NCT214MT3R2G【PDF 17/19 页】IC TEMP SENSOR LOC/REM 10WDFN

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Sheet目录45

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NCT214

http://onsemi.com

" Small variation in h

(50 to 150) that indicates tight

control of V

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 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 self-heating

is negligible. In the case of the NCT214, 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.

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

5 MIL

GND

D+

GND

" 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.

" Place a 0.1 mF bypass capacitor close to the V

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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