LM4858 datasheet(13/17 Pages) NSC

13 / 17 page

Application Information (Continued)

The LM4858’s unity-gain stability allows a designer to maxi-

mize system performance. The LM4858’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

RMS 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.

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 band-

width is dictated by the choice of external components

shown in Figure 1. The input coupling capacitor C

I and

resistor R

I form a first order high pass filter that limits low

frequency response. C

I’s value should be based on the

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:

RMS

•

Load Impedance

Ω

•

Input Level:

RMS

•

Input Impedance:

20k

Ω

•

Bandwidth:

100Hz - 20kHz ± 0.25dB

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

ODTOP and VODBOT are extrapolated

from the Dropout Voltage vs Supply Voltage curve in the

Typical Performance Characteristics section.

(5)

≥ (V

OPEAK +(VODTOP +VODBOT))

(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 LM4858 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)

F /RI =AVD / 2

(8)

From Equation 6, the minimum A

VD is 2.83; use AVD =3.

The desired input impedance was 20k

Ω, and with an A

VD of

3, using Equation 8 results in an allocation of R

I = 20k

Ω and

F = 30k

Ω.

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

bandwidth. To achieve the desired ± 0.25dB pass band

magnitude variation limit, the low frequency response must

extend to at least one−fifth the lower bandwidth limit and the

high frequency response must extend o at least five times

the upper bandwidth limit. The variation for both response

limits is 0.17dB, well within the ± 0.25dB desired limit. This

results in:

L = 100Hz/5= 20Hz

H = 20kHzx5= 100kHz

As stated in the External Components section, R

I in con-

junction with C

I create a highpass filter. Find the coupling

capacitor’s value using Equation 9.

≥ 1/(2πR

IfL)

(9)

≥ 1/(2π x 20kΩ x 20Hz) = 0.397µF

Use a 0.39µF capacitor, the closest standard value.

The high frequency pole is determined by the product of the

desired high frequency pole, f

H, and the differential gain,

VD. With AVD = 3 and fH = 100kHz, the resulting GBWP =

150kHz which is much smaller than the LM4858 GBWP of

10MHz. This difference indicates that a designer can still use

the LM4858 at higher differential gains without bandwidth

limitations.

PCB LAYOUT AND SUPPLY REGULATION

CONSIDERATIONS FOR DRIVING 3

Ω AND 4Ω LOADS

Power dissipated by a load is a function of the voltage swing

across the load and the load’s impedance. As load imped-

ance decreases, load dissipation becomes increasingly de-

pendant on the interconnect (PCB trace and wire) resistance

between the amplifier output pins and the load’s connec-

tions. Residual trace resistance causes a voltage drop,

which results in power dissipated in the trace and not in the

load as desired. For example, 0.1

Ω trace resistance reduces

the output power dissipated by a 4

Ω load from 2.0W to

1.95W. This problem of decreased load dissipation is exac-

erbated as load impedance decreases. Therefore, to main-

tain the highest load dissipation and widest output voltage

swing, PCB traces that connect the output pins to a load

must be as wide as possible.

Poor power supply regulation adversely affects maximum

output power. A poorly regulated supply’s output voltage

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