参数资料
| 型号： | ADT7462ZEVB |
| 厂商： | ON Semiconductor |
| 文件页数： | 30/82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
5.0 V or 12 V
FAN
V CC
several seconds to accumulate a reasonably large and
accurate count. Instead, the period of the fan revolution is
measured by gating an on-chip 90 kHz oscillator into the
PULLUP
TYP < 1 k W
R1
10 k W
TACH
OUTPUT
TACH
ZD1*
ZENER
ADT7462
FAN SPEED
COUNTER
input of a 16-bit counter for N periods of the fan TACH
output (see Figure 47), so the accumulated count is actually
proportional to the fan tachometer period and inversely
proportional to the fan speed.
OR TOTEM-POLE
CLOCK
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 ? V CC
Figure 45. Fan with Strong TACH. Pullup to > V CC
or Totem - Pole Output, Clamped with
PWM
Zener Diode and Resistor
TACH
1
12 V
V CC
2
3
4
Figure 47. Fan Speed Measurement
< 1 k W
TACH
OUTPUT
R1*
TACH
R2*
ADT7462
FAN SPEED
COUNTER
To enable continuous measurement for 3-wire fans, set the
corresponding dc bit for the TACH input in the TACH
configuration register. This bit is set automatically when the
HF PWM is in use with 4-wire fans.
*SEE TEXT
Figure 46. Fan with Strong TACH. Pullup to > V CC
or Totem-Pole Output, Attenuated with R1/R2
Fan Speed Measurement
The method of fan speed measurement when using 3-wire
fans differs from that used with 4-wire fans. When 3-wire
fans are in use, power is continuously applied and removed
from the fan, thereby chopping the TACH information. As
a result, every time a fan speed measurement is to be made,
the fan must be switched on for a long enough period of time
that a measurement can be made. This is called pulse
stretching. With 4-wire fans, power is always applied to the
fan, so fan speed measurements can be made continuously,
and there is no need for pulse stretching. Pulse stretching is
also not necessary when driving a 3-wire fan with a dc input.
The Fan Speed Measurement with Pulse Stretching section,
which describes how fan speed is measured both when pulse
stretching is required and when it is not.
Fan Speed Measurement Without Pulse Stretching
Fan speed is measured by the ADT7462, and the result is
stored in the fan TACH value registers. The fan counter does
not count the fan TACH output pulses directly because the
fan speed can be less than 1000 rpm, and it would take
Fan Speed Measurement with Pulse Stretching
The method for measuring fan speed for 3-wire fans
requiring pulse stretching is similar to the method described
in the Fan Speed Measurement without Pulse Stretching
section for continuous measurements. The main difference
is that the PWM drive must be synchronized to the TACH
input so that the ADT7462 knows that pulse stretching is
taking place while the TACH is being measured.
PWM1 is synchronized with TACH1 and TACH2.
PWM2 is synchronized with TACH3 and TACH4.
PWM3 is synchronized with TACH5 and TACH6.
PWM4 is synchronized with TACH7 and TACH8.
Driving and Measuring the Speed of Two Fans from
One PWM Output
When pulse stretching is enabled, the ADT7462 measures
fan speed once a second. The counter then counts up from
the first to the third TACH pulse; this value is stored in the
TACH value register. The PWM drive returns to its previous
programmed value. Each TACH input is synchronized to a
particular PWM output. The PWM and TACH pins must be
connected as shown in Figure 48 to ensure that pulse
stretching is synchronized between the PWM output and the
TACH inputs, and an accurate fan speed measurement is
made on each fan.
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