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 effect of a larger half supply bypass capacitor

is improved PSRR due to increased half-supply stability.

Typical applications employ a 5V regulator 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

LM4900. The selection of bypass capacitors, especially C

is thus dependent upon desired PSRR requirements, click

and pop performance 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

LM4900 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 than V

current may be greater than the typical value of 0.1µA. 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 LM4900. 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 LM4900 is tolerant to a

variety of external component combinations, consideration

to component values must be used to maximize overall

system quality.

The LM4900 is unity-gain stable, giving a designer maximum

system flexibility. The LM4900 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 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

bandwidth 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 150Hz. In this case using a large

input capacitor may not increase system performance.

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

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

C

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 LM4900 turns

on. The slower the LM4900’s outputs ramp to their quiescent

Choosing C

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

and popless shutdown function. While the device will func-

tion properly, (no oscillations or motorboating), with C

to 0.1µF, the device will be much more susceptible to turn-on

clicks and pops. Thus, a value of C

is recommended in all but the most cost sensitive designs.

AUDIO POWER AMPLIFIER DESIGN

Design a 300 mW/8

Ω Audio Amplifier

Given:

Power Output

300mWrms

Load Impedance

8

Ω

Input Level

1Vrms

Input Impedance

20k

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 dropout voltage. Using this method, the mini-

mum supply voltage would be (V

V

Voltage curve in the Typical Performance Characteristics

section.

(4)

Using the Output Power vs Supply Voltage graph for an 8

Ω

load, the minimum supply rail is 3.5V. But since 5V is a

standard supply voltage in most applications, it is chosen for

the supply rail. Extra supply voltage creates headroom that

allows the LM4900 to reproduce peaks in excess of 700 mW

without producing audible distortion. At this time, the de-

signer must make sure that the power supply choice along

with the output impedance does not violate the conditions

explained in the Power Dissipation section.

Once the power dissipation equations have been addressed,

the required differential gain can be determined from Equa-

tion 5.

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