AD8016 datasheet(18/20 Pages) AD

18 / 20 page

AD8016

Data Sheet

Rev. C | Page 18 of 20

THERMAL ENHANCEMENTS AND PCB LAYOUT

There are several ways to enhance the thermal capacity of the

CO solution. Additional thermal capacity can be created using

enhanced PCB layout techniques such as interlacing (some-

times referred to as stitching or interconnection) of the layers

immediately beneath the line driver. This technique serves to

increase the thermal mass or capacity of the PCB immediately

beneath the driver. The AD8016 in a TSSOP_EP (ARE model)

package can be designed to operate in the CO solution using

prudent measures to manage the power dissipation through

careful PCB design. The ARE package is available for use in

designing the highest density CO solutions. Maximum heat

transfer to the PCB can be accomplished using the ARE

package when the thermal slug is soldered to an exposed

copper pad directly beneath the AD8016. Optimum thermal

performance can be achieved in the ARE package only when

the back of the package is soldered to a PCB designed for

maximum thermal capacity (see Figure 48). Thermal experi-

ments with the ARE package were conducted without soldering

the heat slug to the PCB. Heat transfer was through physical

contact only. The following offers some insight into the AD8016

power dissipation and relative junction temperature, as well as

the effects of PCB size and composition on the junction-to-air

thermal resistance or θJA.

THERMAL TESTING

A wind tunnel study was conducted to determine the relation-

ship between thermal capacity (that is, printed circuit board

copper area), air flow, and junction temperature. Junction-to-

ambient thermal resistance, θJA, was also calculated for the

AD8016 ARE and AD8016 ARB packages. The AD8016 was

operated in a noninverting differential driver configuration,

typical of an xDSL application yet isolated from any other

modem components. Testing was conducted using a 1 oz.

copper board in an ambient temperature of ~24°C over air

flows of 200, 150, 100, and 50 linear feet per minute (LFM)

(0.200 and 400 for AD8016 ARE) and for the ARB packages as

well as in still air. The 4-layer PCB was designed to maximize

the area of copper on the outer two layers of the board, while

the inner layers were used to configure the AD8016 in a

differential driver circuit. The PCB measured 3 inches ×

4 inches in the beginning of the study and was progressively

reduced in size to approximately 2 inches × 2 inches. The

testing was performed in a wind tunnel to control airflow

in units of LFM. The tunnel is approximately 11 inches in

diameter.

AIR FLOW TEST CONDITIONS

DUT Power

A typical DSL DMT signal produces about 1.5 W of power

dissipation in the AD8016 package. The fully biased (PWDN0

and PWDN1 = Logic 1) quiescent current of the AD8016 is

~25 mA. A 1 MHz differential sine wave at an amplitude of

8 V p-p/amplifier into an RLOAD of 100 Ω differential (50 Ω

per side) produces the 1.5 W of power typical in the AD8016

device. (See the Power Dissipation section for details.)

Thermal Resistance

The junction-to-case thermal resistance (θJC) of the AD8016

ARB or SOIC_W_BAT package is 8.6°C/W and for the AD8016

ARE or TSSOP_EP it is 5.6°C/W. These package specifications

were used in this study to determine junction temperature

based on the measured case temperature.

PCB Dimensions of a Differential Driver Circuit

Several components are required to support the AD8016 in a

differential driver circuit. The PCB area necessary for these

components (that is, feedback and gain resistors, ac-coupling

and decoupling capacitors, termination and load resistors)

dictated the area of the smallest PCB in this study, 4.7 square

inches. Further reduction in PCB area, although possible, has

consequences in terms of the maximum operating junction

temperature method of thermal enhancement.) A cooling fan

that draws moving air over the PCB and xDSL drivers, while

not always required, may be useful in reducing the operating

temperature.

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