ADT7483AARQZ-R7 datasheet(22/24 Pages) ONSEMI

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ADT7483A

Rev. 1 | Page 22 of 24 | www.onsemi.com

APPLICATIONS

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 ADT7483A 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 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, the following 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

ADT7483A 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 whose

nf does

not equal 1.008. Consult the processor data sheet for the

values.

(

)

(

)

Kelvin

273

008

–

To factor this in, write the ΔT value to the offset register. It

is then automatically added to, or subtracted from, the

temperature measurement by the ADT7483A.

•

Some CPU manufacturers specify the high and low current

levels of the substrate transistors. The high current level of

the ADT7483A, IHIGH, is 200 μA, and the low level current,

ILOW, is 12 μA. If the ADT7483A current levels do not match

the current levels specified by the CPU manufacturer, it may

be necessary to remove an offset. Refer to the CPU data

sheet to determine whether this offset needs to be removed

and how to calculate it. This offset is programmed to the

offset register. It is important to note that if more than one

offset must be considered, program the algebraic sum of

these offsets to the offset register.

If a discrete transistor is used with the ADT7483A, the best

accuracy is obtained by choosing devices according to the

following criteria:

•

Base-emitter voltage greater than 0.25 V at 6 μA, at the

highest operating temperature.

•

Base-emitter voltage less than 0.95 V at 100 μA, at the

lowest operating temperature.

•

Base resistance less than 100 Ω.

•

Small variation in hFE (50 to 150) that indicates tight

control of VBE characteristics.

Transistors such as 2N3904, 2N3906, or equivalents in SOT-23

packages, are suitable devices to use.

THERMAL INERTIA AND SELF-HEATING

Accuracy depends on the temperature of the remote sensing

diode and/or the local temperature sensor being at the same

temperature as that being measured. A number of factors can

affect this. Ideally, the sensor should be in good thermal contact

with the part of the system being measured. If it is not, the

thermal inertia caused by the sensor’s 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 will

either be 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, however, is often remote from the

processor and only monitors the general ambient temperature

around the package. In practice, the ADT7483A 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. Self-heating, due to

the power dissipated in the ADT7483A 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 ADT7483A, 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. The thermal resistance, θJA, of the

QSOP-16 package is about 150°C/W.

ADT7483AARQZ-R7 Datasheet(PDF) 22 Page - ON Semiconductor

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