MAX9879 datasheet(20/32 Pages) AD

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MAX9879

Stereo Class D Audio Subsystem

with DirectDrive Headphone Amplifier

Maxim Integrated

DirectDrive Headphone Amplifier

Traditional single-supply headphone amplifiers have

outputs biased at a nominal DC voltage (typically half

the supply). Large coupling capacitors are needed to

block this DC bias from the headphone. Without these

capacitors, a significant amount of DC current flows to

the headphone, resulting in unnecessary power dissi-

pation and possible damage to both the headphone

and headphone amplifier.

Maxim’s DirectDrive® architecture uses a charge pump

to create an internal negative supply voltage. This

allows the headphone outputs of the MAX9879 to be

biased at GND while operating from a single supply

(Figure 5). Without a DC component, there is no need

for the large DC-blocking capacitors. Instead of two

large (220µF, typ) capacitors, the MAX9879 charge

pump requires two small ceramic capacitors, conserv-

ing board space, reducing cost, and improving the fre-

quency response of the headphone amplifier. See the

Output Power vs. Load Resistance graph in the

Typical

Operating Characteristics for details of the possible

capacitor sizes. There is a low DC voltage on the ampli-

fier outputs due to amplifier offset. However, the offset of

the MAX9879 is typically ±1.5mV, which, when com-

bined with a 32

Ω load, results in less than 47µA of DC

current flow to the headphones.

In addition to the cost and size disadvantages of the

DC-blocking capacitors required by conventional head-

phone amplifiers, these capacitors limit the amplifier’s

low-frequency response and can distort the audio sig-

nal. Previous attempts at eliminating the output-cou-

pling capacitors involved biasing the headphone return

(sleeve) to the DC bias voltage of the headphone

amplifiers. This method raises some issues:

FREQUENCY (MHz)

160

140

120

100

TEST LIMIT

MAX9879 OUTPUT

TEST LIMIT

180

200

240

260

280

300

220

FREQUENCY (MHz)

600

550

500

450

400

350

300

650

700

800

850

900

1000

950

750

Figure 4. EMI with 152mm of Speaker Cable

DirectDrive is a registered trademark of Maxim Integrated

Products, Inc.

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