LM4842 datasheet(24/31 Pages) NSC

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Audio Power Amplifier Design

AUDIO AMPLIFIER DESIGN: DRIVING 1W INTO AN 8

Ω

LOAD

The following are the desired operational parameters:

Power Output:

1 W

RMS

Load Impedance:

Ω

Input Level:

1 V

RMS

Input Impedance:

20 k

Ω

Bandwidth:

100 Hz−20 kHz ± 0.25 dB

The design begins by specifying the minimum supply voltage

necessary to obtain the specified output power. One way to

find the minimum supply voltage is to use the Output Power

vs Supply Voltage curve in the Typical Performance Char-

acteristics section. Another way, using Equation (9), is to

calculate the peak output voltage necessary to achieve the

desired output power for a given load impedance. To ac-

count for the amplifier’s dropout voltage, two additional volt-

ages, based on the Dropout Voltage vs Supply Voltage in the

Typical Performance Characteristics curves, must be

added to the result obtained by Equation (9). The result is

Equation (10).

(9)

≥ (V

OUTPEAK+(VODTOP +VODBOT))

(10)

The Output Power vs Supply Voltage graph for an 8

Ω load

indicates a minimum supply voltage of 4.6V. This is easily

met by the commonly used 5V supply voltage. The additional

voltage creates the benefit of headroom, allowing the

LM4842 to produce peak output power in excess of 1W

without clipping or other audible distortion. The choice of

supply voltage must also not create a situation that violates

of maximum power dissipation as explained above in the

Power Dissipation section.

After satisfying the LM4842’s power dissipation require-

ments, the minimum differential gain needed to achieve 1W

dissipation in an 8

Ω load is found using Equation (11).

(11)

Thus, a minimum overall gain of 2.83 allows the LM4842’s to

reach full output swing and maintain low noise and THD+N

performance.

The last step in this design example is setting the amplifier’s

−3dB frequency bandwidth. To achieve the desired ±0.25dB

pass band magnitude variation limit, the low frequency re-

sponse must extend to at least one-fifth the lower bandwidth

limit and the high frequency response must extend to at least

five times the upper bandwidth limit. The gain variation for

both response limits is 0.17dB, well within the ±0.25dB

desired limit. The results are an

L = 100Hz/5 = 20Hz

(12)

and an

H = 20kHz x 5 = 100kHz

(13)

As mentioned in the Selecting Proper External Compo-

nents section, R

i (Right & Left) and Ci (Right & Left) create

a highpass filter that sets the amplifier’s lower bandpass

frequency limit. Find the coupling capacitor’s value using

Equation (14).

≥ 1/(2πR

ifL)

(14)

The result is

1/(2

π*20kΩ*20Hz) = 0.397µF

(15)

Use a 0.39µF capacitor, the closest standard value.

The product of the desired high frequency cutoff (100kHz in

this example) and the differential gain A

VD, determines the

upper passband response limit. With A

VD = 3 and fH =

100kHz, the closed-loop gain bandwidth product (GBWP) is

300kHz. This is less than the LM4842’s 3.5MHz GBWP. With

this margin, the amplifier can be used in designs that require

more differential gain while avoiding performance,restricting

bandwidth limitations.

Recommended Printed Circuit

Board Layout

Figures 7 through 9 show the recommended 2 later PC

board layout that is optimized for the MT/MH packaged

LM4842 and associated external components. Figures 10

through 12 show the recommended 2 layer PC board layout

that is optimized for the LQ packaged LM4842 and associ-

ated external components. These circuits are designed for

use with an external 5V supply and 4

Ω or 8Ω speakers.

These circuit boards are easy to use. Apply 5V and ground to

the board’s V

DD and GND pads, respectively. Connect 4

Ω or

Ω speakers between the board’s −OUTA and +OUTA and

OUTB and +OUTB pads.

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