Datasheet

BOOT+
OUT+
AUDIO
INPUT
MAX9744
C
IN
BOOT-
OUT-
IN
VOLUME
CONTROL
9.5dB (max)
CLASS D
20dB
FB
R
IN
R
F
Figure 9. Setting Gain
GND
GND
MAX9744
SHDN
OUT
3.3V
V
DD
PV
DD
IN
1μF
MAX1615
4.5V TO 14.5V
1μF
Figure 10. Using a Linear Regulator to Produce 3.3V from a
Higher Power Supply
MAX9744
20W Stereo Class D Speaker Amplifier
with Volume Control
22 ______________________________________________________________________________________
Component Selection
Gain-Setting Resistors
External feedback resistors set the gain of the
MAX9744. The output stage has an internal 20dB gain
in addition to the externally set input stage gain. Set the
maximum gain by using resistors R
F
and R
IN
(Figure 9)
as follows:
Choose R
F
between 10kΩ and 50kΩ. Note that the
actual gain of the amplifier is dependent on the volume
level setting. For example, with the volume set to
+9.5dB, the amplifier gain would be 9.5dB plus 20dB,
assuming R
IN
= R
F
.
The input amplifier can be configured into a variety of
circuits. The FB terminal is an actual operational ampli-
fier output, allowing the MAX9744 to be configured as a
summing amplifier, a filter, or an equalizer, for example.
Input Capacitor
An input capacitor (C
IN
) in conjunction with the input
impedance of the MAX9744 form a highpass filter that
removes the DC bias from an incoming signal. The AC-
coupling capacitor allows the amplifier to automatically
bias the signal to an optimum DC level. Assuming zero
source impedance, the -3dB point of the highpass filter
is given by:
Choose C
IN
that f
-3dB
is well below the lowest frequen-
cy of interest. Use capacitors whose dielectrics have
low-voltage coefficients, such as tantalum or aluminum
electrolytic. Capacitors with high-voltage coefficients,
such as ceramics, may result in increased distortion at
low frequencies.
DC-Coupled Input
The input amplifier can accept DC-coupled inputs that
are biased to the amplifier’s bias voltage. DC-coupling
eliminates input-coupling capacitors, reducing compo-
nent count to potentially one external component. In this
configuration the highpass filtering effect of the capacitors
is lost, allowing low-frequency signals to be amplified.
Power Supplies
The MAX9744 features separate supplies for each por-
tion of the device, allowing for the optimum combination
of headroom power dissipation and noise immunity. The
speaker amplifier is powered from PVDD and can range
from 4.5V to 14V. The remainder of the device is pow-
ered by V
DD
. Power supplies are independent of each
other so sequencing is not necessary. Power may be
supplied by separate sources or derived from a single
higher source using a linear regulator (Figure 10).
BIAS Capacitor
BIAS is the output of the internally generated DC 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, startup/shutdown,
DC bias waveforms for the speaker amplifiers. Bypass
BIAS with a 2.2µF capacitor to GND.
f
RC
dB
IN IN
−
=
3
1
2π
A
R
R
VV
V
F
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
−30 /