OUTPUT STAGE FILTERING

The LM4680 requires a low pass filter connected between

the amplifier’s bridge output and the load. Figure 1 shows

the recommended LC filter. A minimum value of 27µH is

recommended. As shown in Figure 1, using the values of the

components connected between the amplifier BTL outputs

and the load achieves a 2nd-order lowpass filter response

with a -3dB cutoff frequency of 25kHz.

THD+N MEASUREMENTS AND OUT OF AUDIO BAND

NOISE

THD+N (Total Harmonic Distortion plus Noise) is a very

important parameter by which all audio amplifiers are mea-

sured. Often it is shown as a graph where either the output

power or frequency is changed over the operating range. A

very important variable in the measurement of THD+N is the

bandwidth-limiting filter at the input of the test equipment.

Class D amplifiers, by design, switch their output power

devices at a much higher frequency than the accepted audio

range (20Hz - 20kHz). Alternately switching the output volt-

age between V

much higher efficiency than that achieved by traditional

Class AB amplifiers. Switching the outputs at high frequency

also increases the out-of-band noise. Under normal circum-

stances the output lowpass filter significantly reduces this

out-of-band noise. If the low pass filter is not optimized for a

given switching frequency, there can be significant increase

in out-of-band noise. THD+N measurements can be signifi-

cantly affected by out-of-band noise, resulting in a higher

than expected THD+N measurement. To achieve a more

accurate measurement of THD, the test equipment’s input

bandwidth of the must be limited. Some common upper filter

points are 22kHz, 30kHz, and 80kHz. The input filter limits

the noise component of the THD+N measurement to a

smaller bandwidth resulting in a more real-world THD+N

value.

Recommended Printed Circuit

Board Layout

Figures 2 through 4 show the recommended two-layer PC

board layout that is optimized for the 14-pin SD-packaged

LM4680 and associated external components. This circuit is

designed for use with an external 12V supply and 8W speak-

ers (or load resistors). This circuit board is easy to use. Apply

12V and ground to the board’s V

respectively. Connect speakers (or load resistors) between

the board’s -OUT and +OUT terminals. Apply the input signal

to the input pin labeled -IN.

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