MP7751 – 26V, 2X20W STEREO BTL FIX FREQUENCY CLASS D AUDIO AMPLIFIER

MP7751 Rev. 1.0

www.MonolithicPower.com

17

5/23/2014

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Power Source

For maximum output power, the amplifier circuit

requires a regulated external power source. A

high power-supply voltage can deliver more

power to a given load resistance, but a power-

source voltage exceeding the maximum voltage

of 26V can damage the MP7751. The

MP7751’s power supply rejection is excellent,

though power-supply noise can pass to the

output, so care must be taken to minimize

power supply noise within the pass-band

frequencies. Bypass the power supply with a

large capacitor (typically aluminum electrolytic)

along with two smaller 1μF and 1nF ceramic

capacitors at the MP7751 VCC supply pins.

PCB Layout

Circuit layout is critical for optimal performance,

low output distortion, and noise. Duplicate the

EVB layout for best results. For layout changes,

follow these guidelines and use Figure 7 and

Figure 8 as references.

1) Place the following components as close to

the MP7751 as possible:

Bootstrap Capacitors

pin as possible.

Power Supply Bypass Capacitors

power stage. To avoid overstressing the

MP7751 and excessive output noise, place

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) Make sure that any traces carrying the switch

node (SW) voltages are routed far from any

input signal traces. If the trace must run near

the SW trace near the input, shield the input

with a ground plane between the traces.

Physically separate each channel to prevent

crosstalk. Make sure that all inductors used on

a single circuit board have the same orientation.

Route each power supply 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 Class D

amplifiers, care must be taken to minimize the

effects of electromagnetic interference from the

amplifier. However, proper component selection

and careful attention to circuit layout can

minimize the effects of the EMI due to the

amplifier switching.

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 EM field as well toroidal

inductors, but can achieve a better balance

between good EMI performance with low cost.

The size of high-current loops that carry rapidly

changing currents must be minimized: Make

close to the MP7751 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.