ADT7483A(2012) Ver la hoja de datos (PDF) - ON Semiconductor

Número de pieza

componentes Descripción

Fabricante

ADT7483A
(Rev.:2012)

Dual Channel Temperature Sensor and Over Temperature Alarm

ON Semiconductor 

ADT7483A

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 nf values.

DT + ǒnf * 1.008Ǔń1.008 ǒ273.15 Kelvin ) TǓ (eq. 2)

To factor this in, write the DT 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 mA, and

the low level current, ILOW, is 12 mA. 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 mA, at the

highest operating temperature.

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

lowest operating temperature.

 Base resistance less than 100 W.

 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, qJA, of

the QSOP−16 package is about 150C/W.

Layout Considerations

Digital boards can be electrically noisy environments, and

the ADT7483A measures very small voltages from the

remote sensor, so care must be taken to minimize noise

induced at the sensor inputs. Follow these precautions:

1. Place the ADT7483A 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 4 inches 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 pickup,

a 5 mil track width and spacing is recommended.

Provide a ground plane under the tracks, if

possible.

http://onsemi.com

18

Share Link: