ADT7475 Analog Devices, Inc., ADT7475 Datasheet - Page 28

• Manufacturer Part Number: ADT7475
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

ADT7475
If the fan output has a resistive pull-up to 12 V, or other voltage
greater than 3.6 V, the fan output can be clamped with a Zener
diode, as shown in Figure 37. The Zener diode voltage should
be chosen so that it is greater than V
less than 3.6 V, allowing for the voltage tolerance of the Zener.
A value between 3 V and 3.6 V is suitable.
If the fan has a strong pull-up (less than 1 kΩ) to 12 V or a
totem-pole output, then 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
The fan inputs can have an input resistance of 160 kΩ to 5.1 kΩ
to ground, which should be taken into account when calculat-
ing resistor values.
With a pull-up voltage of 12 V and pull-up resistor less than
1 kΩ, suitable values for R1 and R2 would be 100 kΩ and
33 kΩ, respectively. This gives a high input voltage of 2.95 V.
FAN
Figure 39. Fan with Strong TACH Pull-Up to > V
5V OR 12V
Figure 38. Fan with Strong TACH Pull-Up to > V
Figure 37. Fan with TACH Pull-Up to Voltage > 3.6 V (For Example, 12 V)
12V
2 V < V
12V
PULL-UP TYP
TOTEM POLE
PULL-UP
<1kΩ OR
1
1
PULL-UP
TYPICAL
CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
<1kΩ
4.7kΩ
Clamped with Zener and Resistor
× R2/(R
Clamped with Zener Diode
Attenuated with R1/R2
TACH
OUTPUT
TACH
OUTPUT
TACH
OUTPUT
10kΩ
R1
PULL-UP
R1
1
1
+ R1 + R2) < 3.6 V
SEE TEXT.
R2
ZD1
TACH
ZD1
ZENER
TACH
1
TACH
IH
1
of the TACH input but
1
CC
CC
or Totem-Pole Output,
or Totem-Pole Output,
ADT7475
FAN SPEED
FAN SPEED
ADT7475
FAN SPEED
ADT7475
COUNTER
COUNTER
COUNTER
V
V
V
CC
CC
CC
CC
CC
.
.
Rev. B | Page 28 of 68
Fan Speed Measurement
The fan counter does not count the fan TACH output pulses
directly because the fan speed could 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 meas-
ured by gating an on-chip 90 kHz oscillator into the input of a 16-
bit counter for N periods of the fan TACH output (see Figure 40),
so the accumulated count is actually proportional to the fan
tachometer period and inversely proportional to the fan speed.
N, the number of pulses counted, is determined by the set-
tings of Register 0x7B (TACH Pulses per Revolution register).
This register contains two bits for each fan, allowing one, two
(default), three, or four TACH pulses to be counted.
Measuring Fan TACH
When the ADT7475 starts up, TACH measurements are locked.
In effect, an internal read of the low byte has been made for
each TACH input. The net result of this is that all TACH
readings are locked until the high byte is read from the
corresponding TACH registers. All TACH related interrupts are
also ignored until the appropriate high byte is read.
Once the corresponding high byte has been read, TACH
measurements are unlocked and interrupts are processed as
normal.
Fan Speed Measurement Registers
The fan tachometer readings are 16-bit values consisting of a
2-byte read from the ADT7475.
Register 0x28, TACH1 Low Byte = 0x00 default
Register 0x29, TACH1 High Byte = 0x00 default
Register 0x2A, TACH2 Low Byte = 0x00 default
Register 0x2B, TACH2 High Byte = 0x00 default
Register 0x2C, TACH3 Low Byte = 0x00 default
Register 0x2D, TACH3 High Byte = 0x00 default
Register 0x2E, TACH4 Low Byte = 0x00 default
Register 0x2F, TACH4 High Byte = 0x00 default
CLOCK
TACH
PWM
1
2
3
4
Figure 40. Fan Speed Measurement