LM4868 datasheet(17/28 Pages) NSC

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Application Information (Continued)

Single-Ended Output Power Performance and

Measurement Considerations

The LM4868 delivers clean, low distortion SE output power

into loads that are greater than 10

Ω. As an example, output

power for 16

Ω and 32Ω loads are shown in the Typical

Performance Characteristic curves. For loads less than

Ω, the LM4876 can typically supply 180mW of low distor-

tion power. However, when higher dissipation is desired in

loads less than 10

Ω, a dramatic increase in THD+N may

occur. This is normal operation and does not indicate that

proper functionality has ceased. When a jump from moder-

ate to excessively high distortion is seen, simply reducing

the output voltage swing will restore the clean, low distortion

SE operation.

The dramatic jump in distortion for loads less than 10

Ω

occurs when current limiting circuitry activates. During SE

operation, AMP2A (refer to Figure 4) drives the headphone

sleeve. An on-board circuit monitors this amplifier’s output

current. The sudden increase in THD+N is caused by the

current limit circuitry forcing AMP2A into a high−impedance

output mode. When this occurs, the output waveform has

discontinuities that produce large amounts of distortion. It

has been observed that as the output power is steadily

increased, the distortion may jump from 5% to greater than

35%. Indeed, 10% THD+N may not actually be achievable.

Using the Single−Ended Output for Line Level

Applications

Some samples of the LM4868 may exhibit small amplitude,

high frequency oscillation when the SE output is connected

to a line-level input. This oscillation can be eliminated by

connecting a 5%, 300

Ω resistor between Amp2A’s output pin

and each amplifier, AMP1A and AMP1B, output.

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitor (C

i in Figure 4). A high value capaci-

tor can be expensive and may compromise space efficiency

in portable designs. In many cases, however, the speakers

used in portable systems, whether internal or external, have

little ability to reproduce signals below 150Hz. Applications

using speakers with this limited frequency response reap

little improvement by using large input capacitor.

Besides effecting system cost and size, C

i has an affect on

the LM4868’s click and pop performance. When the supply

voltage is first applied, a transient (pop) is created as the

charge on the input capacitor changes from zero to a quies-

cent state. The magnitude of the pop is directly proportional

to the input capacitor’s size. Higher value capacitors need

more time to reach a quiescent DC voltage (usually V

DD/2)

when charged with a fixed current. The amplifier’s output

charges the input capacitor through the feedback resistor,

f. Thus, pops can be minimized by selecting an input

capacitor value that is no higher than necessary to meet the

desired −3dB frequency and is between 0.14C

B and 0.20CB.

A shown in Figure 4, the input resistor (R

I) and the input

capacitor, C

I produce a −3dB high pass filter cutoff frequency

that is found using Equation (7).

−3dB = 1/(2

πR

INCI)

(7)

As an example when using a speaker with a low frequency

limit of 150Hz, C

i, using Equation (4) is 0.063µF. The 1.0µF

i shown in Figure 4 allows the LM4868 to drive high effi-

ciency, full range speaker whose response extends below

30Hz.

Bypass Capacitor Value Selection

Besides minimizing the input capacitor size, careful consid-

eration should be paid to value of C

B, the capacitor con-

nected to the BYPASS pin. Since C

B determines how fast

the LM4868 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4868’s

outputs ramp to their quiescent DC voltage (nominally 1/2

200267A1

FIGURE 7. The PCDN042 provides additional ESD

protection beyond the 8000V shown in the Absolute

Maximum Ratings for the AMP2A output

20026724

FIGURE 8. Headphone Circuit

(Pin numbers in ( ) are for the 20-pin MTE and MT

packages.)

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