12W+12W, Low-EMI, Spread-Spectrum,

Stereo, Class D Amplifier

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11

Single-Ended Input

The MAX9741 can be configured as single-ended input

amplifiers by capacitively coupling either input to GND

and driving the other input (Figure 4).

Component Selection

Input Filter

An input capacitor, CIN, in conjunction with the input

impedance of the MAX9741, forms a highpass filter that

removes the DC bias from an incoming signal. The AC-

coupling capacitor allows the amplifier to bias the sig-

nal to an optimum DC level. Assuming zero-source

impedance, the -3dB point of the highpass filter is

given by:

Choose CIN so f-3dB is well below the lowest frequency

of interest. Setting f-3dB too high affects the low-fre-

quency response of the amplifier. Use capacitors with

dielectrics that have low-voltage coefficients, such as

tantalum or aluminum electrolytic. Capacitors with high-

voltage coefficients, such as ceramics, may result in

increased distortion at low frequencies.

Charge-Pump Capacitor Selection

Use capacitors with an ESR less than 100m

Ω for opti-

mum performance. Low-ESR ceramic capacitors mini-

mize the output resistance of the charge pump. For

best performance over the extended temperature

range, select capacitors with an X7R dielectric.

Flying Capacitor (C1)

The value of the flying capacitor (C1) affects the load

regulation and output resistance of the charge pump. A

C1 value that is too small degrades the device’s ability

to provide sufficient current drive. Increasing the value

of C1 improves load regulation and reduces the

charge-pump output resistance to an extent. Above

1µF, the on-resistance of the switches and the ESR of

C1 and C2 dominate.

Hold Capacitor (C2)

The output capacitor value and ESR directly affect the rip-

ple at CHOLD. Increasing C2 reduces output ripple.

Likewise, decreasing the ESR of C2 reduces both ripple

and output resistance. Lower capacitance values can be

used in systems with low maximum output power levels.

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, prevent-

ing the unused device inputs from distorting the input

signal. Mute the MAX9741 by driving SS low through an

open-drain output or MOSFET. Driving SS low turns off

the Class D output stage, but does not affect the input

bias levels of the MAX9741. Be aware that during normal

operation, the voltage at SS can be up to 7V, depending

on the MAX9741 supply.

Supply Bypassing/Layout

Proper power-supply bypassing ensures low-distortion

operation. For optimum performance, bypass VDD to

PGND with a 0.1µF or greater capacitor as close to each

VDD pin as possible. In some applications, a 0.1µF

capacitor in parallel with a larger value, low-ESR ceramic

or aluminum electrolytic capacitor provides good results.

A low-impedance, high-current power-supply connection

to VDD is assumed. Additional bulk capacitance should

be added as required depending on the application and

power-supply characteristics. AGND and PGND should

be star connected to system ground. Refer to the

MAX9741 Evaluation Kit for layout guidance.

Class D Amplifier 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 and

consideration of many parameters. This application

note examines Class D amplifiers using general exam-

ples to illustrate good design practices.

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.

f

RC

-3dB

IN

IN

1

2

=

π

MAX9741

IN+

IN-

0.47

µF

0.47

µF

SINGLE-ENDED

AUDIO INPUT

Figure 4. Single-Ended Input