ______________________________________________________________________________________

Table 4. Setting the Time Between Duty-

Cycle Increments

D7:D5, D4:D2

TIME BETWEEN

INCREMENTS (s)

TIME FROM 33%

TO 100% (s)

000

001

0.0625

010

0.125

011

0.25

100

0.5

101

110

160

111

320

Table 5. Setting the Duty-Cycle Change

D7:D4, D3:D0

CHANGE IN DUTY

CYCLE PER

TEMPERATURE

STEP

TEMPERATURE

RANGE FOR FAN

CONTROL

(1?/SPAN>C STEP, 33%

TO 100%)

0000

0001

2/240

0010

4/240

0011

6/240

0100

8/240

0101

10/240

...

1000

...

1111

Table 6. PWM Frequency Select

PWM

FREQUENCY (Hz)

SELECT A SELECT B SELECT C

100

35k

Note: At 35kHz, duty-cycle resolution is decreased from a res-

olution of 2/240 to 4/240.

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