LM4951 datasheet(13/18 Pages) NSC

13 / 18 page

Application Information (Continued)

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 =50Hz/5= 10Hz

(11)

and an

L = 20kHzx5= 100kHz

(12)

As mentioned in the SELECTING EXTERNAL COMPO-

NENTS section, R

i and Ci create a highpass filter that sets

the amplifier’s lower bandpass frequency limit. Find the cou-

pling capacitor’s value using Equation (13).

i =1 / 2

πR

ifL

(13)

The result is

1 / (2

πx20kΩx10Hz) = 0.795µF

Use a 0.82µF 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 = 7 and fH =

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

700kHz. This is less than the LM4951’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 2–4 show the recommended two-layer PC board

layout that is optimized for the SD10A. This circuit is de-

signed for use with an external 7.5V supply 8

Ω (min) speak-

ers.

These circuit boards are easy to use. Apply 7.5V and ground

to the board’s V

DD and GND pads, respectively. Connect a

speaker between the board’s OUT

A and OUTB outputs.

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