Fan 1 and 2 Configuration 2a (11h and 15h)

The following registers apply to the automatic RPM con-

trol mode:

• D[7:4]: Fan RPM (Tachometer) Step-Size A.

Selects the number of tachometer counts the target

value decreases for each temperature step increase

above the fan-start temperature. Value = n + 1 (1

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

• D[3:2]: Temperature Step Size. Selects the temper-

ature increment for fan control. For each temperature

step increase, the target tachometer count decreas-

es by the value selected by D[7:4] (Table 8).

• D1: PWM Output Polarity. PWM output is low at

100% duty cycle when this bit is set to zero. PWM

output is high at 100% duty cycle when this bit is set

to 1.

• D0: Minimum Speed. Selects the value of the mini-

mum fan speed (when temperature is below the fan-

start temperature in the automatic RPM control

mode). Set to zero for 0% fan drive. Set to 1 to deter-

mine the minimum fan speed by the tachometer

count value in registers 22h and 23h (fan maximum

TACH).

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):

• 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].

• 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:

• 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.

• 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 the fan has not reached full

speed), the speed falls at the selected rate-of-

change to the target speed.

• D5: Disable Pulse Stretching. Pulse stretching is

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

ing the fan’s 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

MAX6639 still measures the fan speed but does not

stretch the pulses for measurements, so the fan’s

power supply must not be pulse modulated.

• D[1:0]: PWM Output Frequency. These bits control

the PWM output frequency as shown in Table 9.

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’s 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.

2-Channel Temperature Monitor with Dual,

Automatic, PWM Fan-Speed Controller

______________________________________________________________________________________

15

REGISTER 11h

OR 15h

FAN CONTROL TEMPERATURE

STEP SIZE (

°C)

00

1

01

2

10

4

11

8

Table 8. Temperature Step Size

REGISTERS

13h AND 16h

LOW-FREQUENCY

(Hz) REGISTER

04h D3 = 0

HIGH-FREQUENCY

(kHz) REGISTER

04h D3 = 1

00

20

5

01

33.33

8.33

10

50

12.5

11

100

25

Table 9. Fan PWM Frequency