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.

Charge-Pump Capacitance and 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 off-

set. However, the offset of the MAX9722A is typically

0.5mV, which, when combined with a 32

Ω load, results

in less than 15.6µA of DC current flow to the head-

phones. Previous attempts to eliminate 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:

• 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-

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:

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-

larger, more expensive capacitors. Figure 4 shows the

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

a 16

Ω headphone with a 100µF blocking capacitor is

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

low-frequency attenuation of the reproduced signal.

f

RC

dB

L OUT

-3

1

2

=

π

5V, Differential Input, DirectDrive, 130mW

Stereo Headphone Amplifiers with Shutdown

_______________________________________________________________________________________

9

HEADPHONE DRIVER

MICROPHONE

AMPLIFIER

MICROPHONE

AMPLIFIER

OUTPUT

AUDIO

INPUT

AUDIO

INPUT

MICROPHONE

BIAS

MAX9722

Figure 3. Earbud Speaker/Microphone Combination Headset

Configuration