LM4681 datasheet(17/19 Pages) NSC

17 / 19 page

Application Hints

SUPPLY BYPASSING

Correct power supply bypassing has two important goals.

The first is to reduce noise on the power supply lines and

minimize deleterious effects that the noise may cause to the

amplifier’s operation. The second is to help stabilize an

unregulated power supply and to improve the supply’s tran-

sient response under heavy current demands. These two

goals require different capacitor value ranges. Therefore,

various types and values are recommended for supply by-

passing. For noise de-coupling, generally small ceramic ca-

pacitors (0.01µF to 0.1µF) are recommended. Larger value

(1µF to 10µF) tantalum capacitors are needed for the tran-

sient current demands. These two capacitors in parallel will

do an adequate job of removing most noise from the supply

rails and providing the necessary transient current. These

capacitors should be placed as close as possible to each

IC’s supply pin(s) using leads as short as possible.

The LM4681 has two different set of V

DD pins: a set for

power V

DD (PVDD) and a set for signal VDD (SVDD). The

parallel combination of the low value ceramic (0.1µF) and

high value tantalum (10µF) should be used to bypass the

DD pin. A small value (0.1µF) ceramic or tantalum can be

used to bypass the SV

DD pin.

OUTPUT STAGE FILTERING

The LM4681 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

DD and GND allows the LM4681 to operate at

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.

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