MP7720 – 20W CLASS D MONO SINGLE ENDED AUDIO AMPLIFIER

MP7720 Rev. 1.9

www.MonolithicPower.com

8

3/13/2006

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Output Catch Diode, D1

D1 carries the current over the dead-time while

both MOSFET switches are off. Place D1

between pins 7 (SW) and 8 (PGND) to prevent

the voltage at SW from swinging excessively

below ground.

Input Modulator Capacitor, C3

C3 is used to set the amplifier switching

frequency and is typically on the order of a few

nanofarads. Place C3 as close to the differential

input pins (1 and 2) as possible to reduce

distortion and noise.

Reference Bypass Capacitor, C2

PIN input (pin 1). Place C2 as close to PIN as

possible to improve power supply rejection and

reduce distortion and noise at the output.

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. Power

components (C5, D1, C6 and C8) connect to

the

power

ground.

The

quiet

analog

components (C2, C3, R2, and the input source

ground) connect to the analog ground.

Place the input and feedback resistors R1 and

R4 as close to the NIN input as possible. Make

sure that any traces carrying the switching node

(SW) voltage are separated far from any input

signal traces. If multiple amplifiers are used on

a single board, make sure that each channel is

physically separated to prevent crosstalk. If it is

required to run the SW trace near the input,

shield the input with a ground plane between

the traces. Make sure that all inductors used on

a single circuit board have the same orientation.

If multiple channels 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

capacitor (C5) is as close to the MP7720 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.