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

tion is to use a single-throw switch in conjunction with an

external pull-up resistor. This scheme guarantees 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 when optimizing

device and system performance. Although the LM4923 is

tolerant to a variety of external component combinations,

consideration of component values must be made when

maximizing overall system quality.

The LM4923 is unity-gain stable, giving the designer maxi-

mum system flexibility. The LM4923 should be used in low

closed-loop gain configurations to minimize THD+N values

and maximize signal to noise ratio. Low gain configurations

require large input signals to obtain a given output power.

Input signals equal to or greater than 1Vrms are available

from sources such as audio codecs. Please refer to the

Audio Power Amplifier Design section for a more complete

explanation of proper gain selection. When used in its typical

application as a fully differential power amplifier the LM4923

does not require input coupling capacitors for input sources

with DC common-mode voltages of less than V

allowable input common-mode voltage levels are actually a

function of V

Equation 5:

V

(5)

-R

(6)

Special care must be taken to match the values of the

feedback resistors (R

matching the input resistors (R

Figure 1) more in front. Because of the balanced nature of

differential amplifiers, resistor matching differences can re-

sult in net DC currents across the load. This DC current can

increase power consumption, internal IC power dissipation,

reduce PSRR, and possibly damaging the loudspeaker. The

chart below demonstrates this problem by showing the ef-

fects of differing values between the feedback resistors while

assuming that the input resistors are perfectly matched. The

results below apply to the application circuit shown in Figure

1, and assumes that V

Ω, and the system has

DC coupled inputs tied to ground.

Tolerance R

F1

R

F2

V

I

LOAD

20%

0.8R

1.2R

-0.500V

62.5mA

10%

0.9R

1.1R

-0.250V

31.25mA

5%

0.95R 1.05R -0.125V

15.63mA

1%

0.99R 1.01R -0.025V

3.125mA

0%

RR0

0

Similar results would occur if the input resistors were not

carefully matched. Adding input coupling capacitors in be-

tween the signal source and the input resistors will eliminate

this problem, however, to achieve best performance with

minimum component count it is highly recommended that

both the feedback and input resistors matched to 1% toler-

ance or better.

AUDIO POWER AMPLIFIER DESIGN

Design a 1W/8

Ω Audio Amplifier

Given:

Power Output

1Wrms

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. The supply rail can easily

be found by extrapolating from the Output Power vs Supply

Voltage graphs in the Typical Performance Characteris-

tics section. A second way to determine the minimum supply

rail is to calculate the required V

add the dropout voltages. Using this method, the minimum

supply voltage is (Vopeak + (V

V

DO BOT

and V

Voltage vs Supply Voltage curve in the Typical Perfor-

mance Characteristics section.

(7)

Using the Output Power vs Supply Voltage graph for an 8

Ω

load, the minimum supply rail just about 5V. Extra supply

voltage creates headroom that allows the LM4923 to repro-

duce peaks in excess of 1W without producing audible dis-

tortion. At this time, the designer must make sure that the

power supply choice along with the output impedance does

not violate the conditions explained in the Power Dissipa-

tion section. Once the power dissipation equations have

been addressed, the required differential gain can be deter-

mined from Equation 8.

(8)

R

From Equation 7, the minimum A

sired input impedance was 20k

Ω, a ratio of 2.83:1 of R

results in an allocation of R

Ω for both input resistors

and R

Ω for both feedback resistors. The final design

step is to address the bandwidth requirement which must be

stated as a single -3dB frequency point. Five times away

from a -3dB point is 0.17dB down from passband response

f

The high frequency pole is determined by the product of the

desired frequency pole, f

With a A

150kHz which is much smaller than the LM4923 GBWP of

10MHz. This figure displays that if a designer has a need to

design an amplifier with a higher differential gain, the

LM4923 can still be used without running into bandwidth

limitations.

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