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MAX9722A

5V, Differential Input, DirectDrive, 130mW Stereo Headphone Amplifiers with Shutdown

Maxim Integrated 

5V, Differential Input, DirectDrive, 130mW

Stereo Headphone Amplifiers with Shutdown

DirectDrive

Conventional single-supply headphone amplifiers have

their outputs biased about a nominal DC voltage (typi-

cally half the supply) for maximum dynamic range.

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 head-

phone, resulting in unnecessary power dissipation and

possible damage to both the headphone and the head-

phone amplifier.

Maxim’s patented DirectDrive architecture uses a

charge pump to create an internal negative supply volt-

age, allowing the MAX9722A/MAX9722B outputs to be

biased about GND. With no DC component, there is no

need for the large DC-blocking capacitors. Instead of

two large (220µF, typ) tantalum capacitors, the

MAX9722A/MAX9722B 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

MAX9722A is typically 0.5mV, which, when combined

with a 32Ω load, results in less than 15.6µA of DC cur-

rent flow to the headphones. Previous attempts to elimi-

nate the output-coupling capacitors involved biasing the

headphone return (sleeve) to the DC-bias voltage of the

headphone amplifiers. This method raises some issues:

• The sleeve is typically grounded to the chassis.

Using this biasing approach, the sleeve must be iso-

lated from system ground, complicating product

design.

• During an ESD strike, the amplifier’s ESD structures

are the only path to system ground. Thus, the amplifi-

er must be able to withstand the full ESD strike.

• When using the headphone jack as a line out to other

equipment, the bias voltage on the sleeve may con-

flict with the ground potential from other equipment,

resulting in possible damage to the amplifiers.

• When using a combination microphone and speaker

headset, the microphone typically requires a GND

reference. The amplifier DC bias on the sleeve con-

flicts with the microphone requirements (Figure 3).

Low-Frequency Response

In addition to the cost and size disadvantages of the DC-

blocking capacitors required by conventional head-

MICROPHONE

AMPLIFIER

OUTPUT

MICROPHONE

AMPLIFIER

MICROPHONE

BIAS

AUDIO

INPUT

AUDIO

INPUT

MAX9722

HEADPHONE DRIVER

Figure 3. Earbud Speaker/Microphone Combination Headset

Configuration

phone amplifiers, these capacitors limit the amplifier’s

low-frequency response and can distort the audio signal:

1) The impedance of the headphone load and the DC-

blocking capacitor form a highpass filter with the

-3dB point set by:

f-3dB

=

1

2πRLCOUT

where RL is the impedance of the headphone and

COUT is the value of the DC-blocking capacitor.

The highpass filter is required by conventional single-

ended, single power-supply headphone amplifiers to

block the midrail DC-bias component of the audio sig-

nal from the headphones. The drawback to the filter is

that it can attenuate low-frequency signals. Larger val-

ues of COUT reduce this effect but result in physically

larger, more expensive capacitors. Figure 4 shows the

relationship between the size of COUT and the resulting

low-frequency attenuation. Note that the -3dB point for

a 16Ω headphone with a 100µF blocking capacitor is

100Hz, well within the normal audio band, resulting in

low-frequency attenuation of the reproduced signal.

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