Datasheet

MAX9728A/MAX9728B
Lineout Amplifier and Filter Block
The MAX9728A can be used as an audio line driver
capable of providing 2V
RMS
into 10kΩ loads with a sin-
gle 5V supply (see Figure 3 for the RMS Output Voltage
vs. Supply Voltage plot). 2V
RMS
is a popular audio line
level, first used in CD players, but now common in DVD
and set-top box (STB) interfacing standards. A 2V
RMS
sinusoidal signal equates to approximately 5.7V
P-P
,
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
ability to derive 2V
RMS
from a 5V supply can often sim-
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.
To suppress this noise, and to provide a 2V
RMS
stan-
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
single point on the PC board. Connect PV
SS
to SV
SS
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,
and V
DD
. Connect the exposed paddle to SV
SS
only
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
R
IN
R
IN
R
F
R
F
Figure 2. Gain Setting for the MAX9728A
RMS OUTPUT VOLTAGE
vs. SUPPLY VOLTAGE
MAX9728A/28B fig03
SUPPLY VOLTAGE (V)
RMS OUTPUT 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
f
IN
= 1kHz
R
L
= 10kΩ
1% THD+N
R
L
= 1kΩ
1% THD+N
Figure 3. RMS Output Voltage vs. Supply Voltage