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

will be disabled with shutdown pin voltages less than 0.4

V

0.01 µA.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry which pro-

vides a quick, smooth transition into shutdown. Another solu-

tion is to use a single-pole, single-throw switch in conjunction

with an external pull-down resistor. When the switch is open,

the shutdown pin (1) is connected to ground through the pull-

down resistor (R

If the switch is closed, then V

pin and the LM4876 is enabled. This scheme guarantees

that the shutdown pin will not float thus preventing unwanted

state changes. If an Active Circuit is used to drive the shut-

down pin (1), then the pull-down resistor (R

be necessary.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers is critical to optimize device

and system performance. While the LM4876 is tolerant of

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4876 is unity-gain stable which gives a designer

maximum system flexibility. The LM4876 should be used in

low gain configurations to minimize THD+N values, and

maximize 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 dictated by the choice of external components

shown in

Figure 1. The input coupling capacitor, C

first order high pass filter which limits low frequency re-

sponse. This value should be chosen based on needed fre-

quency response for a few distinct reasons.

Selection Of Input Capacitor Size

Large input capacitors are both expensive and space hungry

for portable designs. Clearly, a certain sized capacitor is

needed to couple in low frequencies without severe attenua-

tion. But in many cases the speakers used in portable sys-

tems, whether internal or external, have little ability to repro-

duce signals below 100 Hz to 150 Hz. Thus, using a large

input capacitor may not increase actual 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 capacitor size, careful consid-

eration should be paid to the bypass capacitor value. Bypass

capacitor, C

turn-on pops since it determines how fast the LM4876 turns

on. The slower the LM4876’s outputs ramp to their quiescent

DC voltage (nominally 1/2 V

Choosing C

(in the range of 0.1 µF to 0.39 µF), should produce a virtually

clickless and popless shutdown function. While the device

will function properly, (no oscillations or motorboating), with

C

to turn-on clicks and pops. Thus, a value of C

1.0 µF is recommended in all but the most cost sensitive de-

signs.

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