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| 型号： | ADM1033ARQZ |
| 厂商： | Analog Devices, Inc. |
| 英文描述： | Thermal Monitor and Fan Speed (RPM) Controller |
| 中文描述： | 热监测和风扇转速（RPM）控制器 |
| 文件页数： | 26/40页 |
| 文件大小： | 1164K |
| 代理商： | ADM1033ARQZ |
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ADM1033
FAN INPUTS
Pin 2 is the TACH input intended for fan speed measurement.
This input is open-drain.
Rev. 0 | Page 26 of 40
Signal conditioning on the ADM1033 accommodates the slow
rise and fall time of typical tachometer outputs. The maximum
input signal range is from 0 V to 5 V, even when V
CC
is less than
5 V. If these inputs are supplied from fan outputs that exceed
0 V to 5 V, either resistive attenuation of the fan signal or diode
clamping must be used to keep the fan inputs within an
acceptable range.
Figure 36 to Figure 38 show examples of possible fan input
circuits.
5V OR 12V
FAN
DRIVE X
FAN SPEED
COUNTER
ADM1033
TACH X
PULL-UP
4.7k
TYP
TACH
OUTPUT
V
CC
V
CC
100k
TYP
0
ZD1*
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8
×
V
CC
Figure 36. Fan with TACH Pull-Up to Voltage > 5 V, Clamped with Zener Diode
If the fan output has a resistive pull-up to 12 V (or another
voltage greater than 5 V), the fan output can be clamped with a
Zener diode, as shown in Figure 36. Select a Zener voltage that
is greater than V
IH
but less than 5 V, allowing for the voltage
tolerance of the Zener. A value of between 3 V and 5 V is
suitable.
12V
ADM1033
FAN SPEED
COUNTER
FAN(0–7)
PULL-UP TYP
<1 k
OR
TOTEM POLE
TACH
OUTPUT
V
CC
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8
×
V
CC
ZD1
ZENER*
R1
10k
0
Figure 37. Fan with Strong TACH Pull-Up to Voltage > V
CC
or Totem Pole
Output, Clamped with Zener and Resistor
If the fan has a strong pull-up (less than 1 k to 12 V) or a
totem-pole output, then a series resistor can be added to limit
the Zener current, as shown in Figure 37.
Or, resistive attenuation can be used, as shown in Figure 38.
R1
and
R2
should be chosen such that
2 V <
V
PULL-UP
×
R2
/(
R
PULL-UP
+ R1 + R2
) < 5 V
The fan inputs have an input resistance of about 160 k to
ground. Consider this when calculating resistor values.
With a pull-up voltage of 12 V and pull-up resistor of less than
1 k, suitable values for R1 and R2 would be 100 k and 47 k.
This gives a high input voltage of 3.83 V.
12V
FAN SPEED
COUNTER
ADM1033
FAN(0–7)
<1 k
TACH
OUTPUT
V
CC
R1*
R2
*SEE TEXT
0
Figure 38. Fan with Strong TACH Pull-Up to Voltage > V
CC
to Totem Pole
Output, Attenuated with R1/R2
FAN SPEED MEASUREMENT
The fan counter does not count the fan TACH output pulses
directly. This is because the fan may be spinning at less than
1,000 rpm and it would take several seconds to accumulate a
large and accurate count. Instead, the period of the fan
revolution is measured by gating an on-chip 81.92 kHz
oscillator into the input of a 16-bit counter for one complete
revolution of the fan. Therefore, the accumulated count value is
actually proportional to the fan tachometer period and inversely
proportional to the fan speed.
The number of poles in the fan must be programmed in
Configuration Register 3 (Address 0x03). This number must be
an even number only, because there cannot be an uneven
number of poles in a fan. A TACH period is output for every
two poles. Therefore, the number of poles must be known so
that the ADM1033 can measure for a full revolution.
Figure 39 shows the fan speed measurement period, assuming
that the fan outputs an ideal TACH signal. In reality, the TACH
signal output by the fan is chopped by the drive signal. However,
because the drive signal and TACH signal are synchronized,
there is enough information available for the ADM1033 to
measure the fan speed accurately.
CLOCK
IDEAL
TACH
FAN
MEASUREMENT
PERIOD
0
Figure 39. Fan Speed Measurement for a 4-Pole Fan
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