LM4879SDBD datasheet(13/28 Pages) TI1

13 / 28 page

LM4879, LM4879MMBD, LM4879SDBD

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SNAS142F – MAY 2004 – REVISED OCTOBER 2004

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 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 3 and add the output voltage. Using this method, the minimum supply voltage

would be (Vopeak + (VODTOP + VODBOT)), where VODBOT and VODTOP are extrapolated from the Dropout Voltage vs

Supply Voltage curve in the TYPICAL PERFORMANCE CHARACTERISTICS section.

(3)

5V is a standard voltage, in most applications, chosen for the supply rail. Extra supply voltage creates headroom

that allows the LM4879 to reproduce peaks in excess of 1W 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 4.

(4)

AVD = (Rf/Ri) 2

(5)

From Equation 4, the minimum AVD is 2.83; use AVD = 3.

Since the desired input impedance was 20 k

Ω, and with a AVD of 3, a ratio of 1.5:1 of Rf to Ri results in an

allocation of Ri = 20 kΩ and Rf = 30 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 −3 dB point is 0.17 dB down from

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

fL = 100 Hz/5 = 20 Hz

fH = 20 kHz * 5 = 100 kHz

As stated in the EXTERNAL COMPONENTS DESCRIPTION section, Ri in conjunction with Ci create a high pass

filter.

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

The high frequency pole is determined by the product of the desired frequency pole, fH, and the differential gain,

AVD. With a AVD = 3 and fH = 100 kHz, the resulting GBWP = 300 kHz which is much smaller than the LM4879

GBWP of 10 MHz. This figure displays that if a designer has a need to design an amplifier with a higher

differential gain, the LM4879 can still be used without running into bandwidth limitations.

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