ADM1026
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.
12V
V
CC
TACH
OUTPUT
FAN(0–7)
FAN SPEED
COUNTER
R1
10kΩ
ZD1*
ZENER
PULL-UP TYP
<1 kΩ OR
TOTEM POLE
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
CC
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.
Figure 46. Fan with Strong Tach Pull-Up to > VCC or Totem Pole Output,
Clamped with Zener and Resistor
12V
V
CC
12V
V
CC
PULL-UP
4.7kΩ
TYP
<1 kΩ
FAN(0–7)
R1*
FAN SPEED
COUNTER
FAN(0–7)
FAN SPEED
COUNTER
TACH
TACH
OUTPUT
OUTPUT
*SEE TEXT
Figure 44. Fan with Tach Pull-Up to +VCC
Figure 47. Fan with Strong Tach Pull-Up to >VCC or Totem Pole Output,
Attenuated with R1/R2
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.
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.
12V
V
CC
PULL-UP
4.7kΩ
TYP
TACH
OUTPUT
FAN(0–7)
FAN SPEED
COUNTER
ZD1*
ZENER
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
CC
22.5kHz
CLOCK
Figure 45. Fan with Tach Pull-Up to Voltage > VCC (e.g. 12 V),
Clamped with Zener Diode
CONFIGURATION
REG. 1 BIT 0
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.
2
3
4
1
FAN0
INPUT
1
2
4
3
FAN1
INPUT
R1 and R2 should be chosen such that
START OF
MONITORING
CYCLE
FAN0
MEASUREMENT
PERIOD
FAN1
MEASUREMENT
PERIOD
R2
2V < V
PULLUP
×
< 3.3 V STBY
(
R
+ R1 + R2
)
PULLUP
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
output.
INT
Rev. A | Page 25 of 56
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