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

Mono/Stereo, Class D Amplifier

14

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Linear Regulator (REG)

The supply voltage range for the MAX9708 is from 10V

to 18V to achieve high-output power. An internal linear

regulator reduces this voltage to 6.3V for use with

small-signal and digital circuitry that does not require a

high-voltage supply. Bypass a 0.01µF capacitor from

REG to GND.

Applications Information

Logic Inputs

All of the digital logic inputs and output have an

absolute maximum rating of +12V. If the MAX9708 is

operating with a supply voltage between 10V and 12V,

outputs must connected to a digital system supply

lower than 12V.

Input Amplifier

Differential Input

The MAX9708 features a differential input structure,

making them compatible with many CODECs, and

offering improved noise immunity over a single-ended

input amplifier. In devices such as flat-panel displays,

noisy digital signals can be picked up by the amplifier’s

inputs. These signals appear at the amplifiers’ inputs as

common-mode noise. A differential input amplifier

amplifies only the difference of the two inputs, while any

signal common to both inputs is attenuated.

Single-Ended Input

The MAX9708 can be configured as a single-ended

input amplifier by capacitively coupling either input to

GND and driving the other input (Figure 4).

Component Selection

Input Filter

impedance of the MAX9708, forms a highpass filter that

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

coupling capacitor allows the amplifier to bias the signal

to an optimum DC level. Assuming zero-source imped-

ance, the -3dB point of the highpass filter is given by:

low-frequency response of the amplifier. Use capaci-

tors 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.

Output Filter

The MAX9708 does not require an output filter.

However, output filtering can be used if a design is fail-

ing radiated emissions due to board layout or cable

length, or the circuit is near EMI-sensitive devices.

Refer to the MAX9708 Evaluation Kit for suggested filter

topologies. The tuning and component selection of the

filter should be optimized for the load. A purely resistor

load (8

Ω) used for lab testing will require different com-

ponents than a real, complex load-speaker load.

Charge-Pump Capacitor Selection

The MAX9708 has an internal charge-pump converter

that produces a voltage level for internal circuitry. It

requires a flying capacitor (C1) and a holding capacitor

(C2). Use capacitors with an ESR less than 100m

Ω for

optimum performance. Low-ESR ceramic capacitors

minimize the output resistance of the charge pump. For

best performance over the extended temperature

range, select capacitors with an X7R dielectric. The

capacitors’ voltage rating must be greater than 36V.

f

RC

dB

IN

IN

−

=

3

1

2

π

Figure 4. Single-Ended Input Connections

INR+

INR-

MAX9708

1

µF

1

µF