LM4846TL datasheet(15/19 Pages) NSC

15 / 19 page

Application Information (Continued)

LM4846’s supply pins and ground. Keep the length of leads

and traces that connect capacitors between the LM4846’s

power supply pin and ground as short as possible. Connect-

ing a 2.2µF capacitor, C

B, between the BYPASS pin and

ground improves the internal bias voltage’s stability and

improves the amplifier’s PSRR. The PSRR improvements

increase as the bypass pin capacitor value increases. Too

large, however, increases turn-on time and can compromise

the amplifier’s click and pop performance. The selection of

bypass capacitor values, especially C

B, depends on desired

PSRR requirements, click and pop performance (as ex-

plained in the section, Proper Selection of External Compo-

nents), system cost, and size constraints.

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitor (C

i in Figures 1 & 2). A high value

capacitor can be expensive and may compromise space

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

speakers used in portable systems, whether internal or ex-

ternal, have little ability to reproduce signals below 150Hz.

Applications using speakers with this limited frequency re-

sponse reap little improvement by using large input capaci-

tor.

The internal input resistor (R

i), nominal 20k

Ω, and the input

capacitor (C

i) produce a high pass filter cutoff frequency that

is found using Equation (15).

c =1/(2

πR

iCi)

(15)

As an example when using a speaker with a low frequency

limit of 150Hz, C

i, using Equation (15) is 0.053µF. The

0.22µF C

i shown in Figure 1 allows the LM4846 to drive high

efficiency, full range speaker whose response extends below

40Hz.

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 bump. Since C

B determines how fast

the LM4846 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4846’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 C

i (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. C

B’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 LM4846 re-

sumes operation after shutdown.

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