MP7748 – 2X20W STEREO SINGLE ENDED CLASS D AUDIO AMPLIFIER

MP7748 Rev. 1.0

www.MonolithicPower.com

14

1/28/2011

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Choosing the Output LC Filter

The Inductor-Capacitor (LC) filter converts the

pulse train at SW to the output voltage that drives

the speaker. There are two kinds of LC filter

structure depending on the output configuration.

SW1/2

Figure 3—SE Filter Configuration

SW1

SW2

Figure 4—BTL Filter Configuration

Where:

2

F

1

F

F

L

L

L

+

=

,

2

Y

1

Y

2

Y

1

Y

X

F

C

C

C

C

C

C

+

×

+

=

,

2

F

1

F

L

L

=

;

2

Y

1

Y

C

C

=

The characteristic frequency of the LC filter

needs to be high enough to allow high frequency

audio to the output, yet needs to be low enough

to filter out high frequency products of the pulses

from SW. The characteristic frequency of the LC

filter is:

F

F

0

C

L

2

1

f

×

×

π

×

=

The quality factor (Q) of the LC filter is important.

If this is too low, output noise will increase, if this is

too high, then peaking may occur at high signal

frequencies reducing the passband flatness. The

circuit Q is set by the load resistance (speaker

resistance, typically 4Ω or 8Ω). The Q is

calculated as:

LOAD

LOAD

0F

0F

RR

Q

L2

f

L

==

ω×

π × ×

between 0.7 and 1.

The actual output ripple and noise is greatly

affected by the type of inductor and capacitor used

in the LC filter. Use a film capacitor and an

inductor with sufficient power handling capability

to supply the output current to the load. The

inductor

should

exhibit

soft

saturation

characteristics. If the inductor exhibits hard

saturation, it should operate well below the

saturation

current.

Gapped

ferrite,

MPP,

Powdered Iron, or similar type toroidal cores are

recommended. If open or shielded bobbin ferrite

cores are used for multi-channel designs, make

sure that the start windings of each inductor line

up (all starts going toward SW pin, or all starts

going toward the output) to prevent crosstalk or

other channel-to-channel interference.

Output Coupling Capacitor For SE Output

block DC voltages and thus passes only the

amplified AC signal from the LC filter to the load.

and the load resistance results in a first-order

selected such that the required minimum

frequency is still allowed to pass. The output

calculated as:

OUT

LOAD

OUT

C

R

2

1

f

×

×

π

×

=

the minimum required frequency.

The output coupling capacitor carries the full load

current, so a capacitor should be chosen such

that its ripple current rating is greater than the

maximum load current. Low ESR aluminum

electrolytic capacitors are recommended.

Input Coupling Capacitor

used to pass only the AC signal at the input. In a

typical system application, the source input signal

is typically centered around the circuit ground,

while the MP7748 input is at half the power

capacitor transmits the AC signal from the source