Pump Capacitance and Load Resistance graph in the

Typical Operating Characteristics for details of the possi-

ble capacitor sizes.

Previous attempts to eliminate 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

isolated from system ground, complicating product

design.

•

During an ESD strike, the driver’s ESD structures

are the only path to system ground. The driver 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

conflict with the ground potential from other equip-

ment, resulting in possible damage to the drivers.

Low-Frequency Response

Large DC-blocking 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 forms a highpass filter with the

-3dB point set by:

highpass filter is required by conventional single-

ended, single power-supply headphone drivers to

block the midrail DC-bias component of the audio

signal from the headphones. The drawback to the

filter is that it can attenuate low-frequency signals.

in physically larger, more expensive capacitors.

Figure 2 shows the relationship between the size of

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.

2) The voltage coefficient of the DC-blocking capacitor

contributes distortion to the reproduced audio sig-

nal as the capacitance value varies as the function

of the voltage across the capacitor changes. At low

frequencies, the reactance of the capacitor domi-

nates at frequencies below the -3dB point and the

voltage coefficient appears as frequency-depen-

dent distortion. Figure 3 shows the THD+N intro-

duced by two different capacitor dielectric types.

Note that below 100Hz, THD+N increases rapidly.

The combination of low-frequency attenuation and fre-

quency-dependent distortion compromises audio

reproduction in portable audio equipment that empha-

sizes low-frequency effects such as multimedia lap-

tops, as well as MP3, CD, and DVD players. These

low-frequency, capacitor-related deficiencies are elimi-

nated by using DirectDrive technology.

Charge Pump

The MAX9721 features a low-noise charge pump. The

580kHz switching frequency is well beyond the audio

range, and does not interfere with the audio signals.

The switch drivers feature a controlled switching speed

that minimizes noise generated by turn-on and turn-off

transients. The di/dt noise caused by the parasitic bond

wire and trace inductance is minimized by limiting the

turn-on/off speed of the charge pump. Additional high-

f

2R C

-3dB

L OUT

=

1

π

1V, Fixed-Gain, DirectDrive, Stereo Headphone

Amplifier with Shutdown

_______________________________________________________________________________________

7

Figure 1. Traditional Driver Output Waveform vs. MAX9721

Output Waveform (Ideal Case)

GND

CONVENTIONAL DRIVER-BIASING SCHEME

DirectDrive BIASING SCHEME

GND