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

MP7751 Rev. 1.0

www.MonolithicPower.com

16

5/23/2014

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APPLICATION INFORMATION

COMPONENT SELECTION

Delay time for input signal absence report

The Recovery time is defined as follows.

[]

=×

×

8

delay

R

T

1.6 10

C sec

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

SW2

Figure 4: 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

=

×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 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

==

ω×

π × ×

is

the

characteristic

frequency

in

with Q between 0.7 and 1.4.

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.

Input Coupling Capacitor

The input coupling capacitor transmits the AC

signal from the source to the MP7751 while

blocking the DC voltage. Choose an input

coupling capacitor such that the corner

frequency. The corner frequency is calculated

as:

IN

IN

IN

C

R

2

1

f

×

×

π

×

=

The impedance of the input resister is different

with the different gain setting. At the same gain

setting, the input impedance from part-to-part

may shift by ±20% due to shifts in the actual

resistance of the input resistors. But please

note that the gain variation from part-to-part is

small.

For design purposes, the input network should

be designed assuming an input impedance of

16 kΩ, which is the absolute minimum input

impedance of the MP7751. At the lower gain

settings, the input impedance could increase as

high as 90 kΩ.