MP7758

– 18V, 2x15W, STEREO BTL, INDUCTORLESS CLASS-D AUDIO AMPLIFIER

MP7758 Rev. 1.0

www.MonolithicPower.com

17

12/3/2020

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© 2020 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Delay Time for Input Signal Absence

Reporting

The delay time can be calculated with Equation

(4):

 

8

DELAY

R

t

1.6 10

C sec







(4)

Selecting the Output EMI Filter

The magnetic bead capacitor filter converts the

pulses at SW to the output voltage that drives

the speaker. Both the PVCC line and output

current flow require magnetic beads to prevent

large radiation from the high slew rate pulses.

Magnetic beads with a higher impedance result

in improved EMI performance.

Selecting the Input Coupling Capacitor

The input coupling capacitor transmits the AC

signal from the source to the MP7758 while

blocking the DC voltage. Choose an input

coupling

capacitor

such

that

the

corner

frequency

(fIN)

is

below

the

passband

frequency.

The

corner

frequency

can

be

estimated with Equation (5):

IN

IN

IN

1

f

2

R

C



 



(5)

The impedance of the input resistor changes

with the gain setting. At the same gain setting,

the input impedance between different parts

may shift by ±20% due to shifts in the actual

resistance of the input resistors. However, the

gain variation between parts is minimal.

For design purposes, the input network should

be designed assuming an input impedance of

16

kΩ (the MP7758’s absolute minimum input

impedance). At lower gain settings, the input

impedance can be as high as 90

kΩ.

Setting the Power Source

For maximum output power, the amplifier circuit

requires a regulated external power source. A

high power supply voltage can deliver more

power to a given load resistance, but a power

source voltage that exceeds the maximum

voltage (about 18V) can damage the MP7758.

The

MP7758

’s power supply rejection is

excellent. However, power supply noise can

pass to the output, so considerations must be

made to minimize power supply noise within the

passband

frequencies.

Bypass

the

power

supply

with

a

large

capacitor

(typically

aluminum electrolytic) along with two smaller

1μF and 1nF ceramic capacitors at the VCC

supply pins.

Electromagnetic Interference (EMI)

Considerations

Due

to

the

switching

nature

of

class-D

amplifiers, care must be taken to minimize the

effects of electromagnetic interference from the

amplifier. Optimal component selection and

careful attention to the circuit layout can

minimize the effects of EMI due to the amplifier

switching.

The magnetic beads block radiated emissions

from

the

SW

nodes.

For

the

best

EMI

performance,

use

high-current

and

high-

impedance magnetic beads.

Murata’s NFZ2M

series Class-D EMI filter is well-suited to work

with the PVCC and four SW outputs.

Minimize the size of the high-current loops that

carry rapidly changing currents. Ensure that the

VCC bypass capacitors are as close to the

MP7758 as possible.

Nodes that carry rapidly changing voltage (e.g.

the SW node) must be as small as possible. If

sensitive traces run near a trace connected to

SW, place a ground shield between the traces.

The

MP7758

’s

evaluation

board

passed

CISPR22 Class B standard with stereo full

power output (see Figure 6).