20W/40W, Filterless, Spread-Spectrum,

Mono/Stereo, Class D Amplifier

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15

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, preventing the unused device inputs from dis-

torting the input signal. Mute the MAX9708 by driving

MUTE low. Driving MUTE low turns off the Class D out-

put stage, but does not affect the input bias levels of

the MAX9708.

Frequency Synchronization

The MAX9708 outputs up to 21W on each channel in

stereo mode. If higher output power or a 2.1 solution is

needed, two MAX9708s can be used. Each MAX9708

is synchronized by connecting SYNCOUT from the first

MAX9708 to SYNCIN of the second MAX9708 (see

Figure 5).

Supply Bypassing/Layout

Proper power-supply bypassing ensures low-distortion

pin as possible. A low-impedance, high-current power-

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 sol-

dered to the ground plane for rated power dissipation

and good ground return. Use wider PC board traces to

lower the parasitic resistance for the high-power output

pins (OUTR+, OUTR-, OUTL+, OUTL-). Refer to the

MAX9708 Evaluation Kit for layout guidance.

Thermal Considerations

Class D amplifiers provide much better efficiency and

thermal performance than a comparable Class AB

amplifier. 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 maxi-

mum output 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

continuous sine wave, the actual thermal impact on the

Class D amplifier is highly reduced. If the thermal per-

formance 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 perfor-

mance will be 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 pos-

sible from this polygon to any adjacent pin on the Class

D amplifier as well as to any adjacent components, pro-

vided these connections are at the same potential.

These copper paths must be as wide as possible. Each

of these paths contributes to the overall thermal capa-

bilities 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 system’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 approximately

10%, but it could make the difference between accept-

able performance and thermal problems.