When evaluating the MAX9737 with a ferrite bead filter
and resistive load, include a series inductor (68µH for
8Ω load and 33µH for 4Ω load) to model typical loud-
speaker’s behavior. Omitting the series inductor
reduces the efficiency, the THD+N performance and
the output power of the MAX9737. When evaluating
with a loudspeaker, no series inductor is required.
Inductor-Based Output Filters
Some applications use the MAX9737 with a full induc-
tor/capacitor-based (LC) output filter. See Figure 4 for
the correct connections of these components.
The load impedance of the speaker determines the fil-
ter component selection (see Table 1).
Inductors L1 and L2 and capacitor C1 form the primary
output filter. Capacitors C2 and C3 provide common-
mode filtering to reduce radiated emissions. Capacitors
C4 and C5, plus resistors R1 and R2, form a Zobel at
the output. A Zobel corrects the output loading to com-
pensate for the rising impedance of the loudspeaker.
Without a Zobel, the filter exhibits peaking near the cut-
off frequency.
Component Selection
Gain-Setting Resistors
The output stage provides a fixed internal gain in addi-
tion to the externally set input stage gain. The fixed-out-
put stage gain is set at 13.6dB (4.8V/V). Set overall gain
by using resistors R
F
and R
IN
(Figure 5) as follows:
where A
V
is the desired voltage gain. Choose R
F
between 10kΩ and 50kΩ.
The PRE terminal is an operational amplifier output,
allowing the MAX9737 to be configured as a filter or an
equalizer.
Input Capacitor
An input capacitor, C
IN
, in conjunction with the input
resistor, R
IN
, of the MAX9737 forms a highpass filter
that removes the DC bias from an incoming signal. The
AC-coupling capacitor allows the amplifier to bias the
signal to an optimum DC level. Assuming negligible
source impedance, the -3dB point of the highpass filter
is given by:
Choose C
IN
such that f
-3dB
is well below the lowest fre-
quency of interest. To reduce low-frequency distortion,
use capacitors whose dielectrics have low-voltage
coefficients. Capacitors with high-voltage coefficients
cause increased distortion close to f
-3dB
. For best click-
and-pop suppression, use a 0.47µF input capacitor.
COM Capacitor
COM is the output of the internally generated DC bias
voltage. Bypass COM with a 1µF capacitor to AGND.
Regulator Capacitor
REG is the output of the internally generated DC bias
voltage. Bypass REG with a 1µF capacitor to AGND.
Power Supplies
The MAX9737 features separate supplies for signal and
power portions of the device, allowing for the optimum
combination of headroom, power dissipation and noise
immunity. The speaker amplifiers are powered from
PVDD and can range from 8V to 28V. The remainder of
the device is powered by an internal 5V regulator, V
S
.
Internal Regulator
The MAX9737 features an internal 5V regulator, V
S
,
powered from PVDD. Bypass V
S
with a 1µF capacitor to
AGND.
