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参数资料
| 型号： | ADM1030ARQZ |
| 厂商： | ANALOG DEVICES INC |
| 元件分类： | 温度/湿度传感器 |
| 英文描述： | DIGITAL TEMP SENSOR-SERIAL, 8BIT(s), 1Cel, RECTANGULAR, SURFACE MOUNT |
| 封装： | MO-137AB, QSOP-16 |
| 文件页数： | 11/28页 |
| 文件大小： | 495K |
| 代理商： | ADM1030ARQZ |
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REV. A
ADM1030
–19–
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 ADM1030 under software control. Once under soft-
ware control, the device may be placed in to RPM Feedback
Mode by writing to Bit 5 of Configuration Register 1. Writing a
1 to Bit 5 selects RPM Feedback Mode for the fan. Once RPM
Feedback Mode has been selected, the required fan RPM may
be written to the Fan Tach High Limit Register (0x10). The
RPM Feedback Mode function allows a fan RPM value to be
programmed into the device, and the ADM1030 will maintain
the selected RPM value by monitoring the fan tach and speed-
ing 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 val-
ues 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 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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