MP7720 – 20W CLASS D MONO SINGLE ENDED AUDIO AMPLIFIER

MP7720 Rev. 1.9

www.MonolithicPower.com

7

3/13/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

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:

R

L

f

2

R

L

Q

0

0

×

×

π

=

×

ω

=

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

The output AC coupling capacitor C9 serves 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,

C9 and the load resistance results in a first-

order high-pass filter. The value of C9 should

be selected such that the required minimum

frequency is still allowed to pass. The output

calculated as:

9

C

R

2

1

f

LOAD

OUT

×

×

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

The input coupling capacitor C1 is 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 MP7720 input is at half the power

capacitor transmits the AC signal from the

source to the MP7720 while blocking the DC

voltage. Choose an input coupling capacitor

the passband frequency. The corner frequency

is calculated as:

1

C

1

R

2

1

×

×

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 operating voltage of 24V, the

MP7720 may sustain damage. The power

supply rejection of the MP7720 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.

Circuit Layout

The circuit layout is critical for optimum

performance and low output distortion and

noise. Place the following components as close

to the MP7720 as possible:

Power Supply Bypass, C5

C5 carries the transient current for the switching

input stage. To prevent overstressing of the

MP7720 and excessive noise at the output,

place the power supply bypass capacitor as

close to pins 6 (VDD) and 8 (PGND) as

possible.