MP7782 – 50W CLASS D MONO BRIDGED AUDIO AMPLIFIER

MP7782 Rev. 1.2

www.MonolithicPower.com

6

8/3/2005

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TM

Choosing the LC Filter

Two identical LC filters are required in the typical

application. The inductor-capacitor (LC) filter is a

second order filter that converts the pulse train at

SW (pins 14, 19) to the output differential signal

that drives the speaker. Typical values for the LC

filters are shown in Figure 1. The characteristic

frequency of the LC filter needs to be high

enough to allow high frequency audio to reach the

output, yet needs to be low enough to filter out

high frequency contents of the pulses from SW.

The characteristic frequency of the LC filter is:

LC

2

1

π

=

where:

2

L

1

L

L

+

=

,

2

L

1

L

=

12

C

7

C

12

C

7

C

30

C

C

+

×

+

=

,

12

C

7

C

=

The quality factor (Q) of the LC filter is

important. If this is too low, output noise will

increase. If the Q factor is too high, then

peaking may occur at high signal frequencies,

reducing the pass-band flatness. The Q value is

calculated as:

C

L

R

Q

=

Where R is the load (speaker) resistance.

Use an LC filter with a Q between 0.7 and 2.

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, 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.

Input Coupling Capacitor

The input coupling capacitors, C6 and C9, are

used to pass only the AC audio signal to the

input of the amplifier. In a typical system

application, the source input signal is usually

centered around the circuit ground, while the

MP7782 input is at half the power supply

transmits the AC signal from the source to the

MP7782 while blocking the DC voltage. This

input coupling capacitor creates a low-pass filter

with the input resistor of the MP7782. Choose

an input coupling capacitor such that the corner

band frequency.

The formula for the corner frequency is:

6

C

3

R

2

1

×

×

π

=

3

R

4

R

=

6

C

9

C

=

R6 are the input resistors and C6 and C9 are

the input AC coupling capacitors.

Power Source

For maximum output power, the amplifier circuit

requires a regulated external power source to

supply power to the amplifier. The higher the

power supply voltage provided, the more power

that can be delivered to a given load resistance.

However, if the power source voltage exceeds the

maximum operating voltage of 24V, the MP7782

may sustain damage.

The power supply rejection of the MP7782 is

excellent; however, noise at the power supply

can get to the output, so care must be taken to

minimize power supply noise within the pass-

band frequencies. Bypass the power supply

pins with a large electrolytic capacitor (typically

aluminum electrolytic) along with smaller 1µF

pins.