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ADM1031

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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 virtu-

ally 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 may be necessary to specify fans with a nominal fan speed of

6250 RPM. However, many of these fans will 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 will allow all of these fans to be programmed to run at the

desired RPM value.

Clearing Bit 7 of Configuration Register 1 (Reg 0x00) to 0

places the ADM1031 under software control. Once under soft-

ware control, the device may be placed in to RPM Feedback

Mode by writing to Bits 5, 6 of Configuration Register 1. Writing

a 1 to Bits 5, 6 selects RPM Feedback Mode for each fan. Once

RPM Feedback Mode has been selected, the required fan RPM

may be written to the Fan Tach High Limit Registers (0x10,

0x11). The RPM Feedback Mode function allows a fan RPM

value to be programmed into the device, and the ADM1031 will

maintain 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 will slow 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 will be measured when the Count value reaches 255.

Since speed range, 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 register, an

8-bit count value is programmed instead. The count to be pro-

grammed is given by:

Count = (f

× 60)/R × N

where:

f = 11.25 kHz

R = desired RPM value

N = Speed Range = 2

Example 1:

If the desired value for RPM Feedback Mode is 5000 RPM,

what value needs to be programmed for Count?

Count = (f

× 60)/R × N

Since the desired RPM value, R, is 5000 RPM, the value for

Count is:

N = 2:

Count = (11250

× 60)/5000 × 2

Count = 675000/10000

Count = 67 (assumes 2 tach pulses/rev).

Example 2:

If the desired value for RPM Feedback Mode is 3650 RPM,

what value needs to be programmed for Count?

Count = (f

× 60)/R × N

Since the desired RPM value, R, is 3650 RPM, the value for

Count is:

N = 2:

Count = (11250

× 60)/3650 × 2

Count = 675000/7300

Count = 92 (assumes 2 tach pulses/rev).

Once the count value has been calculated, it should be written

to the Fan Tach High Limit Register. It should be noted that in

RPM Feedback Mode, there is no high limit register for under-

speed detection that can be programmed as there are in the

other fan speed control modes. The only time each fan will

indicate a fan failure condition is whenever the count reaches

255. Since the speed range, N, = 2, the fan will fail if its speed

drops below 1324 RPM.

Programming RPM Values

1. Choose the RPM value to be programmed.

2. Set speed range value, N, = 2.

3. Calculate count value based on RPM and speed range values

chosen. Use Count Equation to calculate Count Value.

4. Clear Bit 7 of Configuration Register 1 (Reg. 0x00) to place

the ADM1031 under software control.

5. Write a 1 to Bit 5 of Configuration Register 1 to place the

device in RPM Feedback Mode.

6. Write the calculated Count value to the Fan Tach High

Limit Register (Reg. 0x10). The fan speed will now go to the

desired RPM value and maintain that fan speed.

RPM Feedback Mode Limitations

RPM feedback mode only controls Fan RPM over a limited fan

speed range of about 75% to 100%. However, this should be

enough range to overcome fan manufacturing tolerance. In prac-

tice, however, the program must not function at too low an RPM

value for the fan to run at, or the RPM Mode will not operate.

To find the lowest RPM value allowed for a given fan, do the

following:

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