Lineout Amplifier and Filter Block

The MAX9728A can be used as an audio line driver

Ω loads with a sin-

gle 5V supply (see Figure 3 for the RMS Output Voltage

level, first used in CD players, but now common in DVD

which means that the audio system designer cannot

simply run the lineout stage from a (typically common)

5V supply—the resulting output swing would be inade-

quate. A common solution to this problem is to use op

amps driven from split supplies (±5V typically), or to use

a high-voltage supply rail (9V to 12V). This can mean

adding extra cost and complexity to the system power

supply to meet this output level requirement. Having the

plify power-supply design in some systems.

When the MAX9728A is used as a line driver to provide

outputs that feed stereo equipment (receivers, STBs,

notebooks, and desktops) with a digital-to-analog con-

verter (DAC) used as an audio input source, it is often

desirable to eliminate any high-frequency quantization

noise produced by the DAC output before it reaches

the load. This high-frequency noise can cause the input

stages of the line-in equipment to exceed slew-rate lim-

itations or create excessive EMI emissions on the

cables between devices.

dard audio output level from a single 5V supply, the

MAX9728A can be configured as a line driver and

active lowpass filter. Figure 4 shows the MAX9728A

connected as 2-pole Rauch/multiple feedback filter with

a passband gain of 6dB and a -3dB (below passband)

cutoff frequency of approximately 27kHz (see Figure 5

for the Gain vs. Frequency plot).

Layout and Grounding

Proper layout and grounding are essential for optimum

performance. Connect PGND and SGND together at a

and bypass with a 1µF capacitor. Place the power-sup-

ply bypass capacitor and the charge-pump hold

capacitor as close to the MAX9728 as possible. Route

PGND and all traces that carry switching transients

away from SGND and the audio signal path. The thin

QFN package features an exposed paddle that

improves thermal efficiency. Ensure that the exposed

paddle is electrically isolated from PGND, SGND,

when the board layout dictates that the exposed

paddle cannot be left floating.

60mW, DirectDrive, Stereo Headphone

Amplifiers with Shutdown

10

______________________________________________________________________________________

LEFT

AUDIO

INPUT

RIGHT

AUDIO

INPUT

OUTL

INL

INR

OUTR

MAX9728A

Figure 2. Gain Setting for the MAX9728A

RMS OUTPUT VOLTAGE

vs. SUPPLY VOLTAGE

SUPPLY VOLTAGE (V)

5.0

2.7

2.9

3.1

3.3

3.5

3.7

3.9

4.1

2.5

4.5

5.5

Ω

1% THD+N

Ω

1% THD+N

Figure 3. RMS Output Voltage vs. Supply Voltage