ADT7473_11 ONSEMI [ON Semiconductor], ADT7473_11 Datasheet - Page 26

• Manufacturer Part Number: ADT7473_11
• Description: dbCOOL Remote Thermal Monitor and Fan Control
• Manufacturer: ONSEMI [ON Semiconductor]

Driving up to Three Fans from PWM3
particular PWM channels; for example, TACH1 is
synchronized to PWM1. TACH3 and TACH4 are both
synchronized to PWM3, so PWM3 can drive two fans.
Alternatively, PWM3 can be programmed to synchronize
TACH2, TACH3, and TACH4 to the PWM3 output. This
allows PWM3 to drive two or three fans. In this case, the
drive circuitry looks the same, as shown in Figure 38 and
Figure 39. The SYNC bit in Register 0x62 enables this
function.
when used with 4−wire fans.
Bit [4] (SYNC) of Enhanced Acoustics Register 1
(0x62)
PWM3.
TACH Inputs
inputs) are open−drain TACH inputs intended for fan speed
measurement.
accommodates the slow rise and fall times typical of fan
tachometer outputs. The maximum input signal range is 0 V
to 3.6 V. In the event that these inputs are supplied from fan
outputs that exceed 0 V to 3.6 V, either resistive attenuation
ADT7473−1
ADT7473/
ADT7473−1
TACH measurements for fans are synchronized to
Synchronization is not required in high frequency mode
SYNC = 1, synchronizes TACH2, TACH3, and TACH4 to
Pin 4, Pin 6, Pin 7, and Pin 9 (when configured as TACH
Signal conditioning in the ADT7473/ADT7473−1
PWM3 Output Using a Single N−Channel MOSFET
ADT7473/
Figure 38. Interfacing Two Fans in Parallel to the
Figure 39. Interfacing Two Fans in Parallel to the
PWM3 Output UsingLow Cost NPN Transistors
TACH4
TACH3
PWM3
PWM3
3.3V
3.3V
3.3V
3.3V
2.2k
1k
10kΩ
TYPICAL
10kΩ
TYPICAL
10kΩ
TYPICAL
W
W
TACH
3.3V
3.3V
3.3V
Q1
MMBT3904
10k
10k
3.3V
TACH3
+V
W
W
Q1
NDT3055L
5V OR
12V FAN
Q2
MMBT2222
MMBT2222
1N4148
TACH
Q3
12V
+V
5V OR
12V FAN
http://onsemi.com
TACH4
3.3V
26
of the fan signal or diode clamping must be included to keep
inputs within an acceptable range.
TACH outputs.
be connected directly to the fan input, as shown in Figure 40.
voltage greater than 3.6 V), the fan output can be clamped
with a Zener diode, as shown in Figure 41. The Zener diode
voltage should be chosen so that it is greater than V
TACH input, but less than 3.6 V, allowing for the voltage
tolerance of the Zener. A value of between 3.0 V and 3.6 V
is suitable.
a totem−pole output, a series resistor can be added to limit
the Zener current, as shown in Figure 42.
in Figure 43. R1 and R2 should be chosen such that
2 V t V
160 kW to ground, which should be taken into account when
calculating resistor values.
Totem−Pole Output, Clamped with Zener and Resistor
Figure 40 to Figure 43 show circuits for most common fan
If the fan TACH output has a resistive pullup to V
If the fan output has a resistive pullup to 12 V (or other
If the fan has a strong pullup (less than 1 kW) to 12 V or
Figure 42. Fan with Strong TACH Pullup to > V
Alternatively, a resistive attenuator can be used, as shown
The fan inputs have an input resistance of nominally
Figure 41. Fan with TACH Pullup to Voltage > 3.6 V
12V
12V
12V
Figure 40. Fan with TACH Pullup to V
pullup
4.7kΩ OR
TYPICAL
PULLUP
TYPICAL
PULLUP
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 y V
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 y V
TYPICAL
PULLUP
Clamped with Zener Diode
4.7kΩ
R2 R
4.7kΩ
TACH
OUTPUT
pullup
TACH
OUTPUT
TACH
OUTPUT
) R1 ) R2 t 3.6 V
ZD1
ZENER*
ZD1*
TACH
TACH
TACH
ADT7473/
ADT7473-1
ADT7473/
ADT7473-1
FAN SPEED
ADT7473/
ADT7473-1
COUNTER
FAN SPEED
FAN SPEED
COUNTER
COUNTER
V
V
V
CC
CC
CC
CC
CC
CC
CC
IH
CC
(eq. 3)
, it can
of the
or