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参数资料
| 型号： | NCT1008DMT3R2G |
| 厂商： | ON SEMICONDUCTOR |
| 元件分类： | 模拟信号调理 |
| 英文描述： | %2B-1℃ temperature monitor with series resistance cancellation |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, DSO8 |
| 封装： | 2 X 2 MM, 0.50 MM PITCH, LEAD FREE, DFN-8 |
| 文件页数： | 9/20页 |
| 文件大小： | 251K |
| 代理商： | NCT1008DMT3R2G |
NCT1008
http://onsemi.com
17
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 onchip 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. Selfheating due to the power dissipated in
the NCT1008 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 selfheating is negligible. In the case
of the NCT1008, the worstcase 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,
qJA, of
the 8lead DFN is approximately 142
°C/W.
Layout Considerations
Digital boards can be electrically noisy environments, and
the NCT1008 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:
Place the NCT1008 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.
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
GND
D+
D–
GND
5 MIL
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.
Thermocouple effects should not be a major problem as
1
°C corresponds to about 200 mV, and thermocouple
voltages are about 3 mV/
°C of temperature difference.
Unless there are two thermocouples with a big
temperature differential between them, thermocouple
voltages should be much less than 200 mV.
Place a 0.1 mF bypass capacitor close to the VDD pin. In
extremely noisy environments, place an input filter
capacitor across D+ and D close to the NCT1008. 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.
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 NCT1008. 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
NCT1008, 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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