NCT210

http://onsemi.com

13

obtained by choosing devices according to the following

criteria:

1. Base-emitter voltage greater than 0.25 V at 6

mA,

at the highest operating temperature.

2. Base-emitter voltage less than 0.95 V at 100

mA, at

the lowest operating temperature.

3. Base resistance less than 100

W.

Transistors, such as 2N3904, 2N3906, or equivalents, in

SOT−23 package 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, and a

number of factors can affect this. Ideally, the sensor should be

in good thermal contact with the part of the system being

measured, for example the processor. If it is not, the thermal

inertia caused by the mass of the sensor causes a lag in the

response of the sensor to a temperature change. For the

remote sensor, this should not be a problem, because it is

either a substrate transistor in the processor or a small package

device, such as SOT−23, placed in close proximity to it.

The on-chip sensor is, however, often remote from the

processor and only monitors the general ambient

temperature around the package. The thermal time constant

of the QSOP−16 package is approximately 10 seconds.

In practice, the package will have an electrical, and hence

a thermal, connection to the printed circuit board, so the

temperature rise due to self-heating is negligible.

Layout Considerations

Digital boards can be electrically noisy environments, and

because the NCT210 is measuring very small voltages from

the remote sensor, care must be taken to minimize noise

induced at the sensor inputs. The following precautions

should be taken:

1. Place the NCT210 as close as possible to the

remote sensing diode. Provided that the worst

noise sources, such as clock generators,

data/address buses, and CRTs, are avoided, this

distance can be four to eight inches.

2. Route the D+ and D− tracks close together, in

parallel, with grounded guard tracks on each side.

Provide a ground plane under the tracks, if

possible.

3. Use wide tracks to minimize inductance and

reduce noise pickup. 10 mil track minimum width

and spacing is recommended.

4. Try to minimize the number of copper/solder

joints, which can cause thermocouple effects.

Where copper/solder joints are used, ensure they

are in both the D+ and D− paths and at the same

temperature.

Thermocouple effects should not be a major

problem as 1

°C corresponds to about 240 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 240

mV.

5. Place a 0.1

pin, and 2,200 pF input filter capacitors across D+,

D− close to the NCT210.

6. If the distance to the remote sensor is more than

eight inches, the use of twisted pair cable is

recommended. This works up to about 6 to 12 feet.

7. For very long distances (up to 100 feet), use

shielded twisted pair, such as Belden #8451

microphone cable. Connect the twisted pair to D+

and D− and the shield to GND close to the

NCT210. Leave the remote end of the shield

unconnected to avoid ground loops.

Figure 17. Arrangement of Signal Tracks

10 MIL

10 MIL

10 MIL

10 MIL

10 MIL

10 MIL

10 MIL

GND

D−

D+

GND

Because the measurement technique uses switched

current sources, excessive cable and/or filter capacitance

can affect the measurement. When using long cables, the

filter capacitor can be reduced or removed. Cable resistance

can also introduce errors. A series resistance of 1

W

introduces about 1

°C error.

Application Circuits

Figure 18 shows a typical application circuit for the

NCT210, 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 already

provided elsewhere in the system.

The SCLK and SDATA pins of the NCT210 can be

interfaced directly to the SMBus of an I/O chip. Figure 19

shows how the NCT210 might be integrated into a system

using this type of I/O controller.