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2-Channel Temperature Monitor with Dual

Automatic PWM Fan-Speed Controller

MAX6640

Table 7. Temperature Step Size

D[3:2]

00

01

10

11

FAN CONTROL TEMPERATURE

STEP SIZE (掳C)

1

2

4

8

Table 8. Fan PWM Frequency

D[1:0]

00

01

10

11

LOW FREQUENCY (Hz)

20

33.33

50

100

Fan 1 and 2 Configuration 2b (12h and 16h)

The following registers select the tachometer step sizes

and number of steps for step-size A to step-size B

slope changes (see Figure 1):

鈥?/div>

D[7:4]: RPM (Tachometer) Step Size B.

Selects

number of tachometer counts the target value

decreases for each temperature step increase after

the number of steps selected by D[3:0]. Value = n +

1 (1 through 16) where n is the value of D[7:4].

鈥?/div>

D[3:0]:

Selects the number of temperature/tachome-

ter steps above the fan-start temperature at which

step-size B begins.

Fan 1 and Fan 2 Configuration 3 (13h and 17h)

The following registers control fan spin-up, PWM output

frequency, pulse stretching, and

THERM

to fan full-

speed enable:

鈥?/div>

D7: Fan Spin-Up Disable.

Set to zero to enable fan

spin-up. Whenever the fan starts up from zero drive,

it is driven with 100% duty cycle for 2s to ensure that

it starts. Set to 1 to disable the spin-up function.

鈥?/div>

D6:

THERM

to Full-Speed Enable.

When this bit is

1,

THERM

going low (either by being pulled low

externally or by the measured temperature exceed-

ing the THERM limit) forces the fan to full speed. In

all modes, this happens at the rate determined by

the rate-of-change selection. When

THERM

is

deasserted (even if fan has not reached full speed),

the speed falls at the selected rate-of-change to the

target speed.

鈥?/div>

D5: Disable Pulse Stretching.

Pulse stretching is

enabled when this bit is set to zero. When modulat-

ing the fan鈥檚 power supply with the PWM signal, the

PWM pulses are periodically stretched to keep the

tachometer signal available for one full revolution.

Setting this bit to 1 disables pulse stretching. The

MAX6640 still measures the fan speed but does not

stretch the pulses for measurements, so the fan鈥檚

power supply must not be pulse modulated.

鈥?/div>

D[1:0]: PWM Output Frequency.

These bits control

the PWM output frequency as shown in Table 8.

Fan Tach Count 1 and 2 (20h and 21h)

These registers have the latest tachometer measure-

ment of the corresponding channel. This is inversely

proportional to the fan鈥檚 speed. The fan RPM range

should be set so this count falls in the 30 to 160 range

for normal fan operation.

Fan Start Tach Count/Target Tach Count

(22h and 23h)

D[7:0]:

This sets the starting tachometer count for the

fan in automatic RPM mode. Depending on the setting

of the minimum duty-cycle bit, the tachometer count

has this value either at all temperatures below the fan-

start temperature or the count is zero below the fan-

start temperature and has this value when the fan-start

temperature is reached. These registers are the target

tach count when in manual RPM mode.

Fan 1 and 2 Pulses and Min RPM (24h and 25h)

D[7:6]:

This sets the number of tachometer pulses per

revolution for the fan. When set properly, a 2000RPM fan

with two pulses per revolution has the same tachometer

count as a 2000RPM fan with four pulses per revolution.

Table 9 lists tachometer pulses per revolution.

D[5:0]:

This sets the minimum allowable fan tachometer

count (maximum speed). This limits the maximum

speed of the fan to reduce noise at high temperatures.

For reasonable speed resolution, the fan RPM range

should be set so this value is between about 30 and 60.

If a maximum RPM limit is unnecessary, this value can

be set to the full-speed tachometer count.

Fan 1 and 2 Duty Cycle (26h and 27h)

These registers contain the present value of the PWM

duty cycle. In PWM fan-control mode, the desired (tar-

get) value of the PWM duty cycle can be written directly

into this register.

Channel 1 and Channel 2 Fan-Start Temperature

(28h and 29h)

These registers contain the temperatures at which fan

control begins (in automatic RPM mode).

______________________________________________________________________________________

15

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