LM4908MM datasheet(13/21 Pages) NSC

13 / 21 page

Application Information

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATION

The LM4908’s exposed-dap (die attach paddle) package

(LQ) provides a low thermal resistance between the die and

the PCB to which the part is mounted and soldered. This

allows rapid heat transfer from the die to the surrounding

PCB copper traces, ground plane, and surrounding air.

The LQ package should have its DAP soldered to a copper

pad on the PCB. The DAP’s PCB copper pad may be con-

nected to a large plane of continuous unbroken copper. This

plane forms a thermal mass, heat sink, and radiation area.

However, since the LM4908 is designed for headphone ap-

plications, connecting a copper plane to the DAP’s PCB

copper pad is not required. The LM4908’s Power Dissipation

vs Output Power Curve in the Typical Performance Char-

acteristics shows that the maximum power dissipated is just

45mW per amplifier with a 5V power supply and a 32

Ω load.

Further detailed and specific information concerning PCB

layout, fabrication, and mounting an LQ (LLP) package is

available from National Semiconductor’s Package Engineer-

ing Group under application note AN1187.

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a

successful design. Equation 1 states the maximum power

dissipation point for a single-ended amplifier operating at a

given supply voltage and driving a specified output load.

DMAX =(VDD)

2 /(2

π2R

(1)

Since the LM4908 has two operational amplifiers in one

package, the maximum internal power dissipation point is

twice that of the number which results from Equation 1. Even

with the large internal power dissipation, the LM4908 does

not require heat sinking over a large range of ambient tem-

perature. From Equation 1, assuming a 5V power supply and

a32

Ω load, the maximum power dissipation point is 40mW

per amplifier. Thus the maximum package dissipation point

is 80mW. The maximum power dissipation point obtained

must not be greater than the power dissipation that results

from Equation 2:

DMAX =(TJMAX −TA)/

(2)

For package MUA08A,

JA = 210˚C/W. TJMAX = 150˚C for

the LM4908. Depending on the ambient temperature, T

A,of

the system surroundings, Equation 2 can be used to find the

maximum internal power dissipation supported by the IC

packaging. If the result of Equation 1 is greater than that of

Equation 2, then either the supply voltage must be de-

creased, the load impedance increased or T

A reduced. For

the typical application of a 5V power supply, with a 32

Ω load,

the maximum ambient temperature possible without violating

the maximum junction temperature is approximately 133.2˚C

provided that device operation is around the maximum

power dissipation point. Power dissipation is a function of

output power and thus, if typical operation is not around the

maximum power dissipation point, the ambient temperature

may be increased accordingly. Refer to the Typical Perfor-

mance Characteristics curves for power dissipation infor-

mation for lower output powers.

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 0.1µF

supply bypass capacitor, C

connected between the

LM4908’s supply pins and ground. Keep the length of leads

and traces that connect capacitors between the LM4908’s

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

ing a 1.0µF capacitor, C

B, between the IN A(+) / IN B(+) node

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 the amplifier’s turn-on time. The

selection of bypass capacitor values, especially C

B, depends

on desired PSRR requirements, click and pop performance

(as explained in the section, Selecting Proper External

Components), system cost, and size constraints.

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4908’s performance requires properly se-

lecting external components. Though the LM4908 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

The LM4908 is unity-gain stable, giving a designer maximum

design flexibility. The gain should be set to no more than a

given application requires. This allows the amplifier to

achieve minimum THD+N and maximum signal-to-noise ra-

tio. These parameters are compromised as the closed-loop

gain increases. However, low gain demands input signals

with greater voltage swings to achieve maximum output

power. Fortunately, many signal sources such as audio

CODECs have outputs of 1V

RMS (2.83VP-P). Please refer to

the Audio Power Amplifier Design section for more infor-

mation on selecting the proper gain.

Input and Output Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input and output coupling capacitors (C

I and CO in Figure 1).

A high value capacitor can be expensive and may compro-

mise 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 high

value input and output capacitors.

Besides affecting system cost and size, C

i has an effect on

the LM4908’s click and pop performance. The magnitude of

the pop is directly proportional to the input capacitor’s size.

Thus, pops can be minimized by selecting an input capacitor

value that is no higher than necessary to meet the desired

−3dB frequency.

As shown in Figure 1, the input resistor, R

I and the input

capacitor, C

I, produce a −3dB high pass filter cutoff fre-

quency that is found using Equation (3). In addition, the

output load R

L, and the output capacitor CO, produce a -3db

high pass filter cutoff frequency defined by Equation (4).

I-3db=1/2

πR

ICI

(3)

O-3db=1/2

πR

LCO

(4)

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