MAX9729 Maxim Integrated Products, MAX9729 Datasheet - Page 22

• Manufacturer Part Number: MAX9729
• Description: Stereo Headphone Amplifier
• Manufacturer: Maxim Integrated Products

The AC-coupling capacitor (C
form a highpass filter that removes any DC bias from an
input signal. See the Functional Diagram/Typical
Operating Circuit . C
from the input signal source from appearing at the
amplifier outputs. The -3dB point of the highpass filter,
assuming zero source impedance due to the input sig-
nal source, is given by:
Choose C
quency of interest. Setting f
amplifier’s low-frequency response. Use capacitors with
low-voltage coefficient dielectrics. Aluminum electrolytic,
tantalum, or film dielectric capacitors are good choices
for AC-coupling capacitors. Capacitors with high-voltage
coefficients, such as ceramics (non-C0G dielectrics),
can result in increased distortion at low zero frequen-
cies. If a ceramic capacitor is selected due to board
space or cost constraints, use the largest package pos-
sible to minimize voltage coefficient effects. In addition,
use X7R dielectrics as opposed to X5R, Y5V, or Z5U.
The bass boost, low-frequency response when
BassMax is enabled, is set by the ratio of R1 to R2 (see
Figure 2), by the following equation:
where A
quencies. A
the maximum gain setting and the volume setting. The
total gain at low frequencies is equal to:
where A
quencies in dB, A
dB, and ATTEN is the volume attenuation in dB. To
maintain circuit stability, the ratio:
must not exceed 1/2. A ratio equaling 1/3 is recommend-
ed. The switch that shorts BM_ to SGND, when BassMax
is disabled, can have an on-resistance as high as 300Ω.
Stereo Headphone Amplifier with BassMax,
Volume Control, and Input Mux
22
A
V TOTAL BM
______________________________________________________________________________________
_
V_TOTAL_BM
V_BOOST
IN
A
V_BOOST
_
such that f
V BOOST
f
_
−
3
dB
=
BassMax Gain-Setting Components
V_MAX
is the gain boost, in dB, at low fre-
A
=
V MAX
IN
is the total voltage gain at low fre-
2π
is added to the gain realized by
_
=
-3dB
prevents any DC components
20
is the maximum gain setting in
×
R
1
R
R
×
−
+
IN
is well below the lowest fre-
1
Input-Coupling Capacitor
2
log
IN
R
-3dB
ATTEN A
×
2
) and input resistor (R
C
R
R
1
1
IN
too high affects the
+
−
(
R
R
+
Hz
2
2
)
(
V BOOST
dB
_
)
(
dB
IN
)
)
Choose a value for R1 that is greater than 40kΩ to
ensure that positive feedback is negligible when
BassMax is disabled. Table 10 contains a list of R2 val-
ues, with R1 = 47kΩ, and the corresponding low-fre-
quency gain boost values.
The low-frequency boost attained by the BassMax cir-
cuit is added to the gain realized by the maximum gain
setting and volume setting. Select the BassMax gain so
that the output signal will remain within the dynamic
range of the MAX9729. Output signal clipping will occur
at low frequencies if the BassMax gain boost is exces-
sively large. See the Output Dynamic Range section.
Capacitor C4 forms a pole and a zero according to the
following equations:
f
to roll off. f
boost gain no longer affects the transfer function. At
frequencies greater than or equal to f
by the maximum gain setting and the volume control
attenuation dominate. Table 11 contains a list of capac-
itor values and the corresponding poles and zeros for a
given DC gain. See Figure 9 for an example of a gain
profile using BassMax.
Proper layout and grounding are essential for optimum
performance. Connect PGND and SGND together at a
single point (star ground point) on the PCB near the
MAX9729. Connect PV
Place C2 physically close to PV
nect it to PGND. Bypass PV
Connect C3 as close to PV
to SGND with a 1µF capacitor. Place the V
Table 10. BassMax Gain Examples,
R1 = 47kΩ
POLE
R2 (kΩ)
39
33
27
22
15
10
is the frequency at which the gain boost begins
ZERO
f
f
POLE
ZERO
is the frequency at which the bass
=
=
2
2
π
π
×
×
SS
A
R
C
C
R
DD
Layout and Grounding
1
V_BOOST
1
6
6
and SV
−
+
×
×
R
as possible. Bypass V
R
20.6
15.1
11.3
R
R
8.8
5.7
3.7
DD
2
2
1
1
SS
×
×
R
R
and SV
to PGND with C3.
SS
(dB)
2
2
ZERO
(
(
together at C2.
Hz
Hz
)
)
, the gain set
SS
DD
and con-
bypass
DD