MP7745 – 2 x20 W CLASS D STEREO SINGLE ENDED AUDIO AMPLIFIER

MP7745 Rev. 0.9

www.MonolithicPower.com

10

5/11/2009

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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

00

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

to block DC voltages and thus passes only the

amplified AC signal from the LC filter to the

load. The combination of the coupling capacitor,

be 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

×

=

below 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 MP7745 input is at half the

coupling capacitor transmits the AC signal from

the source to the MP7745 while blocking the

DC voltage. Choose an input coupling capacitor

the passband frequency. The corner frequency

is calculated as:

IN

IN

IN

C

R

2

1

f

×

×

π

×

=

Power Source

For maximum output power, the amplifier circuit

requires a regulated external power source to

supply the power to the amplifier. The higher

the power supply voltage, the more power can

be delivered to a given load resistance,

however if the power source voltage exceeds

the maximum voltage of 26V, the MP7745 may

sustain damage. The power supply rejection of

the MP7745 is excellent (typically -59dB),

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 with a

large capacitor (typically aluminum electrolytic)

along with a smaller 1µF ceramic capacitor at

PCB Layout

The circuit layout is critical for optimum

performance and low output distortion and

noise. It is highly recommended to duplicate

EVB layout for optimum performance. If change

is necessary, please follow these guidelines

and take Figure 2 for references.

1) Place the following components as close to

the MP7745 as possible:

Bootstrap Cap

current to the internal high-side MOSFET.

12 and 14 as possible.