MP7720 – 20W CLASS D MONO SINGLE ENDED AUDIO AMPLIFIER

MP7720 Rev. 1.9

www.MonolithicPower.com

5

3/13/2006

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© 2006 MPS. All Rights Reserved.

OPERATION

The MP7720 is a single-ended Class D audio

amplifier. It uses the Monolithic Power Systems

convert the audio input signal into pulses.

These pulses drive an internal high-current

output stage and, when filtered through an

external inductor-capacitor filter, reproduce the

input signal across the load. Because of the

switching

Class

D

output

stage,

power

dissipation in the amplifier is drastically reduced

when compared to Class A, B or A/B amplifiers

while

maintaining

high

fidelity

and

low

distortion.

The amplifier uses a differential input to the

modulator. PIN is the positive input and NIN is

the negative input. The common mode voltage

of the input is set to half the DC power supply

voltage divider formed by R2 and R3. The input

capacitor C1 couples the AC signal at the input.

The amplifier voltage gain is set by the

combination of R1 and R4 and is calculated by

the equation:

1

R

4

R

AV

−

=

The output driver stage uses two 180mΩ

N-Channel MOSFETs to deliver the pulses to

the LC output filter which in turn drives the load.

To fully enhance the high-side MOSFET, the

gate is driven to a voltage higher than the

source by the bootstrap capacitor between SW

and BS. While the output is driven low, the

an internal circuit on the MP7720. The gate of

the high-side MOSFET is driven high from the

voltage at BS, forcing the MOSFET gate to a

MOSFET to fully turn on, reducing power loss in

the amplifier.

Pop Elimination

The capacitor C9 passes only AC currents to

the load. To insure that the amplifier passes low

frequency

signals,

the

time

constant

of

is asserted, the capacitor charges over a long

period and in a normal amplifier can result in a

turn on and/or turn off “pop.” The MP7720

includes integrated circuitry that eliminates the

turn on and turn off pop associated with the

charging of the AC coupling capacitor.

Short Circuit/Overload Protection

The MP7720 has internal overload and short

circuit protection. The currents in both the high-

side and low-side MOSFETs are measured and

if the current exceeds the 5.0A short circuit

current limit, both MOSFETs are turned off. The

MP7720 then restarts with the same power up

sequence that is used for normal starting to

prevent a pop from occurring after a short

circuit condition is removed.

Mute/Enable Function

The MP7720 EN input is an active high enable

control. To enable the MP7720, drive EN with a

2.0V or greater voltage. To disable the

amplifier, drive it below 0.4V. While the MP7720

is disabled, the VDD operating current is less

than 5µA and the output driver MOSFETs are

turned off. The MP7720 requires approximately

500ms from the time that EN is asserted (driven

high) to when the amplifier begins normal

operation.