shutdown function should be virtually clickless and popless.

While the device will function properly, (no oscillations or mo-

torboating), with C

more susceptible to turn on clicks and pops. Thus, a value of

C

most cost sensitive designs.

AUDIO POWER AMPLIFIER DESIGN

Design a Dual 200mW/8

Ω Audio Amplifier

Given:

Power Output

200 mWrms

Load Impedance

8

Ω

Input Level

1 Vrms (max)

Input Impedance

20 k

Ω

Bandwidth

A designer must first determine the needed supply rail to ob-

tain the specified output power. Calculating the required sup-

ply rail involves knowing two parameters, V

the dropout voltage. The latter is typically 530 mV and can

be found from the graphs in the Typical Performance Char-

acteristics. V

(3)

For 200 mW of output power into an 8

Ω load, the required

V

from adding V

voltage in most applications, it is chosen for the supply rail.

Extra supply voltage creates headroom that allows the

LM4881 to reproduce peaks in excess of 200 mW without

clipping the signal. At this time, the designer must make sure

that the power supply choice along with the output imped-

ance does not violate the conditions explained in the Power

Dissipation section. Remember that the maximum power

dissipation point from Equation 1 must be multiplied by two

since there are two independent amplifiers inside the pack-

age.

Once the power dissipation equations have been addressed,

the required gain can be determined from Equation 4.

(4)

A

(5)

From Equation 4, the minimum gain is: A

Since the desired input impedance was 20 k

Ω, and with a

gain of 1.26, a value of 27 k

Ω is designated for R

5% tolerance resistors. This combination results in a nominal

gain of 1.35. The final design step is to address the band-

width requirements which must be stated as a pair of −3 dB

frequency points. Five times away from a −3 dB point is

0.17 dB down from passband response assuming a single

pole roll-off. As stated in the External Components section,

both R

der highpass filters. Thus to obtain the desired frequency low

taken into consideration. The combination of two single order

filters at the same frequency forms a second order response.

This results in a signal which is down 0.34 dB at five times

away from the single order filter −3 dB point. Thus, a fre-

quency of 20 Hz is used in the following equations to ensure

that the response is better than 0.5 dB down at 100 Hz.

C

C

The high frequency pole is determined by the product of the

desired high frequency pole, f

V

. With a closed-loop gain of 1.35 and f

sulting GBWP = 135 kHz which is much smaller than the

LM4881 GBWP of 18 MHz. This figure displays that if a de-

signer has a need to design an amplifier with a higher gain,

the LM4881 can still be used without running into bandwidth

limitations.

www.national.com

9