LM4863M datasheet(11/16 Pages) NSC

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Application Information (Continued)

NO-LOAD DESIGN CONSIDERATIONS

If the outputs of the LM4863 have a load higher than 10k

Ω,

the LM4863 may show a small oscillation at high output lev-

els. To prevent this oscillation, place 5k

Ω resistors from the

power outputs to ground.

AUDIO POWER AMPLIFIER DESIGN

Design a 1W/8

Ω Bridged 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. A second way to determine the minimum sup-

ply rail is to calculate the required V

opeak using Equation 3

and add the dropout voltage. Using this method, the mini-

mum supply voltage would be (V

opeak +(2 * Vod)), where Vod

is extrapolated from the Dropout Voltage vs Supply Voltage

curve in the Typical Performance Characteristics section.

(4)

Using the Output Power vs Supply Voltage graph for an 8

Ω

load, the minimum supply rail is 3.9V. But since 5V is a stan-

dard supply voltage in most applications, it is chosen for the

supply rail. Extra supply voltage creates headroom that al-

lows the LM4863 to reproduce peaks in excess of 1W with-

out 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 Equa-

tion 4.

(5)

f/R i = AVD/2

(6)

From equation 4, the minimum A

VD is 2.83;

use A

VD = 3

Since the desired input impedance was 20 k

Ω, and with a

VD of 3, a ratio of 1.5:1 of Rf to Riresults in an allocation of

i = 20 kΩ and R f = 30 kΩ. The final design step is to ad-

dress the bandwidth requirements which must be stated as a

pair of −3 dB frequency points. Five times away from a pole

gives 0.17 dB down from passband response, which is better

than the required ±0.25 dB specified.

L = 100 Hz/5 = 20 Hz

H = 20 kHz x 5 = 100 kHz

As stated in the External Components section, R

i in con-

junction with C

i create a highpass filter.

i ≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF;

use 0.33 µF

The high frequency pole is determined by the product of the

desired high frequency pole, f

H, and the differential gain, A

. With a A

VD = 3 and fH = 100 kHz, the resulting GBWP =

150 kHz which is much smaller than the LM4863 GBWP of

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

design an amplifier with a higher differential gain, the

LM4863 can still be used without running into bandwidth

problems.

DEMOBOARD CIRCUIT LAYOUT

The demoboard circuit layout is provided here as an ex-

ample of a circuit using the LM4863. If an LM4863MTE is

used with this layout, the exposed-DAP is soldered down to

the copper pad beneath the part. Heat is conducted away

from the part by the two large copper pads in the upper cor-

ners of the demoboard.

This demoboard provides enough heat dissipation ability to

allow an LM4863MTE to output 2.2W into 4

Ω at 25˚C.

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