MP7740 – 15W CLASS D MONO SINGLE ENDED AUDIO AMPLIFIER
MP7740 Rev. 0.9 www.MonolithicPower.com 10
9/22/2009 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2009 MPS. All Rights Reserved.
Output Catch Diodes
D
SH and DSL carry the current over the dead-time
while the MOSFET switches are off. Place the
diodes as close to the MP7740 as possible.
Timing Capacitors
C
INT is used to set the amplifier switching
frequencies and are typically on the order of a
few nF. Place C
INT as close to pins 1 and 2 as
possible to reduce distortion and noise.
Reference Bypass Capacitors
C
R filters the ½ VDD reference voltages. Place CR
as close to the IC as possible to improve power
supply rejection and reduce distortion and noise
at the output.
2) The Inductor-Capacitor (LC) filter converts the
pulse train at SW to the output voltage that drives
the speaker. Please keep the filter capacitor
close to the inductor.
3) 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 (usually
around the bulk bypass capacitor) to prevent
noise injection into the amplifier input to reduce
distortion.
4) Keep the sensitive feedback signal trace on
the input side and shield the trace with the AGND
plane. 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. If multiple
amplifiers are used on a single board, 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.
If multiple amplifiers are used on a single board,
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 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
are as close to the MP7740 as possible.
Nodes that carry rapidly changing voltage, 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.
