參數(shù)資料
型號： ADT7462ACPZ-500RL7
廠商： ON SEMICONDUCTOR
元件分類： 溫度/濕度傳感器
英文描述： DIGITAL TEMP SENSOR-SERIAL, 8BIT(s), 4Cel, SQUARE, SURFACE MOUNT
封裝： 5 X 5 MM, LFCSP-32
文件頁數(shù)： 28/88頁
文件大小： 836K
代理商： ADT7462ACPZ-500RL7
ADT7462
Rev. 2 | Page 34 of 88 | www.onsemi.com
1
2
3
4
CLOCK
PWM
TACH
05
56
9-
03
8
Figure 48. Fan Speed Measurement
To enable continuous measurement for 3-wire fans, set the
corresponding dc bit for the TACH input in the TACH configu-
ration register. This bit is set automatically when the HF PWM
is in use with 4-wire fans.
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 49 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.
PWM1
PWM2
PWM3
PWM4
FAN 1
FAN 2
FAN 3
FAN 4
TACH1
TACH2
TACH3
TACH4
FAN 5
FAN 6
FAN 7
FAN 8
TACH5
TACH6
TACH7
TACH8
0556
9-
087
Figure 49. Synchronizing Fan PWM Output and TACH Inputs
Driving and Measuring the Speed of One Fan from One
PWM Output
If four single fans are being controlled and measured by the
ADT7462, the following configuration should be used. This
applies only to 3-wire fans controlled using low frequency
PWM with pulse stretching enabled.
Fan 1 is driven by PWM1 and measured using TACH1.
Fan 2 is driven by PWM2 and measured using TACH3.
Fan 3 is driven by PWM3 and measured using TACH5.
Fan 4 is driven by PWM4 and measured using TACH7.
PWM1
PWM2
PWM3
PWM4
FAN 1
FAN 2
TACH1
TACH3
FAN 3
FAN 4
TACH5
TACH7
05
569
-0
88
Figure 50. Driving and Measuring the Speed on a Single Fan
The PWM output is pulse stretched until a valid TACH is read
on both TACH inputs synchronized to the particular PWM
output. If one fan is connected to one PWM output, the PWM
output is pulse stretched until the counter has timed out on the
disconnected TACH input. In this case, the pulse is stretching
longer than necessary in an effort to sense a disconnected fan.
The speed of the connected fan may be increased and an
audible change in fan speed may be observed. There are two
options to prevent the PWM output from being stretched
longer than necessary in this case.
Connect the two synchronized TACH inputs together; for
example, if PWM1 is driving a single fan being sensed on
TACH1 only, connect TACH1 and TACH2 together.
Turn off pulse stretching on the unused TACH input; that
is, if PWM1 is driving a single fan being sensed on TACH1
only, turn off pulse stretching on TACH 2 in Register 0x08.
In this register,
Bit 0 controls pulse stretching on TACH1 and TACH5.
Bit 1 controls pulse stretching on TACH2 and TACH6.
Bit 2 controls pulse stretching on TACH3 and TACH7.
Bit 3 controls pulse stretching on TACH4 and TACH8.
Note that the TACH assignments in this register differ
from the TACHs synchronized to each PWM output.
Therefore, if the intention is to drive and sense four fans,
connecting the TACHs together as described in Option 1
allows pulse stretching on all channels.
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