MAX9723
Charge-Pump Capacitance and Load Resistance
graph in the Typical Operating Characteristics.
Charge-Pump Hold Capacitor
The hold capacitor’s value and ESR directly affect the
ripple 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):
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 frequen-
cies is equal to:
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 recom-
mended. 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 low-
frequency gain.
The low-frequency boost attained by the BassMax cir-
cuit 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 signal 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:
f
POLE
is the frequency at which the gain boost begins
to roll off. f
ZERO
is the frequency at which the bass-
boost 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 following equation:
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
