nominal cutoff frequency of 47kHz. This cutoff frequency

ensures that the attenuation is much less than 3dB at 20kHz.

The output filter cutoff frequency and topology are also

optimized for operational efficiency. A higher cutoff frequency

compromises efficiency, whereas a lower cutoff frequency

compromises the high frequencies within the audio fre-

quency range. The filter’s topology also minimizes high fre-

quency peaking, which can also decrease the amplifier’s

efficiency.

The output filter inductors must have a current rating that

exceeds the amplifier’s output current when driving the load

to maximum dissipation. Assuming a load dissipation of 10W

in an 8

Ω load with the amplifier operating on a 14V supply,

the RMS current is 1.1A. In this case, the inductors’ current

rating should be at least 1.2A

If a different output filter cutoff frequency (f

following brief discussion covers the selection of the capaci-

tor and inductor values. In the following equations, R

load resistance and C

three common-mode filter capacitor found between the two

output filter inductors (each inductor is L) as shown in Figure

1. When calculating values for L and C

Ω,

since the LM4668 is specified for 8

Ω loads.

The filter’s two inductors are equal to

L= R

πf

C

(1)

and each of the three capacitors are equal to

C = L / 1.5R

2

(2)

SCHOTTKY DIODE AMPLIFIER OUTPUT OVERDRIVE

PROTECTION

The Schottky diodes shown in Figure 1 provide protection

against an over-voltage condition that may be caused by

inductor-induced transients. These diodes are necessary

when the nominal supply voltage exceeds 12V, the load

impedance falls below 6

Ω or the ambient temperature in the

operating environment rises above 50˚C.

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 MH-packaged

LM4668 and associated external components. Figures 5

through 7 show the recommended two-layer PC board lay-

out that is optimized for the 14-pin LD-packaged LM4668

and associated external components. These circuits are de-

signed for use with an external 12V supply and 8

Ω speakers

(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.

www.national.com

9