ADT7473 Datasheet, PDF (27/76 Pages) Analog Devices – dBCool Remote Thermal Monitor and Fan Controller

ADT7473
first. Care should be taken in designing drive circuits with
transistors and FETs to ensure the PWM pins are not required
to source current and that they sink less than the 8 mA
maximum current specified on the data sheet.
Driving up to Three Fans from PWM3
TACH measurements for fans are synchronized to 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 pro-
grammed 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 35 and Figure 36. The SYNC bit in Register 0x62 enables
this function.
Synchronization is not required in high frequency mode when
used with 4-wire fans.
12V
ADT7473 3.3V
PWM3
1kΩ
2.2kΩ
3.3V
TACH3
3.3V
Q1
MMBT3904
10kΩ
Q2
MMBT2222
TACH4
3.3V
10kΩ
Q3
MMBT2222
Figure 35. Interfacing Two Fans in Parallel to the PWM3 Output Using
Low Cost NPN Transistors
3.3V
10kΩ
TYPICAL
TACH4
3.3V +V
+V
3.3V
ADT7473
TACH3
10kΩ
TYPICAL
TACH
3.3V
3.3V
5V OR ^1N4148
12V FAN
TACH
5V OR
12V FAN
PWM3
10kΩ
TYPICAL
Q1
NDT3055L
Figure 36. Interfacing Two Fans in Parallel to the PWM3 Output Using a
Single N-Channel MOSFET
Bit [4] (SYNC) of Enhance Acoustics Register 1 (0x62)
SYNC = 1, synchronizes TACH2, TACH3, and TACH4 to
PWM3.
TACH Inputs
Pin 4, Pin 6, Pin 7, and Pin 9 (when configured as TACH
inputs) are open-drain TACH inputs intended for fan speed
measurement.
Signal conditioning in the ADT7473 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 of the fan signal or diode
clamping must be included to keep inputs within an acceptable
range.
Figure 37 to Figure 40 show circuits for most common fan
TACH outputs.
If the fan TACH output has a resistive pull-up to VCC, it can be
connected directly to the fan input, as shown in Figure 37.
V_CC
12V
PULL-UP
4.7kΩ
TYPICAL
TACH
OUTPUT
TACH
FAN SPEED
COUNTER
ADT7473
Figure 37. Fan with TACH Pull-Up to VCC
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 38. The Zener diode voltage should
be chosen so that it is greater than VIH of the TACH input but
less than 3.6 V, allowing for the voltage tolerance of the Zener. A
value of between 3 V and 3.6 V is suitable.
12V
V_CC
PULL-UP
4.7kΩ
TYPICAL
TACH
OUTPUT TACH
ZD1*
FAN SPEED
COUNTER
ADT7473
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V_CC
Figure 38. Fan with TACH Pull-Up to Voltage > 3.6 V
Clamped with Zener Diode
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 39.
12V
V_CC
PULL-UP
4.7kΩ OR
TYPICAL
TACH
OUTPUT
TACH
ZD1
ZENER*
FAN SPEED
COUNTER
ADT7473
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V_CC
Figure 39. Fan with Strong TACH Pull-Up to > VCC or Totem-Pole Output,
Clamped with Zener and Resistor
Alternatively, a resistive attenuator can be used, as shown in
Figure 40. R1 and R2 should be chosen such that
2 V < VPULL-UP × R2/(RPULL-UP + R1 + R2) < 3.6 V
Rev. A | Page 27 of 76