MAX9877EWP-TG45 データシートの表示（PDF） - Maxim Integrated

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MAX9877EWP-TG45

Low RF Susceptibility, Mono Audio Subsystem with DirectDrive Headphone Amplifier

Maxim Integrated 

Low RF Susceptibility, Mono Audio

Subsystem with DirectDrive Headphone Amplifier

40

TEST LIMIT

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MAX9877 OUTPUT

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FREQUENCY (MHz)

TEST LIMIT

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MAX9877 OUTPUT

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FREQUENCY (MHz)

Figure 4. EMI with 152mm of Speaker Cable

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 MAX9877 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 MAX9877 charge pump requires

two small ceramic capacitors, conserving board space,

reducing cost, and improving the frequency 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 amplifier outputs

due to amplifier offset. However, the offset of the

MAX9877 is typically ±0.15mV, which, when combined

with a 32Ω load, results in less than 10µA 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:

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