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| 型号： | NCT72CMNR2G |
| 厂商： | ON Semiconductor |
| 文件页数： | 17/20页 |
| 文件大小： | 326K |
| 描述： | IC REMOTE THERMAL SENSOR 8-DFN |
| 标准包装： | 1 |
| 功能： | 温度监控系统（传感器） |
| 传感器类型： | 内部和外部 |
| 感应温度： | 0°C ~ 127°C，-64°C ~ 191°C |
| 精确度： | ±2.5°C |
| 拓扑： | ADC，比较器，多路复用器，寄存器库 |
| 输出类型： | 2 线串行，I²C?/SMBUS? |
| 输出警报： | 是 |
| 输出风扇： | 是 |
| 电源电压： | 2.8 V ~ 3.6 V |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-VFDFN 裸露焊盘 |
| 供应商设备封装： | 8-DFN（3x3） |
| 包装： | 标准包装 |
| 其它名称： | NCT72CMNR2GOSDKR |
NCT72
http://onsemi.com
17
?SPAN class="pst NCT72CMNR2G_2296365_4"> Base resistance less than 100 W
?SPAN class="pst NCT72CMNR2G_2296365_4"> Small variation in h
FE
(50 to 150) that indicates tight
control of V
BE
characteristics
Transistors, such as the 2N3904, 2N3906, or equivalents
in SOT23 packages are suitable devices to use.
Thermal Inertia and Self-heating
Accuracy depends on the temperature of the remote
sensing diode and/or the internal temperature sensor being
at the same temperature as that being measured. Many
factors can affect this. Ideally, place the sensor in good
thermal contact with the part of the system being measured.
If it is not, the thermal inertia caused by the sensors mass
causes a lag in the response of the sensor to a temperature
change. In the case of the remote sensor, this should not be
a problem since it is either a substrate transistor in the
processor or a small package device, such as the SOT23,
placed in close proximity to it.
The on-chip sensor, however, is often remote from the
processor and only monitors the general ambient
temperature around the package. How accurately the
temperature of the board and/or the forced airflow reflects
the temperature to be measured dictates the accuracy of the
measurement. Self-heating due to the power dissipated in
the NCT72 or the remote sensor causes the chip temperature
of the device or remote sensor to rise above ambient.
However, the current forced through the remote sensor is so
small that self-heating is negligible. In the case of the
NCT72, the worst-case condition occurs when the device is
converting at 64 conversions per second while sinking the
maximum current of 1 mA at the ALERT
and THERM
output. In this case, the total power dissipation in the device
is about 4.5 mW. The thermal resistance, q
JA
, of the 8-lead
DFN is approximately 142癈/W.
Layout Considerations
Digital boards can be electrically noisy environments, and
the NCT72 is measuring very small voltages from the
remote sensor, so care must be taken to minimize noise
induced at the sensor inputs. Take the following precautions:
?SPAN class="pst NCT72CMNR2G_2296365_4"> Place the NCT72 as close as possible to the remote
sensing diode. Provided that the worst noise sources,
that is, clock generators, data/address buses, and CRTs
are avoided, this distance can be 4 inches to 8 inches.
?SPAN class="pst NCT72CMNR2G_2296365_4"> Route the D+ and D tracks close together, in parallel,
with grounded guard tracks on each side. To minimize
inductance and reduce noise pickup, a 5 mil track width
and spacing is recommended. Provide a ground plane
under the tracks, if possible.
Figure 22. Typical Arrangement of Signal Tracks
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
GND
D
D+
GND
?SPAN class="pst NCT72CMNR2G_2296365_4"> Try to minimize the number of copper/solder joints that
can cause thermocouple effects. Where copper/solder
joints are used, make sure that they are in both the D+
and D path and at the same temperature.
?SPAN class="pst NCT72CMNR2G_2296365_4"> Thermocouple effects should not be a major problem as
1癈 corresponds to about 200 mV, and thermocouple
voltages are about 3 mV/癈 of temperature difference.
Unless there are two thermocouples with a big
temperature differential between them, thermocouple
voltages should be much less than 200 mV.
?SPAN class="pst NCT72CMNR2G_2296365_4"> Place a 0.1 mF bypass capacitor close to the V
DD
pin. In
extremely noisy environments, place an input filter
capacitor across D+ and D close to the NCT72. This
capacitance can effect the temperature measurement, so
ensure that any capacitance seen at D+ and D is, at
maximum, 1,000 pF. This maximum value includes the
filter capacitance, plus any cable or stray capacitance
between the pins and the sensor diode.
?SPAN class="pst NCT72CMNR2G_2296365_4"> If the distance to the remote sensor is more than
8 inches, the use of twisted pair cable is recommended.
A total of 6 feet to 12 feet is needed.
For really long distances (up to 100 feet), use a shielded
twisted pair, such as the Belden No. 8451 microphone
cable. Connect the twisted pair to D+ and D and the
shield to GND close to the NCT72. Leave the remote
end of the shield unconnected to avoid ground loops.
Because the measurement technique uses switched
current sources, excessive cable or filter capacitance can
affect the measurement. When using long cables, the filter
capacitance can be reduced or removed.
Application Circuit
Figure 23 shows a typical application circuit for the
NCT72, using a discrete sensor transistor connected via a
shielded, twisted pair cable. The pullups on SCLK, SDATA,
and ALERT
are required only if they are not provided
elsewhere in the system.
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