參數(shù)資料
型號(hào)： ADT7466ARQZ
廠商： ANALOG DEVICES INC
元件分類(lèi)： 模擬信號(hào)調(diào)理
英文描述： dBCool Remote Thermal Controller and Voltage Monitor
中文描述： SPECIALTY ANALOG CIRCUIT, PDSO16
封裝： LEAD FREE, MO-137AB, QSOP-16
文件頁(yè)數(shù)： 27/48頁(yè)
文件大小： 1269K
代理商： ADT7466ARQZ
ADT7466
If the fan output has a resistive pull-up to 12 V (or other voltage
greater than 6.5 V), the fan output can be clamped with a Zener
diode, as shown in Figure 37. The Zener diode voltage should
be greater than V
IH
of the tach input but less than 6.5 V,
allowing for the voltage tolerance of the Zener. A value of
between 3 V and 5 V is suitable.
Rev. 0 | Page 27 of 48
0
FAN DRIVE
V
CC
ADT7466
TACH
ZD1*
ZENER
*CHOOSE ZD1 VOLTAGE APPROX. 0.8
×
V
CC
TACH
FAN SPEED
COUNTER
PU4.7k
TYP.
Figure 37. Fan with Tach.
Pull-Up to Voltage >6.5 V, for Example, 12 V Clamped with Zener Diode.
If the fan has a strong pull-up (less than 1 k) to 12 V, or a
totem pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 38. Alternatively, a resistive
attenuator can be used, as shown in Figure 39.
R1 and R2 should be chosen such that
2 V <
V
PULLUP
×
R2
/(
R
PULLUP
+
R1
+
R2
) < 5 V
The fan inputs have an input resistance of nominally 160 k to
ground, which should be taken into account when calculating
resistor values.
With a pull-up voltage of 12 V and pull-up resistor less than
1 k, suitable values for R1 and R2 are 100 k and 47 k. This
gives a high input voltage of 3.83 V.
0
FAN DRIVE
V
CC
ADT7466
TACH
ZD1*
ZENER
*CHOOSE ZD1 VOLTAGE APPROX. 0.8
×
V
CC
TACH
OUTPUT
R1
10k
FAN SPEED
COUNTER
PULL-UP
TYP. < 1k
OR TOTEM POLE
Figure 38. Fan with Strong Tach.
Pull-Up to >V
CC
or Totem Pole Output, Clamped with Zener and Resistor.
0
FAN DRIVE
V
CC
ADT7466
TACH
R1*
R2*
*SEE TEXT
TACH
OUTPUT
FAN SPEED
COUNTER
<1k
Figure 39. Fan with Strong Tach.
Pull-Up to >VCC or Totem Pole Output, Attenuated with R1/R2.
Fan Speed Registers
The fan counter does not count the fan tach output pulses
directly because the fan speed can be less than 1000 rpm; it
would take several seconds to accumulate a reasonably large
and accurate count. Instead, the period of the fan revolution is
measured by gating an on-chip 82 kHz oscillator into the input
of a 16-bit counter for N periods of the fan tach output, as
shown in Figure 40. The accumulated count is actually
proportional to the fan tachometer period and inversely
proportional to the fan speed.
0
CLOCK
TACH
1
2
Figure 40. Fan Speed Measurement
N, the number of pulses counted, is determined by the settings
of Register 0x39 (fan pulses per revolution register). This
register contains 2 bits for each fan, allowing 1, 2 (default), 3 or
4 tach pulses to be counted.
The fan tachometer readings are 16-bit values consisting of a
2-byte read from the ADT7466.
Table 24. Fan Speed Measurement Registers
Register
Description
0x48
TACH1 low byte
0x49
TACH1 high byte
0x4A
TACH2 low byte
0x4B
TACH2 high byte
Reading Fan Speed from the ADT7466
Measuring fan speeds involves a 2-register read for each
measurement. The low byte should be read first, which causes
the high byte to be frozen until both high and low byte registers
are read. This prevents erroneous tach readings.
Default
0xFF
0xFF
0xFF
0xFF
The fan tachometer reading registers report the number of
12.2 μs period clocks (82 kHz oscillator) gated to the fan speed
counter from the rising edge of the first fan tach pulse to the
rising edge of the third fan tach pulse, assuming two pulses per
revolution is being counted. Since the device is essentially
measuring the fan tach period, the higher the count value, the
slower the fan is actually running. A 16-bit fan tachometer
reading of 0xFFFF indicates either that the fan has stalled or
that it is running very slowly (<75 rpm).
A
greater than
comparison is performed when comparing with
the high limit.
The actual fan tach period is being measured in this case.
Therefore, when the fan tach limit is exceeded, a 1 is set for the
appropriate status bit and can be used to generate an ALERT.
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