ADT7463
http://onsemi.com
28
PWM3 so PWM3 can drive 2 fans. Alternatively, PWM2
can be programmed to synchronize TACH2, TACH3, and
TACH4 to the PWM2 output. This allows PWM2 to drive
two or three fans. In this case, the drive circuitry looks the
same as shown in Figures 42 and 43. The SYNC bit in
Register 0x62 enables this function.
Table 33. SYNC: ENHANCE ACOUSTICS REGISTER 1
(REG. 0X62)
Bit
Mnemonic
Description
<4>
SYNC
1 Synchronizes TACH2, TACH3, and
TACH4 to PWM2.
Driving 2wire Fans
Figure 44 shows how a 2wire fan may be connected to
the ADT7463. This circuit allows the speed of a 2wire 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 commutation pulses into a voltage. This is
accoupled into the ADT7463 through the 0.01 mF
capacitor. Onchip 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 W 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 is
developed across the fan and the fan spins faster. Figure 45
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.
Figure 44. Driving a 2wire Fan
ADT7463
Q1
NDT3055L
10 kW
TYPICAL
PWM
3.3 V
+V
5 V
or
12 V
FAN
TACH/AIN
R
SENSE
2 W
TYPICAL
0.01 mF
Figure 45. Fan Speed Sensing Waveform at
TACH/AIN Pin
CH2 5.00mV
CH1 100mV
M 4.00ms
A CH1 2.00mV
CH3 50.0mV
n: 250 mV
@: 258mV
CH4 50.0mV
T -1.00000ms
Laying Out 2wire and 3wire Fans
Figure 46 shows how to lay out a common circuit
arrangement for 2wire and 3wire fans. Some components
are not populated, depending on whether a 2wire or 3wire
fan is being used.
Figure 46. Planning for 2wire or 3wire Fans
on a PCB
FOR 2WIRE FANS:
POPULATE R4, C1
R1, R2, R3 UNPOPULATED
FOR 3WIRE FANS:
POPULATE R1, R2, R3 R4 = 0 W
C1 = UNPOPULATED
Q1
MMBT2222
TACH/AIN
12 V or 5.0 V
1N4148
PWM
R1
R2
R3
R4
R5
3.3 V or 5.0 V
C1
TACH Inputs
Pins 9, 11, 12, and 14 are opendrain TACH inputs
intended for fan speed measurement.
Signal conditioning in the ADT7463 accommodates the
slow rise and fall times typical of fan tachometer outputs.
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