MP7722 – 2 x 20W CLASS D STEREO SINGLE ENDED AUDIO AMPLIFIER
PCB Layout
The circuit layout is critical for optimum
performance and low output distortion and
noise. Place the following components as close
to the MP7722 as possible:
Power Supply Bypass, C
BYP
C
BYP1
and C
BYP2
carry the transient current for
the switching power stage. To prevent
overstressing of the MP7722 and excessive
noise at the output, place C
BYP1
as close to pins
18 (VDD1) and 20 (PGND1) as possible and
also place C
BYP2
as close to pins 13 (VDD2) and
15 (PGND2) as possible.
Output Catch Diodes
D
SH1
, D
SL1
, D
SH2
and D
SL2
carry the current over
the dead-time while the MOSFET switches are
off. Place the diodes as close to the MP7722 as
possible.
Timing Capacitors
C
INT1
and C
INT2
are used to set the amplifier
switching frequencies and are typically on the
order of a few nF. Place C
INT1
as close to pins 2
and 3 as possible to reduce distortion and
noise. Likewise, place C
INT2
as close to pins 7
and 8 as possible.
Reference Bypass Capacitors
C
R1
and C
R2
filter the �½ V
DD
reference voltages.
Place C
R1
and C
R2
as close to the IC as
possible to improve power supply rejection and
reduce distortion and noise at the output.
When laying out the PCB, use two separate
ground planes, analog ground (AGND) and
power ground (PGND), and connect the two
grounds together at a single point to prevent
noise injection into the amplifier input to reduce
distortion.
Make sure that any traces carrying the switch
node (SW) voltages are separated far from any
input signal traces. If it is required to run the
SW trace near the input, shield the input with a
ground plane between the traces. For multiple
channel applications, make sure that each
channel is physically separated to prevent
crosstalk. Make sure that all inductors used on
a single circuit board have the same orientation.
Also, make sure that the power supply is routed
from the source to each channel individually,
not serially. This prevents channel-to-channel
coupling through the power supply input.
Electro-Magnetic Interference (EMI)
Considerations
Due to the switching nature of the Class D
amplifier, care must be taken to minimize the
effects of electromagnetic interference from the
amplifier. However, with proper component
selection and careful attention to circuit layout,
the effects of the EMI due to the amplifier
switching can be minimized.
The power inductors are a potential source of
radiated emissions. For the best EMI
performance use toroidal inductors, since the
magnetic field is well contained inside the core.
However toroidal inductors can be expensive to
wind. For a more economical solution, use
shielded gapped ferrite or shielded ferrite
bobbin core inductors. These inductors typically
do not contain the field as well as toroidal
inductors, but usually can achieve a better
balance of good EMI performance with low
cost.
The size of high-current loops that carry rapidly
changing currents needs to be minimized. To
do this, make sure that the V
DD
bypass
capacitors and the Schottky Catch diodes are
as close to the MP7722 as possible.
Nodes that carry rapidly changing voltages,
such as SW, need to be made as small as
possible. If sensitive traces run near a trace
connected to SW, place a ground shield
between the traces.
MP7722 Rev. 1.3
9/25/2006
www.MonolithicPower.com
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© 2006 MPS. All Rights Reserved.
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