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ADM1026 View Datasheet(PDF) - Analog Devices

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ADM1026 Datasheet PDF : 56 Pages
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Signal conditioning in the ADM1026 accommodates the slow
rise and fall times typical of fan tachometer outputs. The fan
tach inputs have internal 10 kΩ pull-up resistors to 3.3 V STBY.
In the event that these inputs are supplied from fan outputs that
exceed the supply, either resistive attenuation of the fan signal
or diode clamping must be included to keep inputs within an
acceptable range. Figure 44 through Figure 47 show circuits for
common fan tach outputs.
If the fan tach output is open-drain or has a resistive pull-up to
VCC, then it can be connected directly to the fan input, as shown
in Figure 44.
12V
VCC
PULL-UP
4.7k
TYP
FAN(0–7)
TACH
OUTPUT
FAN SPEED
COUNTER
Figure 44. Fan with Tach Pull-Up to +VCC
If the fan output has a resistive pull-up to +12 V (or other
voltage greater than 3.3 V STBY), the fan output can be clamped
with a Zener diode, as shown in Figure 45. The Zener voltage
should be chosen so that it is greater than VIH but less than 3.3 V
STBY, allowing for the voltage tolerance of the Zener.
12V
VCC
PULL-UP
4.7k
TYP
TACH
OUTPUT FAN(0–7)
ZD1*
ZENER
FAN SPEED
COUNTER
* CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × VCC
Figure 45. Fan with Tach Pull-Up to Voltage > VCC (e.g. 12 V),
Clamped with Zener Diode
If the fan has a strong pull-up (less than 1 kΩ) to +12 V, or a
totem pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 46. Alternatively, a resistive
attenuator may be used, as shown in Figure 47.
R1 and R2 should be chosen such that
2V
<
VPULLUP
×
R2
(RPULLUP +
R1
+
R2)
<
3.3
V
STBY
ADM1026
12V
VCC
PULL-UP TYP
<1 kOR
TOTEM POLE
TACH
OUTPUT FAN(0–7)
R1
10k
ZD1*
ZENER
FAN SPEED
COUNTER
* CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × VCC
Figure 46. Fan with Strong Tach Pull-Up to > VCC or Totem Pole Output,
Clamped with Zener and Resistor
12V
VCC
<1 k
R1*
TACH
OUTPUT
FAN(0–7)
FAN SPEED
COUNTER
* SEE TEXT
Figure 47. Fan with Strong Tach Pull-Up to >VCC or Totem Pole Output,
Attenuated with R1/R2
FAN SPEED MEASUREMENT
The fan counter does not count the fan tach output pulses
directly because the fan speed may be less than 1000 RPM and
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 22.5 kHz oscillator into the
input of an 8-bit counter for two periods of the fan tach output,
as shown in Figure 48, so the accumulated count is actually
proportional to the fan tach period and inversely proportional
to the fan speed.
22.5kHz
CLOCK
CONFIGURATION
REG. 1 BIT 0
1
2
3
4
FAN0
INPUT
FAN1
INPUT
1
2
3
4
START OF
MONITORING
CYCLE
FAN0
MEASUREMENT
PERIOD
FAN1
MEASUREMENT
PERIOD
Figure 48. Fan Speed Measurement
The monitoring cycle begins when a 1 is written to the monitor
bit (Bit 0 of Configuration Register 1). The INT_Enable (Bit 1)
should be set to 1 to enable the INT output.
Rev. A | Page 25 of 56
 

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