mance can be compromised. The selection of bypass ca-

pacitor values, especially C

requirements, click and pop performance (as explained in

the section, Proper Selection of External Components),

system cost, and size constraints.

MICRO-POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4876’s shutdown function. Activate micro-power shut-

down by applying a voltage below 400mV to the SHUT-

DOWN pin. When active, the LM4876’s micro-power shut-

down feature turns off the amplifier’s bias circuitry, reducing

the supply current. Though the LM4876 is in shutdown when

400mV is applied to the SHUTDOWN pin, the supply current

may be higher than 0.01µA (typ) shutdown current. There-

fore, for the lowest supply current during shutdown, connect

the SHUTDOWN pin to ground. The relationship between

the supply voltage, the shutdown current, and the voltage

applied to the SHUTDOWN pin is shown in Typical Perfor-

mance Characteristics curves.

There are a few ways to control the micro-power shutdown.

These include using a single-pole, single-throw switch, a

microprocessor, or a microcontroller. When using a switch,

connect an external pull-down resistor between the SHUT-

DOWN pin and GND. Connect the switch between the

SHUTDOWN pin and V

by closing the switch. Opening the switch connects the

SHUTDOWN pin to GND through the pull-down resistor,

activating micro-power shutdown. The switch and resistor

guarantee that the SHUTDOWN pin will not float. This pre-

vents unwanted state changes. In a system with a micropro-

cessor or a microcontroller, use a digital output to apply the

control voltage to the SHUTDOWN pin. Driving the SHUT-

DOWN pin with active circuitry eliminates the pull down

resistor.

SELECTING POWER EXTERNAL COMPONENTS

Optimizing the LM4876’s performance requires properly se-

lecting external components. Though the LM4876 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

The LM4876 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 CO-

DECs have outputs of 1V

Audio Power Amplifier Design section for more informa-

tion on selecting the proper gain.

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitor (C

tor can be expensive and may compromise 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 150 Hz. Applications

using speakers with this limited low frequency response reap

little improvement by using a large input capacitor.

Besides affecting system cost and size, C

LM4876’s click and pop performance. When the supply volt-

age is first applied, a transient (pop) is created as the charge

on the input capacitor changes from zero to a quiescent

state. The magnitude of the pop is directly proportional to the

input capacitor’s size. Higher value capacitors need more

time to reach a quiescent DC voltage (usually V

charged with a fixed current. The amplifier’s output charges

the input capacitor through the feedback resistor, R

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

capacitor, C

that is found using Equation (7).

f

πR

(7)

As an example when using a speaker with a low frequency

limit of 150Hz, Equation (7) gives a value of C

0.1µF. The 0.22µF C

whose response extends down to 75Hz.

Bypass Capacitor Value Selection

Besides minimizing the input capacitor size, careful consid-

eration should be paid to value of, C

nected to the BYPASS pin. Since C

the LM4876 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4876’s

outputs ramp to their quiescent DC voltage (nominally 1/2

V

1.0µF along with a small value of C

0.39µF), produces a click-less and pop-less shutdown func-

tion. As discussed above, choosing C

helps minimize clicks and pops.

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