Datasheet

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 θ
JA
.
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, θ
JA
, 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 R
LOAD
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 (θ
JC
) 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.