MP1740 –3W HIGH EFFCIENCY MONO CLASS-D AUDIO AMPLIFIER

MP1740 Rev. 1.0

www.MonolithicPower.com

9

1/5/2012

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APPLICATION INFORMATION

COMPONENT SELECTION

The MP1740 uses a minimum number of external

components to complete a fully bridged Class D

audio amplifier. Use the following sections to

customize the amplifier for your particular

application.

1. Setting the Voltage Gain

The voltage gain sets the output voltage swing

for a given input voltage swing and is set by the

I

150kΩ

GAIN=2

R

×

(1)

The maximum output voltage swing is limited by

the power supply. To achieve the maximum

output power of the MP1740 amplifier, set the

amplifier gain such that the maximum peak-to-

peak input signal results in at least the maximum

peak-to-peak output voltage swing. Lower gain

allows the MP1740 to operate at its best,

because a higher voltage at the input for the

same output power making the inputs less

susceptible to noise. So setting voltage gain to

2V/V may achieve optimal performance.

The MP1740 is a mono BTL Class-D amplifier

with differential inputs and outputs. This device

can still be used with a single-ended input; but for

some noisy environment (such as a wireless

handset) the MP1740 should be used with

differential inputs to ensure maximum noise

rejection.

The fully differential amplifier allows the inputs to

be biased at voltage other than mid-supply. The

input coupling capacitors are not required if the

design uses a differential source that is biased

from 0.5 V to VCC - 0.8 V (as shown in

Recommended Operating Conditions). If the

input

signal

is

not

biased

within

the

recommended common-mode input range or if

using a single-ended source, the input coupling

capacitors are used to pass only the AC audio

signal to the input of the amplifier as a high pass

filter. The input capacitors create a high-pass

filter with the input resistor. Choose an input

coupling capacitor such that the corner frequency

The formula for the corner frequency is:

C

II

1

f=

2πRC

(2)

Speakers in wireless handsets usually can’t

respond well to low frequencies, so for this

application the corner frequency can be set to

block the low frequencies.

The input coupling capacitance is calculated as:

I

IC

1

C=

2πRf

(3)

If the corner frequency is within the audible band,

the capacitors should have a tolerance of ±10%

or better, because any mismatch in capacitance

causes an impedance mismatch at the corner

frequency and below.

The class-D audio amplifier requires adequate

power supply decoupling to ensure the efficiency

is high and total harmonic distortion (THD) is low.

To carry the higher frequency transient current,

spikes, or digital hash on the line, a good low

ESR ceramic capacitor is necessary. Place a 1μF

decoupling capacitor as close as possible to the

device VCC lead. It is very important for the

efficiency of the class-D amplifier, because any

resistance or inductance in the trace between the

capacitor and the device may cause a loss. A

10μF or greater capacitor placed near the audio

power amplifier would also help for filtering lower-

frequency noise.

In fully differential amplifiers, please use the

matching input resistors. Matching is very

important for the balance of the output on the

reference voltage (more important than overall

tolerance). The higher matching, the batter

CMRR and PSRR performance. Therefore, it is

recommended to use 1% precision resistors or

better to keep the performance optimized. Place

the input resistors as close to the MP1740 as

possible to limit noise injection on the high-

impedance nodes.