MAX9746 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX9746

1.2W, Low-EMI, Filterless, Class D Audio Amplifier

Maxim Integrated 

1.2W, Low-EMI, Filterless,

Class D Audio Amplifier

BUMP

A1

B1

C1

C2

B2

A3

B3

A4

C4

B4

NAME

VDD

IN+

IN-

GND

SHDN

SYNC

PGND

OUT+

OUT-

PVDD

Pin Description

FUNCTION

Analog Power Supply

Noninverting Audio Input

Inverting Audio Input

Analog Ground

Active-Low Shutdown Input. Connect to VDD for normal operation.

Frequency Select and External Clock Input.

SYNC = GND: Fixed-frequency mode with fS = 1100kHz.

SYNC = Float: Fixed-frequency mode with fS = 1450kHz.

SYNC = VDD: Spread-spectrum mode with fS = 1220kHz ±120kHz.

SYNC = Clocked: Fixed-frequency mode with fS = external clock frequency.

Power Ground

Amplifier-Output Positive Phase

Amplifier-Output Negative Phase

H-Bridge Power Supply

Detailed Description

The MAX9746 filterless, Class D audio power amplifier

features several improvements to switch-mode amplifier

technology. The MAX9746 offers Class AB performance

with Class D efficiency, while occupying minimal board

space. A unique filterless modulation scheme, synchro-

nizable switching frequency, and SSM mode create a

compact, flexible, low-noise, efficient audio power

amplifier. The differential input architecture reduces

common-mode noise pickup, and can be used without

input-coupling capacitors. The device can also be con-

figured as a single-ended input amplifier.

Comparators monitor the MAX9746 inputs and com-

pare the complementary input voltages to the sawtooth

waveform. The comparators trip when the input magni-

tude of the sawtooth exceeds their corresponding input

voltage. Both comparators reset at a fixed time after the

rising edge of the second comparator trip point, gener-

ating a minimum-width pulse tON(MIN) at the output of

the second comparator (Figure 1). As the input voltage

increases or decreases, the duration of the pulse at one

output increases (the first comparator to trip) while the

other output pulse duration remains at tON(MIN). This

causes the net voltage across the speaker (VOUT+ -

VOUT-) to change.

Operating Modes

Fixed-Frequency Modulation (FFM) Mode

The MAX9746 features two FFM modes. The FFM modes

are selected by setting SYNC = GND for a 1.1MHz

switching frequency, and SYNC = FLOAT for a 1.45MHz

switching frequency. In FFM mode, the frequency spec-

trum of the Class D output consists of the fundamental

switching frequency and its associated harmonics (see

the Wideband Output Spectrum graph in the Typical

Operating Characteristics). The MAX9746 allows the

switching frequency to be changed by +32%, should the

frequency of one or more of the harmonics fall in a sensi-

tive band. This can be done at any time and does not

affect audio reproduction.

Spread-Spectrum Modulation (SSM) Mode

The MAX9746 features a unique spread-spectrum mode

that flattens the wideband spectral components, improv-

ing EMI emissions that may be radiated by the speaker

and cables by 5dB. Proprietary techniques ensure that

the cycle-to-cycle variation of the switching period does

not degrade audio reproduction or efficiency (see the

Typical Operating Characteristics). Select SSM mode by

setting SYNC = VDD. In SSM mode, the switching fre-

quency varies randomly by ±120kHz around the center

frequency (1.22MHz). The modulation scheme remains

the same, but the period of the sawtooth waveform

changes from cycle to cycle (Figure 2). 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.

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