LM4874 datasheet(18/21 Pages) NSC

18 / 21 page

Application Information (Continued)

As mentioned in the External Components section, the inter-

nal input resistor and C

i create a high pass filter that sets the

amplifier’s lower bandpass frequency limit. Find the coupling

capacitor’s value using Equation (11).

-3dB = 1/2

π(20kΩ)C

(11)

The result is (using the minimum R

IN resistor value to ensure

correct magnitude response at 20Hz)

1/(2

π*20kΩ*20Hz) = 0.398µF

(12)

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 up-

per passband response limit. With A

VD = 3.2 and fH =

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

320kHz. This is less than the LM4874’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 2 through 6 show the recommended four-layer PC

board layout that is optimized for the 20-pin MH-packaged

LM4874 and associated external components. This circuit is

designed for use with an external 5V supply and 3

Ω (or

higher) speakers (or load resistors).

This circuit board is easy to use. Apply 5V and ground to the

board’s V

and GND terminals, respectively. Connect

speakers (or load resistors) between the board’s -OUTA and

+OUTA and -OUTB and +OUTB pads. Apply balanced dif-

ferential stereo input signals to the input pins labeled ’-INA,’

’+INA,’ ’-INB,’ and ’+INB.’

200469D7

FIGURE 2. Recommended MH PC Board Layout:

Component-Side Silkscreen

200469D8

FIGURE 3. Recommended MH PC Board Layout:

Component-Side Layout

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