LM4854 datasheet(22/29 Pages) NSC

22 / 29 page

Application Information (Continued)

Figure 4 shows an optional resistor connected between the

amplifier output that drives the headphone jack sleeve and

ground. This resistor provides a ground path that supressed

power supply hum. This hum may occur in applications such

as notebook computers in a shutdown condition and con-

nected to an external powered speaker. The resistor’s 100

Ω

value is a suggested starting point. Its final value must be

determined based on the tradeoff between the amount of

noise suppression that may be needed and minimizing the

additional current drawn by the resistor (25mA for a 100

Ω

resistor and a 5V supply).

Single-Ended Output Power Performance and Measure-

ment Considerations

The LM4854 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 10

Ω,

the LM4854 can typically supply 180mW of low distortion

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 moderate to

excessively high distortion is seen, simply reducing the out-

put 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, AMP2 (refer to Figure 2) 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 AMP2 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.

ESD Protection

As stated in the Absolute Maximum Ratings, the AMP2

output pin has a maximum ESD susceptibility rating of

8000V. For higher ESD voltages, the addition of a PCDN042

dual transil (from California Micro Devices), as shown in

Figure 4, will provide additional protection.

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitor (Ci in Figure 2). 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.

The LM4854’s advanced output transient suppression cir-

cuitry has eliminated the need to select the input capacitor’s

value in relation to the BYPASS capacitor’s value as was

necessary in some previous Boomer amplifiers. The value of

CI is now strictly determined by the desired low frequency

response.

As shown in Figure 2, the input resistor (R

i) and the input

capacitor (C

i) produce a high pass filter cutoff frequency that

is found using Equation (7).

c =1/(2

πR

iCi)

(7)

As an example when using a speaker with a low frequency

limit of 150Hz, C

i, using Equation (7) is 0.063µF. The 1.0µF

i shown in Figure 2 allows the LM4854 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 CB, the capacitor con-

nected to the BYPASS pin. Since C

B determines how fast

the LM4854 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4854’s

outputs ramp to their quiescent DC voltage (nominally V

DD/

2), the smaller the turn-on pop. Choosing C

B equal to 1.0µF

along with a small value of Ci (in the range of 0.1µF to

0.39µF), produces a click-less and pop-less shutdown func-

tion. As discussed above, choosing C

i no larger than neces-

sary for the desired bandwidth helps minimize clicks and

pops. CB’s value should be in the range of 5 times to 7 times

the value of C

i. This ensures that output transients are

eliminated when power is first applied or the LM4854 re-

sumes operation after shutdown.

20038212

FIGURE 3. Headphone Circuit

200382E8

FIGURE 4. The PCDN042 provides additional ESD

protection beyond the 8000V shown in the Absolute

Maximum Ratings for the AMP2 output

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