Application Information

SHUTDOWN FUNCTION

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

LM4881 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. This shutdown feature turns the am-

plifier off when a logic high is placed on the shutdown pin.

The trigger point between a logic low and logic high level is

typically half supply. It is best to switch between ground and

supply to provide maximum device performance. By switch-

ing the shutdown pin to the V

draw will be minimized in idle mode. While the device will be

disabled with shutdown pin voltages less than V

current may be greater than the typical value of 0.7 µA. In ei-

ther case, the shutdown pin should be tied to a definite volt-

age because leaving the pin floating may result in an un-

wanted shutdown condition. In many applications, a

microcontroller or microprocessor output is used to control

the shutdown circuitry which provides a quick, smooth tran-

sition into shutdown. Another solution is to use a single-pole,

single-throw switch in conjunction with an external pull-up re-

sistor. When the switch is closed, the shutdown pin is con-

nected to ground and enables the amplifier. If the switch is

open, then the external pull-up resistor will disable the

LM4881. This scheme guarantees that the shutdown pin will

not float which will prevent unwanted state changes.

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.

P

(1)

Since the LM4881 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 LM4881 does

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

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

an 8

Ω load, the maximum power dissipation point is 158 mW

per amplifier. Thus the maximum package dissipation point

is 317 mW. The maximum power dissipation point obtained

must not be greater than the power dissipation that results

from Equation 2:

P

(2)

For package MUA08A,

θ

M08A,

θ

JA

= 170˚C/W, and for package N08E,

θ

JA

=

107˚C/W. T

ambient temperature, T

tion 2 can be used to find the maximum internal power dissi-

pation supported by the IC packaging. If the result of Equa-

tion 1 is greater than that of Equation 2, then either the

supply voltage must be decreased, the load impedance in-

creased or T

power supply, with an 8

Ω load, the maximum ambient tem-

perature possible without violating the maximum junction

temperature is approximately 96˚C provided that device op-

eration 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 dissipa-

tion point, the ambient temperature may be increased ac-

cordingly. Refer to the Typical Performance Characteris-

tics curves for power dissipation information for lower output

powers.

POWER SUPPLY BYPASSING

As with any power amplifer, proper supply bypassing is criti-

cal for low noise performance and high power supply rejec-

tion. The capacitor location on both the bypass and power

supply pins should be as close to the device as possible. As

displayed in the Typical Performance Characteristics sec-

tion, the effect of a larger half supply bypass capacitor is im-

proved low frequency PSRR due to increased half-supply

stability. Typical applications employ a 5V regulator with

10 µF and a 0.1 µF bypass capacitors which aid in supply

stability, but do not eliminate the need for bypassing the sup-

ply nodes of the LM4881. The selection of bypass capaci-

tors, especially C

quency PSRR, click and pop performance as explained in

the section, Proper Selection of External Components

section, system cost, and size constraints.

PROPER SELECTION OF EXTERNAL COMPONENTS

Selection of external components when using integrated

power amplifiers is critical to optimize device and system

performance. While the LM4881 is tolerant of external com-

ponent combinations, consideration to component values

must be used to maximize overall system quality.

The LM4881 is unity gain stable and this gives a designer

maximum system flexibility. The LM4881 should be used in

low gain configurations to minimize THD+N values, and

maximum the signal-to-noise ratio. Low gain configurations

require large input signals to obtain a given output power. In-

put signals equal to or greater than 1 Vrms are available

from sources such as audio codecs. Please refer to the sec-

tion, Audio Power Amplifier Design, for a more complete

explanation of proper gain selection.

Besides gain, one of the major considerations is the closed

loop bandwidth of the amplifier. To a large extent, the band-

width is dicated by the choice of external components shown

in

Figure 1. Both the input coupling capacitor, C

put coupling capacitor, C

which limit low frequency response. These values should be

chosen based on needed frequency response for a few dis-

tinct reasons.

Selection of Input and Output Capacitor Size

Large input and output capacitors are both expensive and

space hungry for portable designs. Clearly a certain sized

capacitor is needed to couple in low frequencies without se-

vere attenuation. But in many cases the speakers used in

portable systems, whether internal or external, have little

ability to reproduce signals below 150 Hz. Thus using large

input and output capacitors may not increase system perfor-

mance.

In addition to system cost and size, click and pop perfor-

mance is effected by the size of the input coupling capacitor,

C

reach its quiescent DC voltage (nominally 1/2 V

charge comes from the output via the feedback and is apt to

create pops upon device enable. Thus, by minimizing the ca-

pacitor size based on necessary low frequency response,

turn on pops can be minimized.

Besides minimizing the input and output capacitor sizes,

careful consideration should be paid to the bypass capacitor

value. Bypass capacitor C

minimize turn on pops since it determines how fast the

LM4881 turns on. The slower the LM4881’s outputs ramp to

their quiescent DC voltage (nominally 1/2 V

the turn on pop. Thus choosing C

a small value of C

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