filter that limits low frequency response. This value should be

chosen based on needed frequency response and turn-on

time.

SELECTION OF INPUT CAPACITOR SIZE

Amplifying the lowest audio frequencies requires a high

value input coupling capacitor, Ci. A high value capacitor can

be expensive and may compromise space efficiency in por-

table designs. In many cases, however, the headphones

used in portable systems have little ability to reproduce

signals below 60Hz. Applications using headphones with this

limited frequency response reap little improvement by using

a high value input capacitor. In addition to system cost and

size, turn on time is affected by the size of the input coupling

capacitor Ci. A larger input coupling capacitor requires more

charge to reach its quiescent DC voltage. This charge

comes from the output via the feedback. Thus, by minimizing

the capacitor size based on necessary low frequency re-

sponse, turn-on time can be minimized. A small value of Ci

(in the range of 0.1µF to 0.47µF), is recommended.

AUDIO POWER AMPLIFIER DESIGN

A 25mW/32

Ω Audio Amplifier

Given:

Power Output

10mWrms

Load Impedance

16

Ω

Input Level

0.4Vrms

Input Impedance

20k

Ω

A designer must first choose a mode of operation (SE or

BTL) and determine the minimum supply rail to obtain the

specified output power. By extrapolating from the Output

Power vs. Supply Voltage graphs in the Typical Performance

Characteristics section, the supply rail can be easily found.

3.0V is a standard voltage in most applications, it is chosen

for the supply rail. Extra supply voltage creates headroom

that allows the LM4925to reproduce peak in excess of

10mW without producing audible distortion. 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 Dissipation section. Once the power

dissipation equations have been addressed, the required

gain can be determined from Equation 2.

(4)

From Equation 4, the minimum AV is 1; use A

desired input impedance is 20k, and with a A

ratio of 1:1 results from Equation 1 for R

are chosen with R

is to address the bandwidth requirements which must be

stated as a pair of -3dB frequency points. Five times away

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

f

f

As stated in the External Components section, R

junction with C

C

i

≥ 1/(2π * 20kΩ * 20Hz) = 0.397µF; use 0.39µF.

The high frequency pole is determined by the product of the

desired frequency pole, fH, and the differential gain, A

an AV

which is much smaller than the LM4925GBWP of 3MHz.

This example displays that if a designer has a need to design

an amplifier with higher differential gain, the LM4925can still

be used without running into bandwidth limitations.

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