LM4906 datasheet(10/15 Pages) NSC

10 / 15 page

Application Information (Continued)

i. A larger input coupling capacitor requires more charge to

reach its quiescent DC voltage (nominally 1/2 V

DD). 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.

AUDIO POWER AMPLIFIER DESIGN

A 1W/8

Ω Audio Amplifier

Given:

Power Output

1 Wrms

Load Impedance

Ω

Input Level

1 Vrms

Input Impedance

20 k

Ω

Bandwidth

100 Hz–20 kHz ± 0.25 dB

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 Per-

formance Characteristics section, the supply rail can be

easily found.

Extra supply voltage creates headroom that allows the

LM4906 to reproduce peaks in excess of 1W without pro-

ducing 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.

The gain of the LM4906 is internally set at either 6dB or

12dB.

The final design step is to address the bandwidth require-

ments 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 which is better than the required

±0.25dB specified.

L = 100Hz/5= 20Hz

H = 20kHz*5= 100kHz

As stated in the External Components section, R

in (20k) in

conjunction with C

2 create a highpass filter.

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

200571C0

FIGURE 2. REFERENCE DESIGN BOARD SCHEMATIC

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