MP7770 –2 X 45W STEREO SE OR 90W MONO BTL CLASS D AUDIO AMPLIFIER

MP7770 Rev.1.0

www.MonolithicPower.com

17

12/4/2012

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Choosing the Output LC Filter

The inductor-capacitor (LC) filter converts the

pulses 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 4: SE Filter Configuration

SW1

SW2

Figure 5: BTL Filter Configuration

Where:

FF1

F2

LL

L



,

Y1

Y2

FX

Y1

Y2

CC

CC

CC







,

F1

F2

LL



;

Y1

Y2

CC



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

the

SW

pin.

The

characteristic frequency of the LC filter is:

0

FF

1

f

2L

C



 



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

frequencies and reduce the passband flatness.

The circuit Q is set by the load resistance

(speaker resistance, typically 4Ω or 8Ω). Q is

calculated as:

LOAD

LOAD

0F

0F

RR

Q

L2

f

L





  

is

the

characteristic

frequency

in

with Q between 0.7 and 1.

The type of inductor and capacitor used in the

LC filter.greatly affects the output ripple and

noise. Use a film capacitor and an inductor with

sufficient power rating to supply the output

current to the load. The inductor must exhibit soft

saturation characteristics: If the inductor exhibits

hard saturation, it should operate well below the

saturation current. Use toroidal cores made of

gapped ferrite, MPP, powdered iron, or similar

materials. If using either an open or shielded

bobbin ferrite core for multi-channel designs,

make sure that the start windings of each

inductor align (all starting toward the SW pin, or

all starting toward the output) to prevent

crosstalk

or

other

channel-to-channel

interference.

Output Coupling Capacitor for SE Output

serves to pass only the amplified AC signal

from the LC filter to the load and to block DC

signals. The combination of the coupling

that the required minimum frequency passes.

can be calculated as:

OUT

LOAD

OUT

C

R

2

1

f











below the minimum required frequency.

The output coupling capacitor carries the full

load current, so chose a capacitor such that its

ripple current rating is greater than the

maximum load current. Use low-ESR aluminum

electrolytic capacitors for best results.

Input Coupling Capacitor

pass only the AC signal at the input. For a

typical system application, the source input

signal centers around the circuit ground, while

the MP7770 input is at half the power supply

transmits the AC signal from the source