LM4875 datasheet(30/37 Pages) NSC

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Application Information (Continued)

The maximum power dissipation point given by Equation (9)

must not exceed the power dissipation given by Equation

(10):

DMAX’= (TJMAX -TA)/

(10)

The LM4857’s T

JMAX = 150˚C. In the ITL package, the

LM4857’s

JA is 62˚C/W. At any given ambient temperature

A, use Equation (10) to find the maximum internal power

dissipation supported by the IC packaging. Rearranging

Equation (10) and substituting P

DMAX-TOTAL for PDMAX’ re-

sults in Equation (11). This equation gives the maximum

ambient temperature that still allows maximum stereo power

dissipation without violating the LM4857’s maximum junction

temperature.

A =TJMAX -PDMAX-TOTAL

(11)

For a typical application with a 5V power supply, stereo 8

Ω

loudspeaker load, and the stereo 32

Ω headphone load, the

maximum ambient temperature that allows maximum stereo

power dissipation without exceeding the maximum junction

temperature is approximately 66.4˚C for the ITL package.

JMAX =PDMAX-TOTAL

JA +TA

(12)

Equation (12) gives the maximum junction temperature T

J-

MAX

. If the result violates the LM4857’s 150˚C, reduce the

maximum junction temperature by reducing the power sup-

ply voltage or increasing the load resistance. Further allow-

ance should be made for increased ambient temperatures.

The above examples assume that a device is a surface

mount part operating around the maximum power dissipation

point. Since internal power dissipation is a function of output

power, higher ambient temperatures are allowed as output

power or duty cycle decreases. If the result of Equation (9) is

greater than that of Equation (10), 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

LM4857’s supply pins and ground. Keep the length of leads

and traces that connect capacitors between the LM4857’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)

(13)

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 LM4857

to drive high efficiency, full range speaker whose response

extends below 40Hz.

Output Capacitor Value Selection

Amplifying the lowest audio frequencies also requires the

use of a high value output coupling capacitor (C

O in Figure

1). A high value output capacitor can be expensive and may

compromise space efficiency in portable design.

The speaker load (R

L) and the output capacitor (CO) form a

high pass filter with a low cutoff frequency determined using

Equation (14).

c =1/(2

πR

LCO)

(14)

When using a typical headphone load of R

L =32

Ω with a low

frequency limit of 50Hz, C

O is 99µF.

The 100µF C

O shown in Figure 4 allows the LM4857 to drive

a headphone whose frequency response extends below

50Hz.

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 LM4857 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4857’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 LM4857 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

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