Detailed Description

The MAX9713/MAX9714 filterless, Class D audio power

amplifiers feature several improvements to switch-

mode amplifier technology. The MAX9713 is a mono

amplifier, the MAX9714 is a stereo amplifier. These

devices offer Class AB performance with Class D effi-

ciency, while occupying minimal board space. A

unique filterless modulation scheme and spread-spec-

trum switching mode create a compact, flexible, low-

noise, efficient audio power amplifier. The differential

input architecture reduces common-mode noise pick-

up, and can be used without input-coupling capacitors.

The devices can also be configured as a single-ended

input amplifier.

Comparators monitor the device inputs and compare

the complementary input voltages to the triangle wave-

form. The comparators trip when the input magnitude of

the triangle exceeds their corresponding input voltage.

Operating Modes

Fixed-Frequency Modulation (FFM) Mode

The MAX9713/MAX9714 feature three FFM modes with

different switching frequencies (Table 1). In FFM mode,

the frequency spectrum of the Class D output consists of

the fundamental switching frequency and its associated

harmonics (see the Wideband Output Spectrum (FFM

Mode) graph in the Typical Operating Characteristics).

The MAX9713/ MAX9714 allow the switching frequency

to be changed by

±35%, should the frequency of one or

more of the harmonics fall in a sensitive band. This can

be done at any time and not affect audio reproduction.

Spread-Spectrum Modulation (SSM) Mode

The MAX9713/MAX9714 feature a unique spread-spec-

trum mode that flattens the wideband spectral compo-

nents, improving EMI emissions that may be radiated

by the speaker and cables. This mode is enabled by

setting FS1 = FS2 = H. In SSM mode, the switching fre-

quency varies randomly by

±1.7%kHz around the cen-

ter frequency (335kHz). The modulation scheme

remains the same, but the period of the triangle wave-

form changes from cycle to cycle. Instead of a large

amount of spectral energy present at multiples of the

switching frequency, the energy is now spread over a

bandwidth that increases with frequency. Above a few

megahertz, the wideband spectrum looks like white

noise for EMI purposes (Figure 2).

Efficiency

Efficiency of a Class D amplifier is attributed to the

region of operation of the output stage transistors. In a

Class D amplifier, the output transistors act as current-

steering switches and consume negligible additional

power. Any power loss associated with the Class D out-

put stage is mostly due to the I2R loss of the MOSFET

on-resistance, and quiescent current overhead.

The theoretical best efficiency of a linear amplifier is

78%, however that efficiency is only exhibited at peak

output powers. Under normal operating levels (typical

music reproduction levels), efficiency falls below 30%,

whereas the MAX9714 still exhibits >80% efficiencies

under the same conditions (Figure 3).

Shutdown

The MAX9713/MAX9714 have a shutdown mode that

reduces power consumption and extends battery life.

Driving

SHDN low places the device in low-power

(0.2µA) shutdown mode. Connect

SHDN to a logic high

for normal operation.

Click-and-Pop Suppression

The MAX9713/MAX9714 feature comprehensive click-

and-pop suppression that eliminates audible transients

on startup and shutdown. While in shutdown, the H-

bridge is pulled to GND through 300k

Ω. During startup,

6W, Filterless, Spread-Spectrum

Mono/Stereo Class D Amplifiers

8

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Table 1. Operating Modes

FS1

FS2

SWITCHING MODE

(kHz)

L

L

335

L

H

460

H

L

236

H

H

335

±7%

Figure 1. MAX9714 Outputs with No Input Signal Applied

VIN = 0V

OUT-

OUT+