LM4682 datasheet(16/21 Pages) NSC

16 / 21 page

Application Hints

SUPPLY BYPASSING

The major source of noises to be taken care and applying

bypassing technique in using LM4682 are those transients

response coming from its output stage. During the switching

operations of the output stage of LM4682, the switching

frequencies vary when the internal modulator react to the

input signals. This creates a band of switching transients

giving back to the power supply terminals of LM4682. A

single capacitor may not bypass those transients well. Two

capacitors which values are closed to each other are used to

bypass this range of frequencies to the ground. 10µF tanta-

lum capacitors and 4.7µF ceramic capacitors are needed for

this kind of decoupling of LM4682 switching operation. This

results an improvement in terms of both stability and audio

performance of LM4682. In addition, these capacitors should

be placed as close as possible to each IC’s supply pin(s)

using leads as short as possible. Apart from the power

supply de-coupling capacitors, the four bootstrapping ca-

pacitors (at pins BST1_A, BST2_A, BST1_B and BST2_B)

should also be placed close to their corresponding pins. This

could minimize the undesirable switching noise coupled to

the supply rail.

The LM4682 has two different sets of V

DD pins: a set for

power V

DD (PVDD_A and P VDD _B) and a set for signal VDD

_A and HP_ V

DD. The parallel combination of the low value

ceramic (4.7µF) and high value tantalum (10µF) should be

used to bypass the power V

DD pins. A small value (1µF)

ceramic or tantalum can be used to bypass the signal V

_A and HP_ V

DD pin.

OUTPUT STAGE FILTERING

The LM4682 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 22µH 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

which optimizes the amplifier’s performance within the audio

band.

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 LM4682 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.

RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT

Figures 2 through 6 show the recommended four-layer PCB

board layout that is optimized for the 48-pin LLP packaged

LM4682 and associated external components. This circuit is

designed for use with an external 12V supply and 8

Ω speak-

ers (or load resistors). Apply 12V and ground to board’s V

and GND terminals respectively. And apply 5V to the

VOLV

DD (refer to power supply sequencing for details). Con-

nect speakers (or load resistors) between the board’s

OUTA+ and OUTA-, and between the board’s OUTB+ and

OUTB-. Apply the stereo input signals to IN_A and IN_B.

When designing the layout of the PCB layout, please pay

attention to the output terminals of LM4682.

LM4682 Datasheet(PDF) 16 Page - National Semiconductor (TI)

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