Datasheet

ADT7470 Data Sheet
Rev. E | Page 22 of 40
FAN SPEED MEASUREMENT
TACH INPUTS
Pin 6, Pin 7, Pin 4, and Pin 9 are open-drain tach inputs
intended for fan speed measurement.
Signal conditioning in the ADT7470 accommodates the slow
rise and fall times typical of fan tachometer outputs. The maxi-
mum input signal range is 0 V to 5 V, even where 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 included to keep inputs within an
acceptable range. Figure 20 to Figure 23 show circuits for most
common fan tach outputs.
If the fan tach output has a resistive pull-up to V
CC
, it can be
connected directly to the fan input, as shown in Figure 20.
12V
FAN SPEED
COUNTER
TACH
PULLUP
4.7k
TYP
TACH
OUTPUT
V
CC
ADT7470
04684-0-025
Figure 20. Fan with Tach Pull-Up to V
CC
If the fan output has a resistive pull-up to 12 V (or other voltage
greater than 5 V), the fan output can be clamped with a Zener
diode, as shown in Figure 21. The Zener diode voltage should
be chosen so that it is greater than V
IH
of the tach input 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
FAN SPEED
COUNTER
TACH
TACH
OUTPUT
ZD1*
ZENER
PULLUP
4.7k
TYP
*CHOOSE ZD1 VOLTAGE APPROX. 0.8  V
CC
V
CC
ADT7470
04684-0-026
Figure 21. Fan with Tach.
Pull-Up to voltage > 5 V, for example, 12 V clamped with Zener diode.
If the fan output has a resistive pull-up to 12 V (or other voltage
greater than 5 V), the fan output can be clamped with a Zener
diode, as shown in Figure 21. The Zener diode voltage should
be chosen so that it is greater than V
IH
of the tach input but less
than 5 V, allowing for the voltage tolerance of the Zener. A value
of between 3 V and 5 V is suitable.
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 22. Alternatively, a resistive
attenuator can be used, as shown in Figure 23.
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 nominally 160 kΩ 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 kΩ, suitable values for R1 and R2 are 100 kΩ and 47 kΩ.
This gives a high input voltage of 3.83 V.
12V
FAN SPEED
COUNTER
TACH
PULLUP
TYP. <1k
OR TOTEM-POLE
ZD1
ZENER*
R1
10k
TACH
O/P
*CHOOSE ZD1 VOLTAGE APPROX. 0.8  V
CC
V
CC
ADT7470
04684-0-027
Figure 22. Fan with Strong Tach.
Pull-Up to > V
CC
or Totem-Pole Output, Clamped with Zener and Resistor.
12V
FAN SPEED
COUNTER
TACH
TACH
OUTPUT
R1*
R2*
<1k
V
CC
*SEE TEXT
ADT7470
04684-0-028
Figure 23. Fan with Strong Tach.
Pull-Up to > V
CC
or Totem-Pole Output, Attenuated with R1/R2.
Pulse Stretching
Pulse stretching of the PWM output is performed automatically
in low frequency fan drive mode, to ensure that sufficient tach
readings are taken from the fan.
However, in high frequency fan drive mode, pulse stretching is
disabled. If using 3-wire fans this mode, care should be taken to
ensure that incomplete tach information does not occur at low
PWM duty cycles, or short PWM pulse widths.
Disabling Tach measurement
The tach measurement for each fan can be disabled by writing
to Configuration Register 2 Bits[3:0], at Address 0x74.