ADM1031ARQZ Просмотр технического описания (PDF) - ON Semiconductor
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ADM1031ARQZ Datasheet PDF : 30 Pages
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ADM1031
Programming the Filtered Automatic Fan Speed
Control Loop
1. Program a value for TMIN.
2. Program a value for the slope TRANGE.
3. TMAX = TMIN + TRANGE.
4. Program a value for fan spin−up time.
5. Program the desired automatic fan speed control
mode behavior, that is, which temperature channel
controls the fan.
6. Program a ramp rate for the filtered mode.
7. Program the ADC sample rate in the fan filter
register.
8. Set Bit 0 to enable fan filtered mode for Fan 1.
9. Set Bit 1 to enable the fan filtered mode for Fan 2.
10. Select automatic fan speed control mode by setting
Bit 7 of Configuration Register 1.
PWM Duty Cycle Select Mode
The ADM1031 can operate under software control by
clearing Bit 7 of Configuration Register 1 (Register 0×00).
This allows the user to directly control PWM duty cycle for
each fan.
Clearing Bit 5 and Bit 6 of Configuration Register 1
allows fan control by varying PWM duty cycle. Values of
duty cycle between 0% and 100% can be written to the fan
speed configuration register (0×22) to control the speed of
each fan. Table 10 shows the relationship between hex
values written to the fan speed configuration register and
PWM duty cycle obtained.
Table 10. PWM Duty Cycle Select Mode
Hex Value
00
01
02
03
04
05
06
07
08
09
0A
0B
0C
0D
0E
0F
PWM Duty Cycle
0%
7%
14%
20%
27%
33%
40%
47%
53%
60%
67%
73%
80%
87%
93%
100%
Bits <3:0> set the PWM duty cycle for Fan 1; Bits <7:4>
set the PWM duty cycle for Fan 2.
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 virtually 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 can be necessary to specify fans with
a nominal fan speed of 6250 RPM. However, many of these
fans 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 allows all of these fans to be
programmed to run at the desired RPM value.
Clearing Bit 7 of Configuration Register 1 (Register
0×00) to 0 places the ADM1031 under software control.
Once under software control, the device can be placed into
RPM feedback mode by writing to Bit 5 and Bit 6 of
Configuration Register 1. Writing a 1 to Bit 5 and Bit 6
selects RPM feedback mode for each fan.
Once RPM feedback mode has been selected, the required
fan RPM can be written to the fan tach high limit registers
(0×10, 0×11). The RPM feedback mode function allows a fan
RPM value to be programmed into the device, and the
ADM1031 maintains the selected RPM value by monitoring
the fan tach and speeding 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 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:
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 is measured when the count value
reaches 255.
Since 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 programmed is given by:
Count = (f × 60)/R × N
where:
f = 11.25 kHz
R = desired RPM value
N = speed range 2
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