POWER SUPPLY BYPASSING

As with any 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 LM4871 as possible. The capacitor

connected between the bypass pin and ground improves the

internal bias voltage’s stability, producing improved PSRR.

The improvements to PSRR increase as the bypass pin

capacitor increases. Typical applications employ a 5V regu-

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

supply stability. This does not eliminate the need for bypass-

ing the supply nodes of the LM4871 with a 1µF tantalum

capacitor. The selection of bypass capacitors, especially C

is dependent upon PSRR requirements, click and pop per-

formance as explained in the section, Proper Selection of

External Components, system cost, and size constraints.

SHUTDOWN FUNCTION

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

LM4871 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. This shutdown feature turns the

amplifier 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 V

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

disabled with shutdown pin voltages less then V

current may be greater than the typical value of 0.6µA. In

either case, the shutdown pin should be tied to a definite

voltage to avoid unwanted state changes.

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 so-

lution is to use a single-pole, single-throw switch in conjunc-

tion with an external pull-up resistor. When the switch is

closed, the shutdown pin is connected to ground and en-

ables the amplifier. If the switch is open, then the external

pull-up resistor will disable the LM4871. This scheme guar-

antees that the shutdown pin will not float thus preventing

unwanted state changes.

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 LM4871 is tolerant of

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4871 is unity-gain stable which gives a designer

maximum system flexibility. The LM4871 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.

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 com-

plete 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

frequency 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 attenu-

ation. But in many cases the speakers used in portable

systems, whether internal or external, have little ability to

reproduce signals below 100Hz to 150Hz. 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

capacitor 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 LM4871 turns

on. The slower the LM4871’s outputs ramp to their quiescent

DC voltage (nominally 1/2 V

Choosing C

the range of 0.1µF to 0.39µF), 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

1.0µF is recommended in all but the most cost sensitive

designs.

AUDIO POWER AMPLIFIER DESIGN

Design a 1W/8

Ω Audio Amplifier

Given:

Power Output

1 Wrms

Load Impedance

8

Ω

Input Level

1 Vrms

Input Impedance

20 k

Ω

Bandwidth

A designer must first determine the minimum supply rail to

obtain the specified output power. By extrapolating from the

Output Power vs Supply Voltage graphs in the Typical Per-

formance Characteristics section, the supply rail can be

easily found. A second way to determine the minimum sup-

ply rail is to calculate the required V

and add the output voltage. Using this method, the minimum

supply voltage would be (V

V

age vs Supply Voltage curve in the Typical Performance

Characteristics section.

(3)

Using the Output Power vs Supply Voltage graph for an 8

Ω

load, the minimum supply rail is 4.6V. But since 5V is a

standard voltage in most applications, it is chosen for the

supply rail. Extra supply voltage creates headroom that al-

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