ADT7468 Analog Devices, Inc., ADT7468 Datasheet - Page 31

• Manufacturer Part Number: ADT7468
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

If the fan output has a resistive pull-up to 12 V (or other voltage
greater than 5 V) then the fan output can be clamped with a
Zener diode, as shown in Figure 43. The Zener diode voltage
should be chosen so that it is greater than V
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 (<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 44.
Alternatively, a resistive attenuator can be used, as shown in
Figure 45. R1 and R2 should be chosen such that
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 would be 100 kΩ and
47 kΩ, respectively. This gives a high input voltage of 3.83 V.
FAN
5V OR 12V
Figure 44. Fan with Strong TACH Pull-Up Greater than V
Figure 45. Fan with Strong TACH Pull-Up Greater than V
Figure 43. Fan with TACH Pull-Up Greater than 5 V, for example, 12 V),
12V
2 V < V
12V
PULL-UP TYP
TOTEM POLE
PULL-UP
<1kΩ OR
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
PULL-UP
Output, Clamped with Zener and Resistor
TYPICAL
<1kΩ
4.7kΩ
× R2/(R
Output, Attenuated with R1/R2
Clamped with Zener Diode
TACH
OUTPUT
TACH
OUTPUT
10kΩ
TACH
OUTPUT
R1*
R1
PULL-UP
*SEE TEXT
+ R1 + R2) < 5 V
ZD1
ZENER*
R2*
ZD1*
TACH
TACH
TACH
IH
ADT7468
ADT7468
ADT7468
FAN SPEED
FAN SPEED
FAN SPEED
of the TACH
COUNTER
COUNTER
COUNTER
CC
V
V
V
CC
CC
CC
CC
or Totem-Pole
or Totem-Pole
CC
CC
Rev. A | Page 31 of 84
FAN SPEED MEASUREMENT
The fan counter does not count the fan TACH output pulses
directly, because the fan speed could be less than 1,000 RPM
and it would take several seconds to accumulate a reasonably
large and accurate count. Instead, the period of the fan revo-
lution is measured 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 46), 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 settings
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.
Fan Speed Measurement Registers
The fan tachometer readings are 16-bit values consisting of a
2-byte read from the ADT7468.
Reg. 0x28, TACH1 low byte = 0x00 default
Reg. 0x29, TACH1 high byte = 0x00 default
Reg. 0x2A, TACH2 low byte = 0x00 default
Reg. 0x2B, TACH2 high byte = 0x00 default
Reg. 0x2C, TACH3 low byte = 0x00 default
Reg. 0x2D, TACH3 high byte = 0x00 default
Reg. 0x2E, TACH4 low byte = 0x00 default
Reg. 0x2F, TACH4 high byte = 0x00 default
Reading Fan Speed from the ADT7468
The measurement of fan speeds involves a 2-register read for
each measurement. The low byte should be read first. This
causes the high byte to be frozen until both high and low byte
registers have been read, preventing erroneous TACH readings.
CLOCK
TACH
PWM
1
2
3
4
Figure 46. Fan Speed Measurement
ADT7468