ADM1031
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21
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:
(eq. 7)
Count + (f 60)ńR 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
(eq. 8)
Count + (f 60)ńR N
R + (f 60)ńCount N
R + (11250 60)ń255 2
R + (675000)ń510
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 programmed is given by:
(eq. 9)
Count + (f 60)ńR 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,
the count to be programmed is:
(eq. 10)
Count + (f 60)ńR N
Since the desired RPM value, R is 5000 RPM, the value
for count is:
N = 2:
(eq. 11)
Count + (11250 60)ń5000 2
Count + 675000ń10000
Count + 67 (assumes 2 tach pulsesńrev).
Example 2:
If the desired value for RPM feedback mode is 3650 RPM,
the count to be programmed is:
(eq. 12)
Count + (f 60)ńR N
Since the desired RPM value, R is 3650 RPM, the value
for count is:
N = 2:
(eq. 13)
Count + (11250 60)ń3650 2
Count + 675000ń7300
Count + 92 (assumes 2 tach pulsesńrev).
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 000) 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 010). 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
Register 1 (Register 000) to enter PWM duty
cycle mode. Write 008 to the fan speed
configuration register (Register 022) 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
