MAX9721 Datasheet, PDF(7/14 Page) Maxim Integrated Products – 1V, Fixed-Gain, DirectDrive, Stereo Headphone Amplifier with Shutdown

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MAX9721 Datasheet, PDF (7/14 Pages) Maxim Integrated Products – 1V, Fixed-Gain, DirectDrive, Stereo Headphone Amplifier with Shutdown

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1V, Fixed-Gain, DirectDrive, Stereo Headphone

Amplifier with Shutdown

VDD

VOUT

VDD / 2

GND

CONVENTIONAL DRIVER-BIASING SCHEME

VDD

VOUT

GND

-VDD

DirectDrive BIASING SCHEME

Figure 1. Traditional Driver Output Waveform vs. MAX9721

Output Waveform (Ideal Case)

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:

â¢ The sleeve is typically grounded to the chassis.

Using this biasing approach, the sleeve must be

isolated from system ground, complicating product

design.

â¢ During an ESD strike, the driverâs ESD structures

are the only path to system ground. The driver must

be able to withstand the full ESD strike.

â¢ 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 amplifierâs 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:

f-3dB =

2ÏRLCOUT

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 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.

Larger values of COUT reduce this effect but result

in physically larger, more expensive capacitors.

Figure 2 shows the relationship between the size of

COUT and the resulting low-frequency attenuation.

Note that the -3dB point for a 16â¦ headphone with

a 100ÂµF 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-

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