MP7747 – 20W CLASS D MONO SINGLE ENDED AUDIO AMPLIFIER

MP7747 Rev. 1.0

www.MonolithicPower.com

11

1/28/2011

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==

ω×

π × ×

LOAD

LOAD

00

RR

Q

L2

f

L

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

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

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 MP7747 input is at half the power supply

transmits the AC signal from the source to the

MP7747 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

×

=

Timer Capacitor

The start up source current slew rate is

adjustable by selecting different capacitance of

capacitance is, the smaller the start up current

which capacitance is larger than 312nF, so the

start up current slew rate would be smaller than

20mA/50ms which would help to eliminate the

turn on pop. The recommended 2.2µF capacitor

approximately 20mA/350ms.

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 36V, the MP7747 may sustain

damage. The power supply rejection of the

MP7747 is excellent (typically -60dB), 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

pins.