MP7705 – 12V, 2x5W/2x2.5W CLASS–D STEREO SINGLE ENDED AUDIO AMPLIFIER

MP7705 Rev. 1.1

www.MonolithicPower.com

7

9/18/2009

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The quality factor (Q) of the LC filter is important.

If this is too low output noise will increase, and 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 8Ω) and

is calculated as:

R

L

f

2

R

L

Q

0

0

×

×

π

=

×

ω

=

Where

filter with 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 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 Capacitors

only the amplified AC signal from the LC filter to

the load. The combination of the coupling

capacitor and the load resistance results in a

required minimum frequency is still allowed to

pass. The output corner frequency (-3dB point),

OUT

LOAD

OUT

C

R

2

1

f

×

×

π

×

=

Set the output corner frequency at or below the

minimum required frequency.

The output coupling capacitor carries the full

load current, so the 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 Capacitors

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 MP7705 input is at half the

coupling capacitors transmit the AC signal from

the source to the MP7705 while blocking the

DC

voltage.

Choose

the

input

coupling

less than 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. A higher

power supply voltage allows more power to be

delivered to a given load resistance. It is very

important to bypass the power supply pins with

1µF X7R ceramic capacitors.

Power Supply Pumping

It is also very important to bypass the power

supply with a large aluminum electrolytic

capacitor. It is recommended to use a value of

at least

1000µF. This is necessary to prevent

the supply voltage from getting pumped up to a

level that exceeds the absolute maximum rating.

Supply pumping occurs in single-ended Class–

D amplifiers, and is caused by rapid switch

transitions where current is pumped back up

into the supply line. The large capacitor is

necessary to absorb this current and prevent