ADT7481(2014) Просмотр технического описания (PDF) - ON Semiconductor

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ADT7481
(Rev.:2014)

Dual Channel Temperature Sensor and Overtemperature Alarm

ON Semiconductor 

ADT7481

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 will either be a substrate transistor in the

processor or a small package device, such as an SOT−23,

placed in close proximity to it.

The on-chip sensor, however, will often be remote from

the processor and only monitors the general ambient

temperature around the package. In practice, the ADT7481

package will be in electrical, and hence, thermal contact with

a PCB and may also be in a forced airflow. How accurately

the temperature of the board and/or the forced airflow

reflects the temperature to be measured will also affect the

accuracy of the measurement. Self-heating, due to the power

dissipated in the ADT7481 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. The

worst-case condition occurs when the ADT7481 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

MSOP−10 package is about 142°C/W.

Layout Considerations

Digital boards can be electrically noisy environments, and

the ADT7481 measures very small voltages from the remote

sensor, so care must be taken to minimize noise induced at

the sensor inputs. Take the following precautions:

1. Place the ADT7481 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 range from 4 to 8 inches.

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

parallel, with grounded guard tracks on each side.

To minimize inductance and reduce noise pick up,

a 5 mil track width and spacing is recommended.

Provide a ground plane under the tracks if

possible.

GND

D+

D−

GND

5 MIL

5 MIL

5 MIL

5 MIL

5 MIL

5 MIL

5 MIL

Figure 21. Typical Arrangement of Signal Tracks

3. 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 large

temperature differential between them,

thermocouple voltages should be much less than

200 mV.

4. Place a 0.1 mF bypass capacitor close to the VDD

pin. In extremely noisy environments, an input

filter capacitor may be placed across D+ and D−

close to the ADT7481. This capacitance can affect

the temperature measurement, so care must be

taken to ensure that any capacitance seen at D+

and D− is a maximum of 1,000 pF. This maximum

value includes the filter capacitance, plus any

cable or stray capacitance between the pins and the

sensor diode.

5. 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 of cable

is needed.

For really long distances (up to 100 feet), use

shielded twisted pair, such as Belden No. 8451

microphone cable. Connect the twisted pair to D+

and D− and the shield to GND close to the

ADT7481. 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 22 shows a typical application circuit for the

ADT7481, using discrete sensor transistors. 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 ADT7481 can be

interfaced directly to the SMBus of an I/O controller, such

as the Intel® 820 chipset.

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