Also shown in Figure 2 are the electrical connections for the

headphone jack and plug. A 3-wire plug consists of a Tip,

Ring, and Sleave, where the Tip and Ring are audio signal

conductors and the Sleave is the common ground return.

One control pin for each headphone jack is sufficient to

indicate to the control inputs that a user has inserted a plug

into the jack and that the headphone mode of operation is

desired.

To ensure smooth transition from BTL to SE operation, it is

important to connect HP-IN and R

the Right Output of the headphone jack. The control pin on

the Left Output of the headphone jack should be left open.

Connecting the node between the HP-IN and R

Left Output control pin may cause unwanted state changes

to the HP-IN pin.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers is critical for optimum device

and system performance. While the LM4850 is tolerant to a

variety of external component combinations, consideration

must be given to the external component values that maxi-

mize overall system quality.

The LM4850’s unity-gain stability allows a designer to maxi-

mize system performance. The LM4850’s gain should be set

no higher than necessary for any given application. A low

gain configuration maximizes signal-to-noise performance

and minimizes THD+N. However, a low gain configuration

also requires large input signals to obtain a given output

power. Input signals equal to or greater than 1V

available from sources such as audio codecs. Please refer to

the section, Audio Power Amplifier Design, for a more

complete explanation of proper gain selection.

Selecting Input and Output Capacitor Values

Besides gain, one of the major considerations is the

closed-loop bandwidth of the amplifier. To a large extent, the

bandwidth is dictated by the choice of external components

shown in Figure 1. The input coupling capacitor C

resistor R

frequency response. C

desired frequency response weighed against the following:

Large value input and output capacitors are both expensive

and space consuming for portable designs. Clearly a certain

sized capacitor is needed to couple in low frequencies with-

out severe attenuation. But in many cases the speakers

used in portable systems, whether internal or external, have

little ability to reproduce signals below 150Hz. Thus, large

value input and output capacitors may not increase system

performance.

AUDIO POWER AMPLIFIER DESIGN

Design a 1W / 8

Ω Bridged Audio Amplifier

Given:

•

Power Output:

1W

RMS

•

Load Impedance

8

Ω

•

Input Level:

1V

RMS

•

Input Impedance:

20k

Ω

•

Bandwidth:

A designer must first determine the minimum supply voltage

needed to obtain the specified output power. By extrapolat-

ing from the Output Power vs Supply Voltage graphs in the

Typical Performance Characteristics section, the supply

rail can be easily found. A second way to determine the

minimum supply rail is to calculate the required V

OPEAK

using Equation 5 and add the dropout voltage. This results in

Equation 6, where V

from the Dropout Voltage vs Supply Voltage curve in the

Typical Performance Characteristics section.

(5)

V

DD

≥ (V

(6)

Using the Output Power vs Supply Voltage graph for an 8

Ω

load, the minimum supply rail is 4.7V. But since 5V is a

standard supply voltage in most applications, it is chosen for

the supply rail. Extra supply voltage creates headroom that

allows the LM4850 to reproduce peaks in excess of 1W

without producing audible distortion. However, the designer

must make sure that the chosen power supply voltage and

output load does not violate the conditions explained in the

Power Dissipation section.

Once the power dissipation equations have been addressed,

the required differential gain can be determined from Equa-

tion 7.

(7)

R

(8)

From Equation 6, the minimum A

The desired input impedance was 20k

Ω, and with an A

3, using Equation 8 results in an allocation of R

Ω and

R

Ω.

The final design step is to set the amplifier’s −3dB frequency

magnitude variation limit, the low frequency response must

20001050

FIGURE 2. Headphone Control Circuit

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