ADT7481ARMZ-R7 datasheet(17/20 Pages) ONSEMI

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ADT7481

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Figure 19. Operation of the ALERT and THERM

Interrupts

THERM LIMIT

HIGH TEMP LIMIT

THERM LIMIT−HYSTERESIS

RESET BY MASTER

ALERT

THERM

100°C

TEMPERATURE

90°C

80°C

70°C

60°C

50°C

40°C

• If the measured temperature exceeds the high

temperature limit, the ALERT output will assert low.

• If the temperature continues to increase and exceeds the

THERM limit, the THERM output asserts low. This can

be used to throttle the CPU clock or switch on a fan.

• The THERM output de−asserts (goes high) when the

temperature falls to THERM limit minus hysteresis. In

Figure 19, the default hysteresis value of 10

°C is

shown.

• The ALERT output de−asserts only when the

temperature has fallen below the high temperature

limit, and the master has read the device address and

cleared the status register.

Pin 8 on the ADT7481 can be configured as either an

ALERT output or as an additional THERM output.

THERM2 will assert low when the temperature exceeds the

programmed local and/or remote high temperature limits. It

is reset in the same manner as THERM, and it is not

maskable. The programmed hysteresis value also applies to

THERM2.

Figure 20 shows how THERM and THERM2 might

operate together to implement two methods of cooling the

system. In this example, the THERM2 limits are set lower

than the THERM limits. The THERM2 output could be used

to turn on a fan. If the temperature continues to rise and

exceeds the THERM limits, the THERM output could

provide additional cooling by throttling the CPU.

Figure 20. Operation of the THERM and THERM2

Interrupts

THERM2 LIMIT

THERM LIMIT

THERM2

TEMPERATURE

THERM

90°C

80°C

70°C

60°C

50°C

40°C

30°C

When the THERM2 limit is exceeded, the THERM2

signal asserts low.

• If the temperature continues to increase and exceeds the

THERM limit, the THERM output asserts low.

• , there is no hysteresis value shown.

• As the system cools further, and the temperature falls

below the THERM2 limit, the THERM2 signal resets.

Again, no hysteresis value is shown for THERM2.

The temperature measurement could be either the local or

the remote temperature measurement.

Applications Information

Noise Filtering

For temperature sensors operating in noisy environments,

previous practice was to place a capacitor across the D+ and

D− pins to help combat the effects of noise. However, large

capacitances affect the accuracy of the temperature

measurement, leading to a recommended maximum

capacitor value of 1,000 pF.

Factors Affecting Diode Accuracy

Remote Sensing Diode

The ADT7481 is designed to work with substrate

transistors built into processors or with discrete transistors.

Substrate transistors will generally be PNP types with the

collector connected to the substrate. Discrete types can be

either a PNP or an NPN transistor connected as a diode (base

shorted to collector). If an NPN transistor is used, the

collector and base are connected to D+ and the emitter to D−.

If a PNP transistor is used, the collector and base are

connected to D− and the emitter to D+.

To reduce the error due to variations in both substrate and

discrete transistors, a number of factors should be taken into

consideration:

• The ideality factor, nf, of the transistor is a measure of

the deviation of the thermal diode from ideal behavior.

The ADT7481 is trimmed for an nf value of 1.008. Use

the following equation to calculate the error introduced

at a temperature, T (

°C), when using a transistor where

nf does not equal 1.008. Consult the processor data

sheet for the nf values.

(eq. 2)

DT + nf * 1.008 1.008

273.15 Kelvin ) T

To factor this in, the user can write the

DT value to the

offset register. It will then automatically be added to, or

subtracted from, the temperature measurement by the

ADT7481.

• Some CPU manufacturers specify the high and low

current levels of the substrate transistors. The high

current level of the ADT7481, IHIGH, is 233 mA. The

low level current, ILOW, is 14 mA. If the ADT7481

current levels do not match the current levels specified

by the CPU manufacturer, it may become necessary to

remove an offset. The CPU data sheet will advise

whether this offset needs to be removed and how to

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