Maxim Integrated MAX9778 - Manual (Page 20)

MAX9777/MAX9778
Stereo 3W Audio Power Amplifiers with
Headphone Drive and Input Mux
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BIAS Capacitor
BIAS is the output of the internally generated 2.5VDC
bias voltage. The BIAS bypass capacitor, C
BIAS
,
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.
Supply Bypassing
Proper power-supply bypassing ensures low-noise, low-
distortion performance. Place a 0.1µF ceramic capacitor
from V
DD to GND. Add additional bulk capacitance as
required by the application, typically 100µF. Bypass
PV
DD with a 100µF capacitor to GND. Locate bypass
capacitors as close to the device as possible.
Gain Select
The MAX9777/MAX9778 feature multiple gain settings on
each channel, making available different gain and feed-
back configurations. The gain-setting resistor (R
F ) is con-
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.
Bass Boost Circuit
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
F is an open circuit, and the effective impedance in
the feedback loop (R F(EFF) ) is R F(EFF) = R F1 .
At the frequency:
where the impedance, C F, begins to decrease, and at
high frequencies, the C F is a short circuit. Here the
impedance of the feedback loop is:
Assuming R F1 = R F2 , then R F(EFF) at low frequencies is
twice that of R
F(EFF)
at high frequencies (Figure 11).
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
F . Film or C0G dielectric capacitors are good
choices for C F . Capacitors with high-voltage coeffi-
cients, such as ceramics (non-C0G dielectrics), can
result in increased distortion at low frequencies.
Layout and Grounding
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
R R
R R
F EFF
F F
F F
( )
=
×
+
1 2
1 2
1
2
2
πR C
F F
V BIAS
R IN
R
F2
R F1
C F
Figure 10. Bass Boost Circuit
R F1
R F1 R F2
R IN
R
IN
2 π R
F2
C
F
1
FREQUENCY
GAIN
Figure 11. Bass Boost Response