MP7741 – 10W CLASS D MONO SINGLE ENDED AUDIO AMPLIFIER

MP7741 Rev. 1.0

www.MonolithicPower.com

12

2/17/2011

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PCB Layout

The circuit layout is critical for optimum

performance and low output distortion and noise.

It is highly recommended to duplicate EVB layout

for

optimum

performance.

If

change

is

necessary, please follow these guidelines and

take Figure 4 for references.

1) Place the following components as close to

the MP7741 as possible:

Bootstrap Cap

to pins 8 (BS) and 9 (SW) as possible.

Power Supply Bypass

switching power stage. To prevent overstressing

of the MP7741 and excessive noise at the output,

(PGND) as possible.

Integrated Capacitors

frequencies and are typically on the order of a

(NIN) as possible to reduce distortion and noise.

Reference Bypass Capacitors

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

are as close to the MP7741 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.