LM4928TL datasheet(14/20 Pages) NSC

14 / 20 page

Application Information (Continued)

Special care must be taken to match the values of the input

resistors (R

i1 and Ri2) and (Rf1 and Rf2) to each other.

Because of the balanced nature of differential amplifiers,

resistor matching differences can result in net DC currents

across the load. This DC current can increase power con-

sumption, internal IC power dissipation, reduce PSRR,

CMRR, and possibly damaging the loudspeaker. The chart

below demonstrates this problem by showing the effects of

differing values between the input resistors while assuming

that the feedback resistors are perfectly matched. The re-

sults below apply to the application circuit shown in Figure 1,

and assumes that V

DD =5V, RL =8

Ω, and the system has

DC coupled inputs tied to ground.

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

Similar results would occur if the feedback resistors were not

carefully matched. Adding input coupling resistors in be-

tween the signal source and the input resistors will eliminate

this problem, however. To achieve best performance with

minimum component count, it is highly recommended that

both the feedback and input resistors matched to 1% toler-

ance or better for best performance.

AUDIO POWER AMPLIFIER DESIGN

Design a 1W/8

Ω Audio Amplifier

Given:

Power Output

1Wrms

Load Impedance

Ω

Maximum Input Level

1Vrms

Maximum 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 VDO

BOT

and V

DO TOP are extrapolated from the Dropout Voltage

vs Supply Voltage curve in the Typical Performance Char-

acteristics section.

(7)

Using the Output Power vs Supply Voltage graph for an 8

Ω

load, the minimum supply rail just about 4.5V. Extra supply

voltage creates headroom that allows the LM4928 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 8, the minimum A

VD is 2.83. With Rf = 40k

Ω,

a ratio of R

f to Ri of 2.83 gives Ri = 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 =

283kHz which is much smaller than the LM4928 GBWP of

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

design an amplifier with a higher differential gain, the

LM4928 can still be used without running into bandwidth

limitations.

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