LM4766 datasheet(17/19 Pages) NSC

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Application Information (Continued)

AUDIO POWER AMPLlFIER DESIGN

Design a 30W/8

Ω Audio Amplifier

Given:

Power Output

30Wrms

Load Impedance

Ω

Input Level

1Vrms(max)

Input Impedance

47k

Ω

Bandwidth

20Hz−20kHz

±0.25dB

A designer must first determine the power supply require-

ments in terms of both voltage and current needed to obtain

the specified output power. V

OPEAK can be determined from

Equation (4) and I

OPEAK from Equation (5).

(4)

(5)

To determine the maximum supply voltage the following

conditions must be considered. Add the dropout voltage to

the peak output swing V

OPEAK, to get the supply rail at a

current of I

OPEAK. The regulation of the supply determines

the unloaded voltage which is usually about 15% higher. The

supply voltage will also rise 10% during high line conditions.

Therefore the maximum supply voltage is obtained from the

following equation.

Max supplies

≈ ± (V

OPEAK +VOD) (1 + regulation) (1.1)

For 30W of output power into an 8

Ω load, the required

OPEAK is 21.91V. A minimum supply rail of 25.4V results

from adding V

OPEAK and VOD. With regulation, the maximum

supplies are ±32V and the required I

OPEAK is 2.74A from

Equation (5). It should be noted that for a dual 30W amplifier

into an 8

Ω load the I

OPEAK drawn from the supplies is twice

2.74A

PK or 5.48APK. At this point it is a good idea to check

the Power Output vs Supply Voltage to ensure that the

required output power is obtainable from the device while

maintaining low THD+N. In addition, the designer should

verify that with the required power supply voltage and load

impedance, that the required heatsink value

SA is feasible

given system cost and size constraints. Once the heatsink

issues have been addressed, the required gain can be de-

termined from Equation (6).

(6)

From Equation (6), the minimum A

V is:

≥ 15.5.

By selecting a gain of 21, and with a feedback resistor, R

f =

20k

Ω, the value of R

i follows from Equation (7).

i =Rf (AV − 1)

(7)

Thus with R

i =1k

Ω a non-inverting gain of 21 will result.

Since the desired input impedance was 47k

Ω, a value of

47k

Ω was selected for R

IN. The final design step is to

address the bandwidth requirements 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. This fact re-

sults in a low and high frequency pole of 4Hz and 100kHz

respectively. As stated in the External Components sec-

tion, R

i in conjunction with Ci create a high-pass filter.

≥ 1/(2π *1kΩ * 4Hz) = 39.8µF;

use 39µF.

The high frequency pole is determined by the product of the

desired high frequency pole, f

H, and the gain, AV. With a

V = 21 and fH = 100kHz, the resulting GBWP is 2.1MHz,

which is less than the guaranteed minimum GBWP of the

LM4766 of 8MHz. This will ensure that the high frequency

response of the amplifier will be no worse than 0.17dB down

at 20kHz which is well within the bandwidth requirements of

the design.

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