ADM1031ARQZ-R7 datasheet(19/30 Pages) ONSEMI

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ADM1031

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Programming the Filtered Automatic Fan Speed

Control Loop

1. Program a value for TMIN.

2. Program a value for the slope TRANGE.

3. TMAX = TMIN + TRANGE.

4. Program a value for fan spin−up time.

5. Program the desired automatic fan speed control

mode behavior, that is, which temperature channel

controls the fan.

6. Program a ramp rate for the filtered mode.

7. Program the ADC sample rate in the fan filter

8. Set Bit 0 to enable fan filtered mode for Fan 1.

9. Set Bit 1 to enable the fan filtered mode for Fan 2.

10. Select automatic fan speed control mode by setting

Bit 7 of Configuration Register 1.

PWM Duty Cycle Select Mode

The ADM1031 can operate under software control by

clearing Bit 7 of Configuration Register 1 (Register 0

×00).

This allows the user to directly control PWM duty cycle for

each fan.

Clearing Bit 5 and Bit 6 of Configuration Register 1

allows fan control by varying PWM duty cycle. Values of

duty cycle between 0% and 100% can be written to the fan

speed configuration register (0

×22) to control the speed of

each fan. Table 10 shows the relationship between hex

values written to the fan speed configuration register and

PWM duty cycle obtained.

Table 10. PWM Duty Cycle Select Mode

Hex Value

PWM Duty Cycle

14%

20%

27%

33%

40%

47%

53%

60%

67%

73%

80%

87%

93%

100%

Bits <3:0> set the PWM duty cycle for Fan 1; Bits <7:4>

set the PWM duty cycle for Fan 2.

RPM Feedback Mode

The second method of fan speed control under software is

RPM feedback mode. This involves programming the

desired fan RPM value to the device to set fan speed. The

advantages include a very tightly maintained fan RPM over

the fan’s life, and virtually no acoustic pollution due to fan

speed variation.

Fans typically have manufacturing tolerances of

±20%,

meaning a wide variation in speed for a typical batch of

identical fan models. If it is required that all fans run at

exactly 5000 RPM, it can be necessary to specify fans with

a nominal fan speed of 6250 RPM. However, many of these

fans run too fast and make excess noise. A fan with nominal

speed of 6250 RPM could run as fast as 7000 RPM at 100%

PWM duty cycle. RPM mode allows all of these fans to be

programmed to run at the desired RPM value.

Clearing Bit 7 of Configuration Register 1 (Register

×00) to 0 places the ADM1031 under software control.

Once under software control, the device can be placed into

RPM feedback mode by writing to Bit 5 and Bit 6 of

Configuration Register 1. Writing a 1 to Bit 5 and Bit 6

selects RPM feedback mode for each fan.

Once RPM feedback mode has been selected, the required

fan RPM can be written to the fan tach high limit registers

×10, 0×11). The RPM feedback mode function allows a fan

RPM value to be programmed into the device, and the

ADM1031 maintains the selected RPM value by monitoring

the fan tach and speeding up the fan as necessary, should the

fan start to slow down. Conversely, should the fan start to

speed up due to aging, the RPM feedback slows the fan down

to maintain the correct RPM speed. The value to be

programmed into each fan tach high limit register is given by:

Count = (f

× 60)/R × N

where:

f = 11.25 kHz

R = desired RPM value

N = speed range; must be set to 2

The speed range, N, really determines what the slowest

fan speed measured can be before generating an interrupt.

The slowest fan speed is measured when the count value

reaches 255.

Since N = 2

Count = (f

× 60)/R × N

R = (f

× 60)/Count × N

R = (11250

× 60)/255 × 2

R = (675000)/510

R = 1324 RPM, fan fail detect speed

Programming RPM Values in RPM Feedback Mode

Rather than writing a value such as 5000 to a 16−bit

count to be programmed is given by:

Count = (f

× 60)/R × N

where:

f = 11.25 kHz

R = desired RPM value

N = speed range 2

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