ADM1030
http://onsemi.com
21
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 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 ADM1030 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 practice, 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:
1. Run the fan at 53% PWM duty cycle in Software
Mode. Clear Bits 5 and 7 of Configuration
Register 1 (Reg 0x00) to enter PWM duty cycle
mode. Write 0x08 to the Fan Speed Config
Register (Reg 0x22) to set the PWM output to
53% duty cycle.
2. Measure the fan RPM. This represents the fan
RPM below which the RPM mode will fail 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.
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 ADM1030 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):
(eq. 14)
Fan RPM + (f 60)ńCount N
For a 6-pole fan (3 tach pulses/rev):
(eq. 15)
Fan RPM + (f 60)ń(Count N 1.5)
For a 8-pole fan (4 tach pulses/rev):
(eq. 16)
Fan RPM + (f 60)ń(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
will need to handle the higher current requirements. The
only other stipulation is that the MOSFET should have a gate
voltage drive, V
GS
< 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.
Figure 34 shows how a 3-wire fan may be driven using
PWM control.
Figure 34. Interfacing the ADM1030 to a 3-wire Fan
ADM1030
5 V OR 12 V
FAN
10 kW
TYPICAL
TACH/AIN
TACH
3.3 V
PWM_OUT
10 kW
TYPICAL
3.3 V
+V
Q1
NDT3055L
The NDT3055L n-type MOSFET was chosen since it has
3.3 V gate drive, low on-resistance, and can handle 3.5 A of
current. Other MOSFETs may be substituted based on the
system’s fan drive requirements.
