参数资料
| 型号： | ADT7468ZEVB |
| 厂商： | ON Semiconductor |
| 文件页数： | 30/81页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR ADT7468 |
| 标准包装： | 1 |
| 其它名称： | EVAL-ADT7468EB EVAL-ADT7468EB-ND |
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ADT7468
Driving 2-Wire Fans
The ADT7468 can support 2-wire fans only when the low
frequency PWM mode is selected in Configuration Register 5,
Bit 2. If this bit is not set to 1, the ADT7468 cannot measure the
speed of 2-wire fans.
Figure 39 shows how a 2-wire fan can be connected to the
ADT7468. This circuit allows the speed of a 2-wire fan to be
measured, although the fan has no dedicated TACH signal. A
series resistor, R SENSE , in the fan circuit converts the fan
commutation pulses into a voltage, which is ac-coupled into the
ADT7468 through the 0.01 μF capacitor. On-chip signal
conditioning allows accurate monitoring of fan speed. The
For fans that draw more current, such as larger desktop or
server fans, R SENSE can be reduced for the same programmed
threshold. The smaller the threshold programmed the better,
because more voltage is developed across the fan and the fan
spins faster. Note that when the voltage spikes (either negative
or positive) are more than 40 mV in amplitude, the fan speed
can be reliably determined.
LAYING OUT 2-WIRE AND 3-WIRE FANS
Figure 41 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 used.
12V OR 5V
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 ? R SENSE value is suitable when the
threshold is programmed as 40 mV.
R1
R2
1N4148
3.3V OR 5V
R5
+V
TACH
C1
Q1
MMBT2222
PWM
ADT7468
3.3V
10k ?
5V OR
12V FAN
1N4148
R3
R4
FOR 3-WIRE FANS:
POPULATE R1, R2, R3
R4 = 0W
C1 = UNPOPULATED
TYPICAL
PWM
TACH
Q1
NDT3055L
0.01 μ F
R SENSE
2 ?
TYPICAL
Figure 39. Driving a 2-Wire Fan
FOR 2-WIRE FANS:
POPULATE R4, C1
R1, R2, R3 UNPOPULATED
Figure 41. Planning for 2-Wire or 3-Wire Fans on a PCB
TACH Inputs
Pins 9, 11, 12, and 14 (when configured as TACH inputs) are
open-drain TACH inputs intended for fan speed measurement.
Signal conditioning in the ADT7468 accommodates the slow
Figure 40 shows a typical plot of the sensing waveform at the
TACH/AIN pin.
rise and fall times typical of fan tachometer outputs. The
maximum input signal range is 0 V to 5 V, even when V CC is less
than 5 V. When 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.
Figure 42 to Figure 45 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 42.
V CC
12V
PULL-UP
4.7k ?
TYPICAL
TACH
OUTPUT
TACH
FAN SPEED
COUNTER
ADT7468
Figure 42. Fan with TACH Pull-Up to V CC
Figure 40. Fan Speed Sensing Waveform at TACH/AIN Pin
Rev. 3 | Page 30 of 81 | www.onsemi.com
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