C

T

1.0

120

2.2

120

4.7

200

10

440

In order eliminate "clicks and pops", all capacitors must be

discharged before turn-on. Rapidly switching V

allow the capacitors to fully discharge, which may cause

"clicks and pops".

There is a relationship between the value of C

IN

and

C

is applied or the shutdown mode is deactivated. Best perfor-

mance is achieved by setting the time constant created by

C

given value of C

AUDIO POWER AMPLIFIER DESIGN

Audio Amplifier Design: Driving 3W into a 4

Ω load

The following are the desired operational parameters:

Power Output

3W

RMS

Load Impedance

4

Ω

Input Level

0.3V

Input Impedance

20k

Ω

Bandwidth

The design begins by specifying the minimum supply voltage

necessary to obtain the specified output power. One way to

find the minimum supply voltage is to use the Output Power

vs Power Supply Voltage curve in the Typical Performance

Characteristics section. Another way, using Equation (8), is

to calculate the peak output voltage necessary to achieve

the desired output power for a given load impedance. To

account for the amplifier’s dropout voltage, two additional

voltages, based on the Clipping Dropout Voltage vs Power

Supply Voltage in the Typical Performance Characteris-

tics curves, must be added to the result obtained by Equa-

tion (8). The result is Equation (9).

(6)

V

(7)

The Output Power vs. Power Supply Voltage graph for an 8

Ω

load indicates a minimum supply voltage of 11.8V. The com-

monly used 12V supply voltage easily meets this. The addi-

tional voltage creates the benefit of headroom, allowing the

LM4940 to produce an output power of 3W without clipping

or other audible distortion. The choice of supply voltage must

also not create a situation that violates of maximum power

dissipation as explained above in the Power Dissipation

section. After satisfying the LM4940’s power dissipation re-

quirements, the minimum differential gain needed to achieve

3W dissipation in a 4

Ω BTL load is found using Equation

(10).

(8)

Thus, a minimum gain of 11.6 allows the LM4940’s to reach

full output swing and maintain low noise and THD+N perfor-

mance. For this example, let A

BTL gain is set using the input (RIN

resistors of the first amplifier in the series BTL configuration.

Additionaly, A

R

Ω,

the feedback resistor is found using Equation (11).

R

(9)

The value of R

Ω. The nominal output power is 3W.

The last step in this design example is setting the amplifier’s

pass band magnitude variation limit, the low frequency re-

sponse must extend to at least one-fifth the lower bandwidth

limit and the high frequency response must extend to at least

five times the upper bandwidth limit. The gain variation for

desired limit. The results are an

f

(10)

and an

f

(11)

As mentioned in the SELECTING EXTERNAL COMPO-

NENTS section, R

create a highpass filter that sets the amplifier’s lower band-

pass frequency limit. Find the coupling capacitor’s value

using Equation (14).

C

πR

(12)

The result is

1 / (2

πx20kΩx20Hz) = 0.398µF = C

IN

and

1 / (2

πx4Ωx20Hz) = 1989µF = C

OUT

Use a 0.39µF capacitor for C

C

The product of the desired high frequency cutoff (100kHz in

this example) and the differential gain A

upper passband response limit. With A

100kHz, the closed-loop gain bandwidth product (GBWP) is

1.2mHz. This is less than the LM4940’s 3.5MHz GBWP. With

this margin, the amplifier can be used in designs that require

more differential gain while avoiding performance restricting

bandwidth limitations.

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