LM4927 datasheet(12/14 Pages) NSC

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Application Information (Continued)

Tolerance

02 -V01

LOAD

20%

0.8R

1.2R

-0.500V

62.5mA

10%

0.9R

1.1R

-0.250V

31.25mA

0.95R

1.05R

-0.125V

15.63mA

0.99R

1.01R

-0.025V

3.125mA

Since the same variations can have a significant effect on

PSRR and CMRR performance, it is highly recommended

that the input resistors be matched to 1% tolerance or better

for best performance.

AUDIO POWER AMPLIFIER DESIGN

Design a 1W/8

Ω Audio Amplifier

Given:

Power Output

1Wrms

Load Impedance

Ω

Input Level

1Vrms

Input Impedance

20k

Ω

Bandwidth

100Hz–20kHz ± 0.25dB

A designer must first determine the minimum supply rail to

obtain the specified output power. The supply rail can easily

be found by extrapolating from the Output Power vs Supply

Voltage graphs in the Typical Performance Characteris-

tics section. A second way to determine the minimum supply

rail is to calculate the required V

OPEAK using Equation 7 and

add the dropout voltages. Using this method, the minimum

supply voltage is (Vopeak + (V

DO TOP +(VDO BOT )), where

DO BOT

and V

DO TOP are extrapolated from the Dropout

Voltage vs Supply Voltage curve in the Typical Perfor-

mance Characteristics section.

(7)

Using the Output Power vs Supply Voltage graph for an 8

Ω

load, the minimum supply rail just about 5V. Extra supply

voltage creates headroom that allows the LM4927 to repro-

duce peaks in excess of 1W without producing audible dis-

tortion. 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 Dissipa-

tion section. Once the power dissipation equations have

been addressed, the required differential gain can be deter-

mined from Equation 8.

(8)

f /Ri =AVD

From Equation 7, the minimum A

VD is 2.83. A ratio of Rf to Ri

of 2.83 gives R

i = 14k

Ω. The final design step is to address

the bandwidth requirement which must be stated as a single

-3dB frequency point. Five times away from a -3dB point is

0.17dB down from passband response which is better than

the required ±0.25dB specified.

H = 20kHz*5= 100kHz

The high frequency pole is determined by the product of the

desired frequency pole, f

H , and the differential gain, AVD .

With a A

VD = 2.83 and fH = 100kHz, the resulting GBWP =

150kHz which is much smaller than the LM4927 GBWP of

10MHz. This figure displays that if a designer has a need to

design an amplifier with a higher differential gain, the

LM4927 can still be used without running into bandwidth

limitations.

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