2.3W, Ultra-Low-EMI, Filterless,

Class D Audio Amplifier

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Detailed Description

The MAX9705 ultra-low-EMI, filterless, Class D audio

power amplifier features several improvements to switch-

mode amplifier technology. The MAX9705 features output

driver active emissions limiting circuitry to reduce EMI.

Zero dead time technology maintains state-of-the-art effi-

ciency and THD+N performance by allowing the output

FETs to switch simultaneously without cross-conduction.

A unique filterless modulation scheme, synchronizable

switching frequency, and spread-spectrum mode create

a compact, flexible, low-noise, efficient audio power

amplifier while occupying minimal board space. The dif-

ferential input architecture reduces common-mode noise

pickup with or without the use of input-coupling capaci-

tors. The MAX9705 can also be configured as a single-

ended input amplifier without performance degradation.

Thermal-overload and short-circuit protection prevent the

MAX9705 from being damaged during a fault condition.

The amplifier is disabled if the die temperature reaches

+125°C. The die must cool by 10°C before normal opera-

tion can continue. The output of the MAX9705 shuts down

if the output current reaches approximately 2A. Each out-

put FET has its own short-circuit protection. This protec-

tion scheme allows the amplifier to survive shorts to either

supply rail. After a thermal overload or short circuit, the

device remains disabled for a minimum of 50µs before

attempting to return to normal operation. The amplifier will

shut down immediately and wait another 50µs before turn-

ing on if the fault condition is still present. This operation

will cause the output to pulse during a persistent fault.

Comparators monitor the MAX9705 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-

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

Operating Modes

Fixed-Frequency Modulation (FFM) Mode

The FFM mode is selected by setting SYNC = GND for a

1.1MHz switching frequency. In FFM mode, the frequen-

cy spectrum of the Class D output consists of the funda-

mental switching frequency and its associated

harmonics (see the Wideband Output Spectrum Fixed-

Frequency Mode graph in the

Typical Operating

Characteristics).

Pin Description

PIN

BUMP

TDFN

UCSP

NAME

FUNCTION

1A1

Analog Power Supply

2

B1

IN+

Noninverting Audio Input

3

C1

IN-

Inverting Audio Input

4

C2

GND

Analog Ground

5B2

SHDN

6

A3

SYNC

Frequency Select and External Clock Input.

±120kHz.

7

B3

PGND

Power Ground

8

A4

OUT+

Amplifier-Output Positive Phase

9

C4

OUT-

Amplifier-Output Negative Phase

10

B4

H-Bridge Power Supply

——

EP

Exposed Pad. Internally connected to ground. Connect to a large ground plane to

maximize thermal performance. Not intended as an electrical connection point

(TDFN only).