参数资料
| 型号： | ADT7462ZEVB |
| 厂商： | ON Semiconductor |
| 文件页数： | 28/82页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION FOR ADT7462 |
| 产品变化通告： | MFG CHG Notification ADI to ON Semi |
| 标准包装： | 1 |
| 类型： | 温度传感器 |
| 适用于相关产品： | ADT7462 |
| 所含物品： | 评估板 |
| 其它名称： | EVAL-ADT7462EBZ EVAL-ADT7462EBZ-ND |
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ADT7462
(or on/off ratio) of a square wave applied to the fan to vary
the fan speed. The advantage of using PWM control is that
it uses a very simple external circuit. The specific circuit
used depends upon the type of fan.
There are three main fan types in use: 2-wire fans, 3-wire
fans, and 4-wire fans. The 2-wire fan has only power and
ground connections. The 3-wire fan has power and ground
connections and a TACH output to indicate the speed of the
fan. The 4-wire fan has power and ground connections, a
TACH output, and a PWM input. The PWM input is
connected directly to the PWM drive of the ADT7462 and
is used to control the speed of the fans.
For 2-wire and 3-wire fans, the low frequency PWM drive
signal should be selected. For 4-wire fans, the high
frequency PWM drive signal should be selected.
Using the ADT7462 with 2-wire Fans
Figure 39 shows the most typical circuit used with a
2-wire fan and illustrates how a 2-wire fan can be connected
to the ADT7462. The low frequency PWM mode must be
selected when using a 2-wire fan.
+V
current required by the fan being driven. A typical PC fan
can draw a nominal current ranging from a few hundred
milliamps to over an amp of current. Depending on the
current rating of the fan, a SOT device can be used where
board space is a concern. If several fans in parallel are driven
from a single PWM output or if larger server fans are driven,
the MOSFET must handle the higher current requirements.
The only other stipulation is that the MOSFET should have
a gate voltage drive, V GS < 3.3 V, for direct interfacing to the
PWM pins. V GS can be greater than 3.3 V as long as the
pullup on the gate is tied to 5.0 V. The MOSFET should also
have a low on resistance to ensure that there is not a
significant voltage drop across the FET, which would reduce
the voltage applied across the fan and reduce the full speed
of the fan.
Figure 40 uses a 10 k W pullup resistor for the TACH
signal. This assumes that the TACH signal is an
open-collector from the fan. In all cases, the TACH signal
from the fan must be kept below 5.0 V maximum to prevent
damaging the ADT7462. If in doubt as to whether the fan
used has an open-collector or totem-pole TACH output, use
one of the input signal conditioning circuits shown in the Fan
Speed Measurement section.
ADT7462
PWM
TACH
3.3 V
10 k W
TYPICAL
0.01 m F
5V
or
12 V
FAN
Q1
NDT3055L
R SENSE
2 W
TYPICAL
Driving a 3-wire fan with a PWM signal makes the fan
speed measurement more difficult because the TACH signal
is chopped by the PWM drive signal. Pulse stretching is
required in this case to make accurate fan speed
measurements. For more information, see the Fan Speed
Measurement section.
Using the ADT7462 with 4-wire Fans
Figure 41 shows the most typical circuit used with 4-wire
fans.
12 V
12 V
Figure 39. Driving a 2-wire Fan
Using the ADT7462 with 3-wire Fans
Figure 40 shows the most typical circuit used with a
TACH
10 k W
4.7 k W
10 k W
TACH
12 V, 4-WIRE FAN
V CC
TACH
PWM
3-wire fan.
12 V
12 V
ADT7462
3.3 V or 5.0 V
2 k W
TACH/AIN
10 k W
10 k W
12 V
FAN
PWM
ADT7462
4.7 k W
3.3 V
Figure 41. Driving a 4-wire Fan
PWM
10 k W
Q1
NDT3055L
Because the electronics in a 4-wire fan are powered
continuously, unlike previous PWM driven/powered fans,
4-wire fans tend to perform better than 3-wire fans,
especially for high frequency applications. 4-wire frames
also eliminate the requirement for pulse stretching, because
Figure 40. Driving a 3-wire Fan
The external circuitry required is very simple. A
MOSFET, such as the NDT3055L, is used as the pass device.
The specifications of the MOSFET depend on the maximum
the TACH signal is always available.
Driving Two Fans from Each PWM
Note that the ADT7462 has up to eight TACH inputs
available for fan speed measurement, but only four PWM
http://onsemi.com
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