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

MP7745 Rev. 0.9

www.MonolithicPower.com

8

5/11/2009

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OPERATION

The MP7745 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 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

the AC signal at the input.

The amplifier voltage gain is set by the

equation:

IN

FB

R

R

AV

−

=

The

MP7745

includes

four

high-power

MOSFETs wherein for each channel the output

driver stage uses two 250mΩ 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 bootstrap

internal circuit on the MP7745. 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

signal and pass only AC signals to the load. To

insure that the amplifier passes low frequency

However, when EN 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 MP7745 includes integrated circuit

that eliminate the turn on and turn off pop

associated with the charging of the AC coupling

capacitor.

Short Circuit/Overload Protection

The MP7745 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 4.5A short circuit

current limit, both MOSFETs are turned off. The

MP7745 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.

Two schottky diodes (for example, B340

manufactured by Diodes Inc) are required for

short-circuit

protection,

with

the

cathode

connected to SW and the anode connected to

PGND. Please place the diode as close to the

MP7745 as possible.

If short-circuit protection is not needed, the

Schottky diode can be omitted.

Enable Function

The MP7745 EN input is an active high enable

control. To enable the MP7745, drive EN with a

2.5V or greater voltage. To disable the

amplifier, drive it below 0.8V. While the MP7745

is disabled, the VDD operating current is less

than 100µA and the output driver MOSFETs are

turned off.