Application Information

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a

successful design. Equation 1 states the maximum power

dissipation point for a single-ended amplifier operating at a

given supply voltage and driving a specified output load.

P

(1)

Since the LM4808 has two operational amplifiers in one

package, the maximum internal power dissipation point is

twice that of the number which results from Equation 1. Even

with the large internal power dissipation, the LM4808 does

not require heat sinking over a large range of ambient tem-

perature. From Equation 1, assuming a 5V power supply and

a32

Ω load, the maximum power dissipation point is 40 mW

per amplifier. Thus the maximum package dissipation point

is 80 mW. The maximum power dissipation point obtained

must not be greater than the power dissipation that results

from Equation 2:

P

(2)

For package MUA08A,

θ

M08A,

θ

pending on the ambient temperature, T

roundings, Equation 2 can be used to find the maximum in-

ternal power dissipation supported by the IC packaging. If

the result of Equation 1 is greater than that of Equation 2,

then either the supply voltage must be decreased, the load

impedance increased or T

tion of a 5V power supply, with a 32

Ω load, the maximum

ambient temperature possible without violating the maximum

junction temperature is approximately 131.6˚C provided that

device operation is around the maximum power dissipation

point. Power dissipation is a function of output power and

thus, if typical operation is not around the maximum power

dissipation point, the ambient temperature may be increased

accordingly. Refer to the Typical Performance Character-

istics curves for power dissipation information for lower out-

put powers.

POWER SUPPLY BYPASSING

As with any power amplifer, proper supply bypassing is criti-

cal for low noise performance and high power supply rejec-

tion. The capacitor location on both the bypass and power

supply pins should be as close to the device as possible. As

displayed in the Typical Performance Characteristics sec-

tion, the effect of a larger half supply bypass capacitor is im-

proved low frequency PSRR due to increased half-supply

stability. Typical applications employ a 5V regulator with

10 µF and a 0.1 µF bypass capacitors which aid in supply

stability, but do not eliminate the need for bypassing the sup-

ply nodes of the LM4808. The selection of bypass capaci-

tors, especially C

quency PSRR, click and pop performance as explained in

the section, Proper Selection of External Components

section, system cost, and size constraints.

PROPER SELECTION OF EXTERNAL COMPONENTS

Selection of external components when using integrated

power amplifiers is critical to optimize device and system

performance. While the LM4808 is tolerant of external com-

ponent combinations, consideration to component values

must be used to maximize overall system quality.

The LM4808 is unity gain stable and this gives a designer

maximum system flexibility. The LM4808 should be used in

low gain configurations to minimize THD+N values, and

maximize the signal-to-noise ratio. Low gain configurations

require large input signals to obtain a given output power. In-

put signals equal to or greater than 1 Vrms are available

from sources such as audio codecs. Please refer to the sec-

tion, Audio Power Amplifier Design, for a more complete

explanation of proper gain selection.

Besides gain, one of the major considerations is the closed

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

width is dicated by the choice of external components shown

in

Figure 1. Both the input coupling capacitor, C

put coupling capacitor, C

which limit low frequency response. These values should be

chosen based on needed frequency response for a few dis-

tinct reasons.

Selection of Input and Output Capacitor Size

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 150 Hz. Thus using

large input and output capacitors may not increase system

performance.

In addition to system cost and size, click and pop perfor-

mance is affected by the size of the input coupling capacitor,

C

reach its quiescent DC voltage (nominally 1/2 V

charge comes from the output via the feedback and is apt to

create pops upon device enable. Thus, by minimizing the ca-

pacitor size based on necessary low frequency response,

turn on pops can be minimized.

Besides minimizing the input and output capacitor sizes,

careful consideration should be paid to the bypass capacitor

value. Bypass capacitor C

minimize turn on pops since it determines how fast the

LM4808 turns on. The slower the LM4808’s outputs ramp to

their quiescent DC voltage (nominally 1/2 V

the turn on pop. While the device will function properly, (no

oscillations or motorboating), with C

vice will be much more susceptible to turn on clicks and

pops. Thus, a value of C

mended in all but the most cost sensitive designs.

AUDIO POWER AMPLIFIER DESIGN

Design a Dual 70mW/32

Ω Audio Amplifier

Given:

Power Output

70 mW

Load Impedance

32

Ω

Input Level

1 Vrms (max)

Input Impedance

20 k

Ω

Bandwidth

A designer must first determine the needed supply rail to ob-

tain the specified output power. Calculating the required sup-

ply rail involves knowing two parameters, V

the dropout voltage. The latter is typically 300mV and can be

found from the graphs in the Typical Performance Charac-

teristics. V

(3)

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