ADT7421 datasheet(16/20 Pages) ONSEMI

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ADT7421

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inductance and reduce noise pickup, a 5−mil track

width and spacing is recommended. Provide a ground

plane under the tracks, if possible.

Figure 16. Typical Arrangement of Signal Tracks

GND

D+

D–

GND

5MIL

• 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

°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 ADT7421.

This capacitance can effect the temperature

measurement, so ensure that any capacitance seen at D+

and D− is, at maximum, 2200 pF. This maximum value

includes the filter capacitance, plus any cable or stray

capacitance between the pins and the sensor transistor.

• 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 ADT7421. 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 17 shows a typical application circuit for the

ADT7421, using a discrete sensor transistor connected via

a shielded, twisted pair cable. The pull−ups on SCLK,

SDATA, and ALERT are required only if they are not

provided elsewhere in the system.

Figure 17. Typical Application Circuit

5V OR 12V

SMBUS

CONTROLLER

FAN CONTROL

CIRCUIT

CPU THERMAL

DIODE

2N3906

D+

D–

VDD

SCLK

SDATA

ALERT/

THERM2

THERM

GND

ADT7421

0.1mF

VDD

TYP 10k

FAN ENABLE

3V TO 3.6V

TYP 10k

SHIELD

CPU

ADT7421 Register Details

Table 10. Status/Configuration Registers

Read Address

(Hex)

Write Address

(Hex)

Name

Power−On Default

Binary

Hex

Decimal

Not Applicable

Address Pointer

Undefined

Not Applicable

Status

Undefined

Configuration

0000 1000

0x08

Conversion Rate

0000 0110

0x06

Not Applicable

One−Shot

Consecutive ALERT

0000 0001

0x01

Not Applicable

Device ID

0010 0001

0x21

Not Applicable

Manufacturer ID

0100 0001

0x41

Not Applicable

Die Revision Code

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