LM4851 datasheet(17/22 Pages) NSC

17 / 22 page

Application Information (Continued)

power, higher ambient temperatures are allowed as output

power or duty cycle decreases. If the result of Equation (5) is

greater than that of Equation (6), then decrease the supply

voltage, increase the load impedance, or reduce the ambient

temperature. If these measures are insufficient, a heat sink

can be added to reduce

JA. The heat sink can be created

using additional copper area around the package, with con-

nections to the ground pin(s), supply pin and amplifier output

pins. External, solder attached SMT heatsinks such as the

Thermalloy 7106D can also improve power dissipation.

When adding a heat sink, the

JA is the sum of

JC,

CS, and

SA.(

JC is the junction-to-case thermal impedance,

CS is

the case-to-sink thermal impedance, and

SA is the sink-to-

ambient thermal impedance.) Refer to the Typical Perfor-

mance Characteristics curves for power dissipation informa-

tion at lower output power levels.

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

LM4851’s supply pins and ground. Keep the length of leads

and traces that connect capacitors between the LM4851’s

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

ing a 1µF 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 Figure 1). 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 internal input resistor (R

i) and the input capacitor (Ci)

produce a high pass filter cutoff frequency that is found using

Equation (9).

c =1/(2

πR

iCi)

(9)

As an example when using a speaker with a low frequency

limit of 150Hz, C

i, using Equation (9) is 0.063µF. The 0.22µF

i shown in Figure 1 allows the LM4851 to drive high effi-

ciency, 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 LM4851 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4851’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 LM4851 re-

sumes operation after shutdown.

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