ARA05050S12 Datasheet, PDF (9/12 Pages) ANADIGICS, Inc – CATV Reverse Amplifier w/step Attenuator

ARA05050S12
Advanced Product Information - Rev. 6
Application Note
LAYOUT CONSIDERATIONS
There are two issues that must be taken into
consideration when doing the PCB layout.
The first is thermal management, and the
second is RF related.
THERMAL LAYOUT CONSIDERATIONS
The ARA05050 will typically dissipate 0.9W,
and as high as 1.2W. Since the interior of
most set-top boxes, and cable modems as
well typically are at +70°C, consideration
must be given to providing an adequate heat
sink for the die to obtain the maximum MTF
possible. To this end the ARA05050 incorpo-
rates a heat slug in the bottom of the pack-
age. This provides a low thermal resistance
path from the die to the outside of the pack-
age. The typical thermal rise from the heat
slug to the junction is 35°C/W. However, this
is only half of the equation. Adequate heat
sinking must be applied to the heat slug for
thermal dissipation. Providing a metalized pad
with via holes under the package will do this
(see Figure 14). The via holes should connect
to the ground plane of the PCB. The part is
then soldered to this pad during assembly.
EXTERNAL CIRCUITRY
Output Disconnect Switch:
For MCNS/DOCSIS applications an external
switch to disconnect the output of the
ARA05050 from the diplexer is required. This
switch is needed because of the output noise
requirement between bursts, and because of the
requirement that any shutdown transient not
exceed 7mV. The switch shown in Figure 12
meets these conditions because it does not
switch any current, or voltage, on the output
line. The series FET provides 35 dB of isolation,
while the shunt FET insures that the diplexer
remains terminated into a 75 ohm impedance.
Since the switch does not draw any current, it
may be driven directly from a low power CMOS
logic inverter; however the control voltages must
A
be +5V. When the switch is in the open state, it
is good general practice to set the program-
mable attenuator to its maximum attenuation
setting. This will increase the isolation between
the cm output and upstream modulator, and
provide the first stage amplifier with a 75 ohm
termination.
Shutdown of the ARA05050:
In some applications it may be desirable to shut
the ARA05050 down for power saving. This can
be done by applying a negative voltage to pin 10
to shut down the input stage, and to pin 24 to
shut down output stage (see Figure 13). Shut-
ting down both amplifier stages will reduce the
current drawn from the +5V supply to typically
10 mA. If only one stage is shutdown, it is
recommended that the programmable attenuator
be set to a minimum of 16 dB to provide a good
impedance match to the remaining stage.
RF LAYOUT CONSIDERATIONS
The ARA05050 is a power amplifier designed for
driving a 75ohm load. Since this part connects
the transmitter to the cable system, typically via
a diplexer, it is an analog device operating at RF
frequencies. This means that the layout of the
PCB will have an effect on the system perfor-
mance. The first consideration in RF layout are
the connections to ground. These must be low
impedance, and as short as possible. The best
way to do this is to use as large a via hole as
possible, located as close as possible to
connect to the ground plane. Specifically, care
should be given to the layout of the following
connections (the traces leading from the
following pins to their respective components
should be as low as impedance as is practical.):
Pins 5 & 19: the 20 ohm chip resistor should be
as close as possible to the pins and the 1 uF
capacitor should be kept close to the 20 ohm
resistor.
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