LM4844 datasheet(17/21 Pages) NSC

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Application Information (Continued)

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection. Applications that employ a 5V regulator typically

use a 10µF in parallel with a 0.1µF filter capacitors to stabi-

lize the regulator’s output, reduce noise on the supply line,

and improve the supply’s transient response. However, their

presence does not eliminate the need for a local 1.0µF

tantalum bypass capacitance connected between the

LM4844’s supply pins and ground. Keep the length of leads

and traces that connect capacitors between the LM4844’s

power supply pin and ground as short as possible.

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires a high

value input coupling capacitor (C

i in Figure 1). In many

cases, however, the speakers used in portable systems,

whether internal or external, have little ability to reproduce

signals below 50Hz. Applications using speakers with this

limited frequency response reap little improvement; by using

a large input capacitor.

The internal input resistor (R

i) and the input capacitor (Ci)

produce a high pass filter cutoff frequency that is found using

Equation (13).

c =1/(2

πR

iCi)(11)

As an example when using a speaker with a low frequency

limit of 50Hz and R

i = 20k

Ω,C

i, using Equation (13) is

0.19µF. The 0.22µF C

i shown in Figure 4 allows the LM4844

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 pin. Since C

B determines how fast

the LM4844 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4844’s

outputs ramp to their quiescent DC voltage (nominally V

DD/

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

B equal to 2.2µF

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 10

times the value of C

i. This ensures that output transients are

eliminated when the LM4844 transitions in and out of shut-

down mode. Connecting 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 im-

provements increase as the bypass pin capacitor value in-

creases. However, increasing the value of C

B will increase

wake-up time. The selection of bypass capacitor value, C

depends on desired PSRR requirements, click and pop per-

formance, wake-up time, system cost, and size constraints.

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