PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers is critical to optimize device

and system performance. While the LM4864 is tolerant to a

variety of external component combinations, consideration

to component values must be used to maximize overall sys-

tem quality.

The LM4864 is unity-gain stable, giving a designer maximum

system flexibility. The LM4864 should be used in low gain

configurations to minimize THD+N values, and maximize the

signal to noise ratio. Low gain configurations require large in-

put signals to obtain a given output power. Input signals

equal to or greater than 1 Vrms are 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.

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

first order high pass filter which limits low frequency re-

sponse. This value should be chosen based on needed fre-

quency response for a few distinct reasons.

Selection of Input Capacitor Size

Large input capacitors are both expensive and space hungry

for portable designs. Clearly, a certain sized capacitor is

needed to couple in low frequencies without severe attenua-

tion. But in many cases the speakers used in portable sys-

tems, whether internal or external, have little ability to repro-

duce signals below 150 Hz. In this case using a large input

capacitor may not increase system performance.

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

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

C

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 capacitor size, careful consid-

eration should be paid to the bypass capacitor value. Bypass

capacitor, C

turn-on pops since it determines how fast the LM4864 turns

on. The slower the LM4864’s outputs ramp to their quiescent

Choosing C

(in the range of 0.1 µF to 0.39 µF), should produce a click-

less and popless shutdown function. While the device will

function properly, (no oscillations or motorboating), with C

B

equal to 0.1 µF, the device will be much more susceptible to

turn-on clicks and pops. Thus, a value of C

or larger is recommended in all but the most cost sensitive

designs.

AUDIO POWER AMPLIFIER DESIGN

Design a 300 mW/8

Ω Audio Amplifier

Given:

Power Output

300 mWrms

Load Impedance

8

Ω

Input Level

1 Vrms

Input Impedance

20 k

Ω

Bandwidth

A designer must first determine the minimum supply rail to

obtain the specified output power. By extrapolating from the

Output Power vs Supply Voltage graphs in the Typical Per-

formance Characteristics section, the supply rail can be

easily found. A second way to determine the minimum sup-

ply rail is to calculate the required V

and add the dropout voltage. Using this method, the mini-

mum supply voltage would be (V

V

age curve in the Typical Performance Characteristics sec-

tion.

(4)

Using the Output Power vs Supply Voltage graph for an 8

Ω

load, the minimum supply rail is 3.5V. But since 5V is a stan-

dard supply voltage in most applications, it is chosen for the

supply rail. Extra supply voltage creates headroom that al-

lows the LM4864 to reproduce peaks in excess of 500 mW

without producing audible distortion. At this time, the de-

signer must make sure that the power supply choice along

with the output impedance 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 5.

(5)

R

(6)

From Equation 5, the minimum A

Since the desired input impedance was 20 k

Ω, and with a

A

R

bandwidth requirements which must be stated as a pair of

−3 dB frequency points. Five times away from a pole gives

0.17 dB down from passband response which is better than

f

f

As stated in the External Components section, R

junction with C

C

use 0.39 µF

The high frequency pole is determined by the product of the

desired high frequency pole, f

A

= 100 kHz which is much smaller than the LM4864 GBWP of

18 MHz. This figure displays that if a designer has a need to

design an amplifier with a higher differential gain, the

LM4864 can still be used without running into bandwidth

problems.

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