ADM1031ARQZ-R7 datasheet(20/30 Pages) ONSEMI

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ADM1031

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Example 1:

If the desired value for RPM feedback mode is 5000 RPM,

the count to be programmed is:

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,

the count to be programmed is:

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 underspeed detection that can be programmed as there

are in the other fan speed control modes. The only time each

fan indicates a fan failure condition is whenever the count

reaches 255. Since the speed range N = 2, the fan fails 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 the count equation to

calculate the count value.

4. Clear Bit 7 of Configuration Register 1 (Register

×00) 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 (Register 0

×10). The fan speed

now goes to the desired RPM value and maintains

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 practice, however, the program must not function at too

low an RPM value for the fan to run at, or the RPM mode does

not operate.

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

the following:

1. Run the fan at 53% PWM duty cycle in software

mode. Clear Bit 5 and Bit 7 of Configuration

×00) to enter PWM duty

cycle mode. Write 0

×08 to the fan speed

configuration register (Register 0

×22) to set the

PWM output to 53% duty cycle.

2. Measure the fan RPM. This represents the fan

RPM below which the RPM mode fails to operate.

Do not program a lower RPM than this value when

using RPM feedback mode.

3. Ensure that speed range N = 2 when using RPM

feedback mode.

Fan Drive and Speed Measurement

Fans come in a variety of different options. One

distinguishing feature of fans is the number of poles that a

fan has internally. The most common fans available have

four, six, or eight poles. The number of poles the fan has

generally affects the number of pulses per revolution the fan

outputs.

If the ADM1031 is used to drive fans other than 4−pole

fans that output 2 tach pulses/revolution, then the fan speed

measurement equation needs to be adjusted to calculate and

display the correct fan speed, and also to program the correct

count value in RPM feedback mode.

Fan Speed Measurement Equations

For a 4−pole fan (2 tach pulses/rev):

Fan RPM = (f

× 60)/Count × N

For a 6−pole fan (3 tach pulses/rev):

Fan RPM = (f

× 60)/(Count × N × 1.5)

For an 8−pole fan (4 tach pulses/rev):

Fan RPM = (f

× 60)/(Count × N × 2)

If in doubt as to the number of poles the fans used have,

or the number of tach output pulses/rev, consult the fan

manufacturer’s data sheet, or contact the fan vendor for

more information.

Fan Drive Using PWM Control

The external circuitry required to drive a fan using PWM

control is extremely simple. A single NMOS FET is the only

drive transistor required. The specifications of the MOSFET

depend on the maximum current required by the fan being

driven. Typical notebook fans draw a nominal 170 mA, and

so SOT devices can be used where board space is a

constraint. If driving several fans in parallel from a single

PWM output, or driving larger server fans, the MOSFET

needs to handle the higher current requirements. The only

other stipulation is that the MOSFET should have a gate

voltage drive, VGS <3.3 V, for direct interfacing to the

PWM_OUT pin. The MOSFET should also have a low

on−resistance to ensure that there is not significant voltage

drop across the FET. This would reduce the maximum

operating speed of the fan.

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