Datasheet

Sharing Input Sources
In certain systems, a single audio source can be shared
by multiple devices (speaker and headphone amplifiers).
When sharing inputs, it is common to mute the unused
device, rather than completely shutting it down. This pre-
vents the unused device inputs from distorting the input
signal. Mute the MAX9709 by driving MUTE low. Driving
MUTE low turns off the Class D output stage, but does not
affect the input bias levels of the MAX9709.
Frequency Synchronization
The MAX9709 outputs up to 27W on each channel in
stereo mode. If higher output power or a 2.1 solution is
needed, two MAX9709s can be used. Each MAX9709
is synchronized by connecting SYNCOUT from the first
MAX9709 to SYNCIN of the second MAX9709 (see
Figure 5).
Supply Bypassing/Layout
Proper power-supply bypassing ensures low distortion
operation. For optimum performance, bypass PV
DD
to
PGND with a 0.1µF capacitor as close to each PV
DD
pin as possible. A low-impedance, high-current power-
supply connection to PV
DD
is assumed. Additional bulk
capacitance should be added, as required, depending on
the application and power-supply characteristics. GND
and PGND should be star-connected to system ground.
For the TQFN package, solder the exposed paddle (EP)
to the ground plane using multiple-plated through-hole
vias. The exposed paddle must be soldered to the ground
plane for rated power dissipation and good ground return.
Use wider PC board traces to lower the parasitic resis-
tance for the high-power output pins (OUTR+, OUTR-,
OUTL+, OUTL-). Refer to the MAX9709 evaluation kit for
layout guidance.
Thermal Considerations
Class D amplifiers provide much better efficiency and
thermal performance than a comparable Class AB ampli-
fier. However, the system’s thermal performance must be
considered with realistic expectations along with its many
parameters.
Continuous Sine Wave vs. Music
When a Class D amplifier is evaluated in the lab, often
a continuous sine wave is used as the signal source.
While this is convenient for measurement purposes, it
represents a worst-case scenario for thermal loading on
the amplifier. It is not uncommon for a Class D amplifier
to enter thermal shutdown if driven near maximum out-
put power with a continuous sine wave. The PC board
must be optimized for best dissipation (see the PC Board
Thermal Considerations section).
Audio content, both music and voice, has a much lower
RMS value relative to its peak output power. Therefore,
while an audio signal may reach similar peaks as a con-
tinuous sine wave, the actual thermal impact on the Class
D amplifier is highly reduced. If the thermal performance
of a system is being evaluated, it is important to use actual
audio signals instead of sine waves for testing. If sine
waves must be used, the thermal performance is less than
the system’s actual capability for real music or voice.
PC Board Thermal Considerations
The exposed pad is the primary route for conducting heat
away from the IC. With a bottom-side exposed pad, the
PC board and its copper becomes the primary heatsink for
the Class D amplifier. Solder the exposed pad to a copper
polygon. Add as much copper as possible from this poly-
gon to any adjacent pin on the Class D amplifier as well
as to any adjacent components, provided these connec-
tions are at the same potential. These copper paths must
be as wide as possible. Each of these paths contributes to
the overall thermal capabilities of the system.
The copper polygon to which the exposed pad is attached
should have multiple vias to the opposite side of the PC
board, where they connect to another copper polygon.
Make this polygon as large as possible within the sys-
tem’s constraints for signal routing.
Additional improvements are possible if all the traces from
the device are made as wide as possible. Although the IC
pins are not the primary thermal path out of the package, they
do provide a small amount. The total improvement would not
exceed about 10%, but it could make the difference between
acceptable performance and thermal problems.
MAX9709 25W/50W, Filterless, Spread-Spectrum,
Stereo/Mono, Class D Amplier
www.maximintegrated.com
Maxim Integrated
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