ADT7460 Datasheet, PDF (24/48 Pages) Analog Devices – dB COOL Remote Thermal Controller and Fan Controller

ADT7460
Driving 2-Wire Fans
Figure 32 shows how a 2-wire fan may be connected to the
ADT7460. This circuit allows the speed of a 2-wire fan to be
measured, even though the fan has no dedicated TACH signal.
A series resistor, R_SENSE, in the fan circuit converts the fan commu-
tation pulses into a voltage. This is ac-coupled into the ADT7460
through the 0.01 µF capacitor. On-chip signal conditioning
allows accurate monitoring of fan speed. The value of R_SENSE
chosen depends upon the programmed input threshold and the
current drawn by the fan. For fans drawing approximately 200 mA,
a 2 Ω R_SENSE value is suitable when the threshold is programmed as
40 mV. For fans that draw more current, such as larger desktop
or server fans, R_SENSE may be reduced for the same programmed
threshold. The smaller the threshold programmed the better,
since more voltage will be developed across the fan and the fan
will spin faster. Figure 33 shows a typical plot of the sensing
waveform at a TACH/AIN pin. The most important thing is that
the voltage spikes (either negative going or positive going) are
more than 40 mV in amplitude. This allows fan speed to be
reliably determined.
ADT7460
PWM
TACH/AIN
3.3V
+V
5V OR
12V FAN
1N4148
10k⍀
TYPICAL
Q1
NDT3055L
0.01␮F
R_SENSE
2⍀
TYPICAL
Figure 32. Driving a 2-Wire Fan
LAYING OUT 2-WIRE AND 3-WIRE FANS
Figure 34 shows how to lay out a common circuit arrangement
for 2-wire and 3-wire fans. Some components will not be popu-
lated, depending on whether a 2-wire or 3-wire fan is being used.
12V OR 5V
R1
1N4148
3.3V OR 5V
R2
R5
TACH/AIN
C1
Q1
MMBT2222
PWM
R3
R4 _{FOR 3-WIRE FANS:}
POPULATE R1, R2, R3
R4 = 0⍀
C1 = UNPOPULATED
FOR 2-WIRE FANS:
POPULATE R4, C1
R1, R2, R3 UNPOPULATED
Figure 34. Planning for 2-Wire or 3-Wire Fans on a PCB
TACH Inputs
Pins 4, 6, 7, and 9 are open-drain TACH inputs intended for
fan speed measurement.
Signal conditioning in the ADT7460 accommodates the slow rise
and fall times typical of fan tachometer outputs. The maximum
input signal range is 0 V to 5 V, even where V_CC is less than 5 V.
In the event that 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.
Figures 35a to 35d 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 35a.
V_CC
12V
PULL-UP
4.7k⍀
TYP
TACH
OUTPUT
TACH
ADT7460
FAN SPEED
COUNTER
Figure 33. Fan Speed Sensing Waveform at
TACH/AIN Pin
Figure 35a. 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 35b. 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.
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