MP7742 – 2 x 15W CLASS D STEREO SINGLE ENDED AUDIO AMPLIFIER

MP7742 Rev. 0.91

www.MonolithicPower.com

7

1/25/2010

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OPERATION

The MP7742 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

MP7742

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 MP7742. 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 MP7742 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 MP7742 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

MP7742 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.

For VDD > 24V, two schottky diodes are

required for short-circuit protection, with the

cathodes connected to SW1/2 and the anodes

connected to PGND1/2. The diodes should

have output current of at least 1A and a

minimum of 40V dc blocking voltage rating.

Please place the diodes as close to the

MP7742 as possible.

For VDD < 24V, the

schottky diodes are not required for short circuit

protection.

If short-circuit protection is not needed, the

Schottky diodes can be omitted.

Enable Function

The MP7742 EN input is an active high enable

control. To enable the MP7742, drive EN with a

2.5V or greater voltage. To disable the amplifier,

drive it below 0.8V. While the MP7742 is

disabled, the VDD operating current is less than

100µA and the output driver MOSFETs are

turned off.