ADM1031ARQZ View Datasheet(PDF) - ON Semiconductor
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ADM1031ARQZ Datasheet PDF : 30 Pages
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ADM1031
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
0×00) 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
Register 1 (Register 0×00) 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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