MP7748 – 2X20W STEREO SINGLE ENDED CLASS D AUDIO AMPLIFIER

MP7748 Rev. 1.0

www.MonolithicPower.com

11

1/28/2011

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2011 MPS. All Rights Reserved.

OPERATION

The MP7748 is a Class D Audio Amplifier for

driving

stereo

speakers

in

single-ended

configuration or a mono speaker in bridge-tied-

load configuration. It uses the Monolithic Power

Systems

patented

Analog

Adaptive

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.

REF1/2 are the positive inputs of the two

amplifiers. They are set to half the DC power

signal at the input.

The amplifier voltage gain is set by the

equation:

IN

FB

R

R

AV

−

=

The

MP7748

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 MP7748. 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 MP7748 integrates a source current

function to charge the AC coupling capacitor

at the start up moment. The start up source

current slew rate is adjustable by selecting

different

capacitance

of

timer

capacitor

capacitor is, the smaller the start up current

slew rate is. The recommended 2.2µF timer

capacitor results in a start up current slew rate

of approximately 20mA/350ms which would

help to minimize the turn on pop.

After driving EN pin low, output SW will be set

to high impedance immediately which would

help to eliminate the turn off pop.

Short Circuit/Overload Protection

The MP7748 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

MP7748 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.

Enable Function

The MP7748 EN input is an active high enable

control. To enable the MP7748, drive EN with a

2.0V or higher voltage. To disable the amplifier,

drive it below 0.4V. While the MP7748 is

disabled, the VDD operating current is less than

140µA and the output driver MOSFETs are

turned off.

Programmable UVP

MP7748 integrate programmable UVP function,

which can be used to shutdown the MP7748 to

escape the pop, by controlling the UVP node

voltage. The corresponding circuit is shown in

the followed figure 2. If the UVP pin is NC, the

default VDD shutdown voltage (rising threshold)

is 8.4V since there is internal voltage divided

circuit. The VDD shutdown voltage can be

flexibly adjusted by controlling UVP pin voltage

The recommended VDD shutdown voltage is

from 9.5V to power supply. As shown in the

internal 500kΩ and 550kΩ resistor, the VDD

shutdown voltage (rising threshold) can be

calculated by the equation: