LM4924

www.ti.com

SNAS272B – OCTOBER 2004 – REVISED APRIL 2013

SELECTION OF INPUT CAPACITOR SIZE

can be expensive and may compromise space efficiency in portable 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 response,

turn-on time can be minimized. A small value of Ci (in the range of 0.1µF to 0.39µF), is recommended.

USING EXTERNAL POWERED SPEAKERS

The LM4924 is designed specifically for headphone operation. Often the headphone output of a device will be

used to drive external powered speakers. The LM4924 has a differential output to eliminate the output coupling

speakers that are designed to have single-ended signals at the input, the click and pop circuitry will not be able

to eliminate the turn-on/turn-off click and pop. Unless the inputs to the powered speakers are fully differential the

turn-on/turn-off click and pop will be very large.

AUDIO POWER AMPLIFIER DESIGN

A 30mW/32

Ω Audio Amplifier

Given:

Power Output

30mWrms

Load Impedance

32

Ω

Input Level

1Vrms

Input Impedance

20k

Ω

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 Performance Characteristics section, the supply

rail can be easily found.

Since 3.3V is a standard supply voltage in most applications, it is chosen for the supply rail in this example. Extra

supply voltage creates headroom that allows the LM4924 to reproduce peaks in excess of 30mW without

producing audible distortion. At this time, the designer must make sure that the power supply choice along with

the output impedance does no 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 Equation 3.

(3)

20k

Ω.

The last step in this design example is setting the amplifier's

−3dB frequency bandwidth. To achieve the desired

±0.25dB pass band magnitude variation limit, the low frequency response must extend to at least one-fifth the

lower bandwidth limit and the high frequency response must extend to at least five times the upper bandwidth

limit. The gain variation for both response limits is 0.17dB, well within the ±0.25dB desired limit. The results are

an

(4)

and an

(5)

sets the amplifier's lower bandpass frequency limit. Find the coupling capacitor's value using Equation 4.

(6)

Copyright © 2004–2013, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LM4924