vice or the shutdown function may cause the “click and pop”

circuitry to not operate fully, resulting in increased “click and

pop” noise.

The value of C

tain size for C

excessive attenuation. But in many cases, the speakers

used in portable systems have little ability to reproduce sig-

nals below 100 Hz to 150 Hz. In this case, using a large input

and output coupling capacitor may not increase system per-

formance. In most cases, choosing a small value of C

range of 0.1 µF to 0.33 µF, along with C

should produce a virtually clickless and popless turn-on. In

cases where C

geous to increase the value of C

stood that increasing the value of C

and pops” at the expense of a longer device turn-on time.

AUDIO POWER AMPLIFIER DESIGN

Design a 250 mW/8

Ω Audio Amplifier

Given:

Power Output

250 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 530mV and can be

found from the graphs in the Typical Performance Charac-

teristics. V

(3)

For 250 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

LM4882 to reproduce peaks in excess of 300 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.

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 1.4, a value of 28 k

Ω is designated for R

5% tolerance resistors. This combination results in a nominal

gain of 1.4. The final design step is to address the bandwidth

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

quency 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

i

in conjunction with C

pass filters. Thus to obtain the desired frequency low re-

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 frequency of

20 Hz is used in the following equations to ensure that the re-

sponse 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 1.4 and f

ing GBWP = 140 kHz which is much smaller than the

LM4882 GBWP of 12.5Mhz. This figure displays that if a de-

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

the LM4882 can still be used without running into bandwidth

limitations.

9

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