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參數(shù)資料
| 型號： | NCT214MT3R2G |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 模擬信號調(diào)理 |
| 英文描述： | SPECIALTY ANALOG CIRCUIT, QCC10 |
| 封裝： | 3 X 3 MM, 0.50 MM PITCH, LEAD FREE, WQFN-10 |
| 文件頁數(shù)： | 9/19頁 |
| 文件大?。?/td> | 269K |
| 代理商： | NCT214MT3R2G |
NCT214
http://onsemi.com
17
Thermal Inertia and SelfHeating
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 sensor’s 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 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 NCT214 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 NCT214, 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.
Layout Considerations
Digital boards can be electrically noisy environments, and
the NCT214 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 NCT214 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 NCT214. 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 NCT214. 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
NCT214, 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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