LM4902 datasheet(17/24 Pages) TI

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LM4902

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SNAS150D – DECEMBER 2001 – REVISED APRIL 2013

Selection of Input Capacitor Size

Large input capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized

capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the speakers

used in portable systems, whether internal or external, have little ability to reproduce signals below 150Hz. In this

case using a large input capacitor may not increase system performance.

In addition to system cost and size, click and pop performance is effected by the size of the input coupling

capacitor, Ci. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally

½ VDD). This charge comes from the output via the feedback and is apt to create pops upon device enable. Thus,

by minimizing the capacitor size based on necessary low frequency response, turn-on pops can be minimized.

Besides minimizing the input capacitor size, careful consideration should be paid to the bypass capacitor value.

Bypass capacitor, CB, is the most critical component to minimize turn-on pops since it determines how fast the

LM4902 turns on. The slower the LM4902's outputs ramp to their quiescent DC voltage (nominally ½ VDD), the

smaller the turn-on pop. Choosing CB equal to 1.0 μF along with a small value of Ci (in the range of 0.1μF to

0.39

μF), should produce a clickless and popless shutdown function. While the device will function properly, (no

oscillations or motorboating), with CB equal to 0.1μF, the device will be much more susceptible to turn-on clicks

and pops. Thus, a value of CB equal to 1.0μF or larger is recommended in all but the most cost sensitive

designs.

AUDIO POWER AMPLIFIER DESIGN

Design a 300 mW/8

Ω Audio Amplifier

Given:

Power Output

300mWrms

Load Impedance

Input Level

1Vrms

Input Impedance

20k

Bandwidth

100Hz–20 kHz ± 0.25dB

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. A second way to determine the minimum supply rail is to calculate the required Vopeak

using Equation 5 and add the dropout voltage. Using this method, the minimum supply voltage would be (Vopeak

+ (2*VOD)), where VOD is extrapolated from the Dropout Voltage vs Supply Voltage curve in the Typical

Performance Characteristics section.

(5)

Using the Output Power vs Supply Voltage graph for an 8

Ω load, the minimum supply rail is 3.5V. But since 5V is

a standard supply voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates

headroom that allows the LM4902 to reproduce peaks in excess of 700 mW 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 differential gain can be determined

from Equation 6.

(6)

RF/Ri = AVD/2

(7)

From Equation 6, the minimum AVD is 1.55; use AVD = 2.

Since the desired input impedance was 20 k

Ω, and with a AVD of 2, a ratio of 1:1 of RF to Ri results in an

allocation of Ri = RF = 20 kΩ. The final design step is to address the bandwidth requirements which must be

stated as a pair of

−3 dB frequency points. Five times away from a pole gives 0.17 dB down from passband

response which is better than the required ±0.25 dB specified.

fL = 100Hz/5 = 20Hz

(8)

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