AD8392

Rev. 0 | Page 13 of 16

THERMAL CONSIDERATIONS

When using a quad, high output current amplifier, such as the

AD8392, special consideration should be given to system level

thermal design. In applications such as ADSL/ADSL2+, the

AD8392 could be required to dissipate as much as 1.4 W or

more on chip. Under these conditions, particular attention

should be paid to the thermal design in order to maintain safe

operating temperatures on the die. To aid in the thermal design,

the thermal information in the Thermal Resistance section can

be combined with what follows here.

The information in Table 4 and Figure 3 is based on a standard

JEDEC 4-layer board and a maximum die temperature of

150°C. To provide additional guidance and design suggestions, a

thermal study was performed under a set of conditions more

closely aligned with an actual ADSL/ADSL2+ application.

In a typical ADSL/ADSL2+ line card, component density usu-

ally dictates that most of the copper plane used for thermal

dissipation be internal. Additionally, each ADSL/ADSL2+ port

may be allotted only 1 square inch, or even less, of board space.

For these reasons, a special thermal test board was constructed

for this study. The 4-layer board measured approximately

4 inches × 4 inches and contained two internal 1 oz copper

ground planes, each measuring 2 inches × 3 inches. The top

layer contained signal traces and an exposed copper strip

¼ inch × 3 inches to accommodate heat sinking, with no other

copper on the top or bottom of the board.

Three 28-lead TSSOPs were placed on the board representing

six ADSL channels, or one channel per square inch of copper,

with each channel dissipating 700 mW on-chip (1.4 W per

package). The die temperature is then measured in still air and

in a wind tunnel with calibrated airflow of 100 LFM, 200 LFM,

and 400 LFM. Figure 36 shows the power dissipation versus the

ambient temperature for each airflow condition. The figure

assumes a maximum die temperature of 135°C. No heat sink

was used.

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5

15

25354555657585

AMBIENT TEMPERATURE (

°C)

STILL AIR

100LFM

200LFM

400LFM

Figure 36. Power Dissipation vs. Ambient

Temperature and Air Flow 28-Lead TSSOP/EP

This data is only provided as guidance to assist in the thermal

design process. Due diligence should be performed with regards

to power dissipation because there are many factors that can

affect thermal performance.

TYPICAL ADSL/ADSL2+ APPLICATION

In a typical ADSL/ADSL2+ application, a differential line driver

is used to take the signal from the analog front end (AFE) and

drive it onto the twisted pair telephone line. Referring to the

typical circuit representation in Figure 37, the differential input

output is transformer coupled to the telephone line at tip and

ring. The common-mode operating point, generally midway

R4

R4

R3

R3

R2

R1

R2

1:N

TIP

RING

Figure 37. Typical ADSL/ADSL2+ Application Circuit

In ADSL/ADSL2+ applications, it is common practice to

conserve power by using positive feedback to synthesize the

output resistance, thereby lowering the required ohmic value of

somewhat unique in that the positive feedback introduced via

R3 has the effect of synthesizing the input resistance as well.

The following definitions and equations can be used to calculate

the resistor values necessary to obtain the desired gain, input

resistance, and output resistance for a given application. For

simplicity the following calculations assume a lossless

transformer.

The following values are used in the design equations and are

assumed already known or chosen by the designer.

Differential input voltage

Desired differential input resistance

N

Transformer turns ratio

Differential output voltage at tip and ring

Each is typically 5% to 15% of the transformer reflected

line impedance

R2

Recommended in the amplifier data sheet

Voltage at the + inputs to the amplifier, approximately ½

Transformer reflected line impedance