Datasheet

Charge-Pump Hold Capacitor
The hold capacitor’s value and ESR directly affect the rip-
ple at PV
SS
. Ripple is reduced by increasing the value of
the hold capacitor. Choosing a capacitor with lower ESR
reduces ripple and output impedance. Lower capacitance
values can be used in systems with low maximum output
power levels. See the Output Power vs. Charge-Pump
Capacitance and Load Resistance graph in the Typical
Operating Characteristics.
BassMax Gain-Setting Components
The bass-boost low-frequency response, when BassMax
is enabled, is set by the ratio of R1 to R2 by the following
equation (see Figure 2):
V_BOOST
R1 R2
A 20 log
R1 R2
+
= ×
−
where A
V_BOOST
is the voltage gain boost in dB at low
frequencies. A
V_BOOST
is added to the gain realized by
the volume setting. The absolute gain at low frequencies
is equal to:
V_TOTAL V _VOL V_BOOST
A AA= +
where A
V_VOL
is the gain due to the volume setting, and
A
V_TOTAL
is the absolute gain at low frequencies. To
maintain circuit stability, the ratio:
R2/(R1 + R2)
must not exceed 1/2. A ratio equaling 1/3 is recommend-
ed. The switch that shorts BB_ to SGND, when BassMax
is disabled, can have an on-resistance as high as 300Ω.
Choose a value for R1 that is greater than 40kΩ to ensure
that positive feedback is negligible when BassMax is
disabled. Table 12 contains a list of R2 values, with R1 =
47kΩ, and the corresponding lowfrequency gain.
The low-frequency boost attained by the BassMax circuit
is added to the gain realized by the volume setting. Select
the BassMax gain so that the output signal will remain
within the dynamic range of the MAX9723. Output sig-
nal clipping will occur at low frequencies if the BassMax
gain boost is excessively large (see the Output Dynamic
Range section).
Capacitor C3 forms a pole and a zero according to the
following equations:
POLE
ZERO
R1 R2
f
2 C3 R1 R2
R1 R2
f
2 C3 R1 R2
−
=
π× × ×
+
=
π× × ×
f
POLE
is the frequency at which the gain boost begins to
roll off. f
ZERO
is the frequency at which the bassboost
gain no longer affects the transfer function and the
volume-control gain dominates. Table 13 contains a list of
capacitor values and the corresponding poles and zeros
for a given DC gain. See Figure 11 for an example of a
gain profile using BassMax.
Custom Maximum Gain Setting Using
BassMax
The circuit in Figure 12 uses the BassMax function to
increase the maximum gain of the MAX9723. The gain
boost created with the circuit in Figure 12 is added to the
maximum gain selected by Bit 5 in the command register.
Set the maximum gain with RA and RB using the follow-
ing equation:
V_TOTAL V _VOL
RA RB
A A 20 log
RA RB
+

= +×

−

where A
V_VOL
is the gain due to the volume setting, and
A
V_TOTAL
is the absolute passband gain in dB.
Capacitor CA blocks any DC offset from being gained,
but allows higher frequencies to pass. CA creates a pole
that indicates the low-frequency point of the pass band.
Choose CA so that the lowest frequencies of interest are
not attenuated. For a typical application, set f
POLE
equal
to or below 20Hz.
Figure 11. BassMax, Gain Profile Example
GAIN PROFILE WITH AND
WITHOUT BassMax
FREQUENCY (Hz)
A
V
(dB)
1k10010
-8
-6
-4
-2
0
2
4
6
8
10
-10
1 10k
MAX9723A
CMD REGISTER
CODE = 0xFF
R1 = 47kΩ
R2 = 22kΩ
C3 = 0.1mF
f
POLE
f
ZERO
WITH
BassMax
WITHOUT
BassMax
MAX9723 Stereo DirectDrive Headphone Amplier
with BassMax, Volume Control, and I
2
C
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