Datasheet

MAX9742
Single-/Dual-Supply, Stereo 16W,
Class D Amplifier with Differential Inputs
______________________________________________________________________________________ 27
A
V
-
R
F
R
IN_
R
IN1
= R
IN2
, R
F1
= R
F2
DUAL-SUPPLY CONFIGURATION
MID
R
F2
R
F2
C
FBR
R
F1
C
IN
R
IN1
MAX9742
C
IN
R
IN2
R
F1
INL-
INL+
C
IN
R
IN2
C
IN
R
IN1
INR+
INR-
C
FBL
OUTL
+
+
-
-
+
-
+
-
C
FBL
C
FBR
OUTR
L
F
C
F
L
F
C
F
LEFT-CHANNEL
AUDIO INPUT
FBL
FBR
RIGHT-CHANNEL
AUDIO INPUT
V
DD
V
SS
Figure 14. Circuit Configuration for Minimizing Supply Pumping
Worst-case supply pumping occurs at high output pow-
ers with low-frequency signals and small load resis-
tances. Since the period is longer for low-frequency
signals, the continuous output current has more time to
pump up the supply rails during each cycle of the
audio signal. Additionally, for most stereo audio
sources the low-frequency audio content (bass) is pri-
marily monophonic. This means both output channels
are basically equal in magnitude and in phase at low
frequencies causing twice as much pump-up current to
flow into the supply bypass capacitors and therefore
doubling the supply pump-up voltages. Assuming
purely sinusoidal output signals, the worst-case supply
voltage increase due to supply pumping can be
approximated using the following equation:
where V
PUMP_MAX
is the magnitude increase of the
supply rail, V
SUPPLY
is the nominal voltage magnitude
of the respective supply, f
OUT
is the frequency of the
audio signal, and C
SUPPLY
is the value of the respec-
tive supply bypass capacitor. The above equation
shows that increasing the value of the supply bypass
capacitor decreases the supply voltage variations due
to supply pumping. Using large bypass capacitors
helps minimize supply voltage variations by providing
sufficient supply decoupling at low output frequencies.
To prevent the MAX9742 from entering supply overvolt-
age protection mode at low output frequencies (as low
as 20Hz), use supply bypass capacitors with values of
at least 1000µF for dual-supply operation and 660µF for
single-supply operation.
Alternate Methods for Mitigating Supply Pumping
Using the BTL configuration minimizes the supply
pumping effect since the outputs are driven 180° out-
of-phase with each other. Driving the outputs 180° out-
of-phase causes each half-bridge to pump up and
draw current from opposite supplies, which reduces
the magnitude of the of the supply pumping.
For the single-ended output configuration, the supply
pumping can be minimized by driving the channels
180° out-of-phase and reversing the polarity of one
speaker connection (see Figure 14). Reversing the
polarity of one speaker minimizes any adverse affects
on the audio quality by ensuring that the physical dis-
placement of the speaker cones matches the physical
displacement of the speakers when driven with in
phase signals.
V
V
2
1
f R C
PUMP_MAX
SUPPLY
2
OUT SPKR SUPPLY
=
⎛
⎝
⎜
⎞
⎠
⎟
×
××
⎛
⎝
⎜
⎞
⎠
⎟
π