EVAL-ADT7463EB ON Semiconductor, EVAL-ADT7463EB Datasheet - Page 27

• Manufacturer Part Number: EVAL-ADT7463EB
• Manufacturer: ON Semiconductor

Specifications of EVAL-ADT7463EB

• Lead Free Status / RoHS Status: Supplier Unconfirmed

Driving 2-Wire Fans
Figure 33 shows how a 2-wire fan may be connected to the
ADT7463. 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
commutation pulses into a voltage. This is ac-coupled into the
ADT7463 through the 0.01 µF capacitor. On-chip signal
conditioning allows accurate monitoring of fan speed. The
value of R
threshold and the current drawn by the fan. For fans drawing
approximately 200 mA, a 2 Ω R
threshold is programmed as 40 mV. For fans that draw more
current, such as larger desktop or server fans, R
reduced for the same programmed threshold. The smaller the
threshold programmed the better, since more voltage is developed
across the fan and the fan spins faster. Figure 34 shows a typical
plot of the sensing waveform at the 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.
REV. C
Figure 34. Fan Speed Sensing Waveform at TACH/AIN Pin
SENSE
ADT7463
TACH/AIN
Figure 33. Driving a 2-Wire Fan
chosen depends upon the programmed input
PWM
SENSE
, in the fan circuit converts the fan
3.3V
10k
TYPICAL
0.01 F
SENSE
12V FAN
5V OR
value is suitable when the
+V
Q1
NDT3055L
R
2
TYPICAL
SENSE
SENSE
1N4148
may be
–27–
LAYING OUT 2-WIRE AND 3-WIRE FANS
Figure 35 shows how to lay out a common circuit arrangement
for 2-wire and 3-wire fans. Some components are not populated,
depending on whether a 2-wire or 3-wire fan is being used.
TACH Inputs
Pins 9, 11, 12, and 14 are open-drain TACH inputs intended
for fan speed measurement.
Signal conditioning in the ADT7463 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
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 36a to 36d show circuits for most common fan
TACH outputs.
If the fan TACH output has a resistive pull-up to V
be connected directly to the fan input, as shown in Figure 36a.
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 36b. 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.
Figure 35. Planning for 2-Wire or 3-Wire Fans on a PCB
12V
TACH/AIN
Figure 36a. Fan with TACH Pull-Up to V
PULL-UP
4.7k
TYP
R1
R2
R3
TACH
OUTPUT
C1
12V OR 5V
R4
TACH
Q1
MMBT2222
FOR 3-WIRE FANS:
POPULATE R1, R2, R3
R4 = 0
C1 = UNPOPULATED
FOR 2-WIRE FANS:
POPULATE R4, C1
R1, R2, R3 UNPOPULATED
1N4148
R5
3.3V OR 5V
IH
CC
ADT7463
ADT7463
FAN SPEED
of the TACH
COUNTER
is less than 5 V.
V
CC
PWM
CC
CC
, it can