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Stereo 3W Audio Power Amplifiers with
Headphone Drive and Input Mux
BIAS is the output of the internally generated 2.5VDC
bias voltage. The BIAS bypass capacitor, C
improves PSRR and THD+N by reducing power supply
and other noise sources at the common-mode bias
node, and also generates the clickless/popless, start-
up/shutdown DC bias waveforms for the speaker ampli-
fiers. Bypass BIAS with a 1µF capacitor to GND.
Proper power-supply bypassing ensures low-noise, low-
distortion performance. Place a 0.1µF ceramic capacitor
from V
to GND. Add additional bulk capacitance as
DD
required by the application, typically 100µF. Bypass
PV
with a 100µF capacitor to GND. Locate bypass
DD
capacitors as close to the device as possible.
The MAX9777/MAX9778 feature multiple gain settings on
each channel, making available different gain and feed-
back configurations. The gain-setting resistor (R
nected between the amplifier output (OUT_+) and the
gain set point (GAIN_). An internal multiplexer switches
between the different feedback resistors depending on
the status of the gain control input. The stereo
MAX9777/MAX9778 feature two gain options per chan-
nel. See Tables 1a and 1b for the gain-setting options.
Headphones typically have a poor low-frequency
response due to speaker and enclosure size limitations.
A bass boost circuit compensates the poor low-frequen-
cy response (Figure 10). At low frequencies, the capaci-
tor C
is an open circuit, and the effective impedance in
F
the feedback loop (R
F(EFF)
At the frequency:
πR C
2
R
IN
V
BIAS
Figure 10. Bass Boost Circuit
20
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
BIAS Capacitor
Supply Bypassing
Gain Select
) is con-
F
Bass Boost Circuit
) is R
= R
.
F(EFF)
F1
1
F
2
F
C
F
R
F2
R
F1
where the impedance, C
high frequencies, the C
,
impedance of the feedback loop is:
BIAS
Assuming R
F1
twice that of R
Thus, the amplifier has more gain at lower frequencies,
boosting the system's bass response. Set the gain roll-
off frequency based upon the response of the speaker
and enclosure.
To minimize distortion at low frequencies, use capaci-
tors with low-voltage coefficient dielectrics when select-
ing C
. Film or C0G dielectric capacitors are good
F
choices for C
cients, such as ceramics (non-C0G dielectrics), can
result in increased distortion at low frequencies.
Good PC board layout is essential for optimizing perfor-
mance. Use large traces for the power-supply inputs
and amplifier outputs to minimize losses due to para-
sitic trace resistance, as well as route heat away from
the device. Good grounding improves audio perfor-
mance, minimizes crosstalk between channels, and
prevents any digital switching noise from coupling into
the audio signal. If digital signal lines must cross over
or under audio signal lines, ensure that they cross per-
pendicular to each other.
The MAX9777/MAX9778 TQFN package features an
exposed thermal pad. This pad lowers the package's
thermal resistance by providing a direct heat conduc-
tion path from the die to the PC board. Connect the pad
to signal ground (0V) by using a large pad or multiple
vias to the ground plane.
R
F1
R
IN
R
R
F1
F2
R
IN
Figure 11. Bass Boost Response
begins to decrease, and at
F,
is a short circuit. Here the
F
×
R
R
F
1
F
2
=
R
F EFF
(
)
+
R
R
F
1
F
2
= R
, then R
at low frequencies is
F2
F(EFF)
at high frequencies (Figure 11).
F(EFF)
. Capacitors with high-voltage coeffi-
F
Layout and Grounding
GAIN
1
2π R
C
F2
F
FREQUENCY
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