LM4868 datasheet(19/28 Pages) NSC

19 / 28 page

Application Information (Continued)

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 and Ci create a highpass filter that sets the

amplifier’s lower bandpass frequency limit. Find the coupling

capacitor’s value using Equation (12).

≥ 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 LM4868’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 9 through 13 show the recommended four-layer PC

board layout that is optimized for the 24-pin LQ-packaged

LM4868 and associated external components. Figures 14

through 18 show the recommended four-layer PC board

layout that is optimized for the 24-pin MTE-packaged

LM4868 and associated external components. Figures 19

through 21 show the recommended two-layer PC board

layout that is optimized for the 20-pin MT-packaged LM4868

and associated external components. These circuits are de-

signed for use with an external 5V supply and 4

Ω speakers

(or greater) for the MT-packaged LM4868 or 3

Ω speakers (or

greater) for the LQ- and MTE-packaged LM4868.

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

the board’s V

DD and GND pads, respectively. Connect 4

Ω

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

OUTB and +OUTB pads.

20026778

FIGURE 9. Recommended LQ PC Board Layout:

Component-Side Silkscreen

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