Datasheet

Bootstrap Diode (D
BOOT
)
To provide sufficient gate drive voltage to the high-side
transistor of the half-bridge output stage, an external
diode (D
BOOT
) and capacitor (C
BOOT
) are needed for
the internal bootstrapping circuitry (see Figure 2). To
maintain high power efficiencies and maximum output
power at low audio frequencies, use fast-recovery
switching diodes for D
BOOT
. Silicon diodes equivalent
to 1N914, BAS16, or 1N4148 work well.
Capacitor (C
BOOT
)
For most applications, use a C
BOOT
capacitor ≥ 0.1µF
and ≤ 0.22µF. For proper operation, use capacitors with
low ESR and voltage ratings greater than 7V for C
BOOT
.
Output-Coupling Capacitors
(C
OUT
, Single-Ended, Single-Supply Operation)
The MAX9742 requires output-coupling capacitors for
single-supply operation. Since the MAX9742 outputs
switch between V
DD
and ground in single-supply opera-
tion, there is a DC component equal to 0.5 x V
DD
pre-
sent at the outputs. The output-coupling capacitor
blocks this DC component, preventing DC current from
flowing into the load. The output capacitor and the load
resistance of the speaker form a highpass filter. The
-3dB point of the highpass filter can be approximated by:
where f
-3dB
is the -3dB cutoff frequency of the filter,
R
SPKR
is the DC resistance of the speaker, and C
OUT
is
the value of the output-coupling capacitor. As with the
input capacitor, choose C
OUT
such that f
-3dB
is well
below the lowest frequency of interest. Setting f
-3dB
too
high affects the amplifier‘s low-frequency response.
Select capacitors with low ESR to minimize power loss-
es. Since the output-coupling capacitor has a large
amplitude AC current (resulting average output current
due to the LC filter) flowing through it at high output
powers, it is important to select an output-coupling
capacitor that has an appropriate ripple current rating.
To prevent damage to the output-coupling capacitor,
use the following equation to calculate the required
RMS ripple current rating for C
OUT
:
where I
RMS_RIPPLE
is the minimum required RMS ripple
current rating for C
OUT
and R
SPKR
is the DC resistance
of the speaker. The ripple current ratings of capacitors
are frequency dependent, so be sure to select a
capacitor based on its ripple current rating within the
audio frequency range.
Select output-coupling capacitors with DC voltage rat-
ings greater than V
DD
.
In single-supply operation with single-ended outputs, the
leakage current of C
OUT
can affect the startup time of
the MAX9742. To minimize startup time delays due to
C
OUT
, use capacitors with leakage current ratings less
than 1µA for C
OUT
. See the Startup Time Considerations
section for more information on optimizing the startup
time of the MAX9742.
Setting V
MID
The voltage present at the MID input biases the internal
amplifiers and should be set to the average value of
V
DD
and V
SS
for maximum dynamic range. For dual-
supply operation, connect MID to ground. For single-
supply operation, set MID to 0.5 x V
DD
through an
external resistive divider. To minimize power dissipation
while providing enough input bias current for the MID
input, select divider-resistors with values greater than
or equal to 10kΩ and less than or equal to 20kΩ.
Connect a decoupling network between MID and the
SGND plane (see the Supply Bypassing/Layout sec-
tion) to provide a sufficient low- and high-frequency AC
ground for the internal amplifiers. Figure 11 shows the
recommended decoupling networks for bypassing the
MID input.
I
V
2.83 R
RMS_RIPPLE
DD
SPKR
=
×
()A
f
1
2R C
3dB
SPKR OUT
−
=
××
()
π
Hz
MAX9742
Single-/Dual-Supply, Stereo 16W,
Class D Amplifier with Differential Inputs
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