LM4811 datasheet(12/18 Pages) NSC

12 / 18 page

Application Information (Continued)

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 LM4811 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 LM4811 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 predicted by

Equation 2:

DMAX =(TJMAX −TA)/

(2)

For the MSOP package,

JA = 194˚C/W, and for the LD

package,

JA = 63˚C/W. TJMAX = 150˚C for the LM4811. For

a given ambient temperature, T

A, of the system surround-

ings, 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 decreased, the load im-

pedance increased, or T

A reduced. For the MSOP package

in a typical application of a 5V power supply and a 32

Ω load,

the maximum ambient temperature possible without violating

the

maximum

junction

temperature

approximately

134.5˚C. This assumes the device operates at maximum

power dissipation and uses surface mount packaging. Inter-

nal power dissipation is a function of output power. If typical

operation is not around the maximum power dissipation

point, operation at higher ambient temperatures is possible.

Refer to the Typical Performance Characteristics curves

for power dissipation information for lower output power

levels.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATION

The LM4811’s exposed-dap (die attach paddle) package

(LD) 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 LD 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 LM4811 is designed for headphone ap-

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

copper pad is not required. The LM4811’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 LD (LLP) package is

available from National Semiconductor’s Package Engineer-

ing Group under application note AN1187.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection. The capacitor location on both the bypass and

power supply pins should be as close to the device as

possible. The value of the bypass capacitor directly affects

the LM4811’s half-supply voltage stability and PSRR. The

stability and supply rejection increase as the bypass capaci-

tor’s value increases. Typical applications employ a 5V regu-

lator with 10µF and a 0.1µF bypass capacitors which aid in

supply stability, but do not eliminate the need for bypassing

the supply nodes of the LM4811. The selection of bypass

capacitors, especially C

B, is thus dependent upon desired

low frequency PSRR, click and pop performance, (explained

in the section, Proper Selection of External Components),

system cost, and size constraints.

SHUTDOWN FUNCTION

In order to reduce power consumption while not is use, the

LM4811 features amplifier bias circuitry shutdown. This shut-

down function is activated by applying a logic high to the

SHUTDOWN pin. The trigger point is 1.4V minimum for a

logic high level, and 0.4V maximum for a logic low level. It is

best to switch between ground and V

DD to ensure optimal

shutdown operation. By switching the SHUTDOWN pin to

DD, the LM4811 supply current draw will be minimized in

idle mode. Whereas the device will be disabled with shut-

down voltages less than V

DD, the idle current may be greater

than the typical value of 0.3µA. In either case, the SHUT-

DOWN pin should be tied to a fixed voltage to avoid un-

wanted state changes.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry. This provides

a quick, smooth shutdown transition. Another solution is to

use a single-pole, single-throw switch in conjunction with an

external pull-up resistor. When the switch is closed, the

SHUTDOWN pin is connected to ground and enables the

amplifier. If the switch is open, the external pull-up resistor,

PU, will disable the LM4811. This scheme guarantees that

the SHUTDOWN pin will not float, thus preventing unwanted

state changes.

PROPER SELECTION OF EXTERNAL COMPONENTS

Selection of external components when using integrated

power amplifiers is critical for optimum device and system

performance. While the LM4811 is tolerant of external com-

ponent combinations, consideration must be given to the

external component values that maximize overall system

quality.

The LM4811’s unity-gain stability allows a designer to maxi-

mize system performance. Low gain settings maximize

signal-to-noise performance and minimizes THD+N. Low

gain configurations require large input signals to obtain a

given output power. Input signals equal to or greater than 1

Vrms are available from sources such as audio codecs.

Please refer to the section, Audio Power Amplifier Design,

for a more complete explanation of proper gain selection.

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