lm4682 National Semiconductor Corporation, lm4682 Datasheet - Page 16

• Manufacturer Part Number: lm4682
• Description: 10 Watt Stereo Class D Audio Power Amplifier With Stereo Headphone Amplifier And Dc Volume Control
• Manufacturer: National Semiconductor Corporation

www.national.com
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
power V
_A and HP_ V
ceramic (4.7µF) and high value tantalum (10µF) should be
used to bypass the power V
ceramic or tantalum can be used to bypass the signal V
_A and HP_ V
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.
DD
(PV
DD
DD
DD
. The parallel combination of the low value
_A and P V
pin.
DD
DD
_B) and a set for signal V
pins. A small value (1µF)
DD
pins: a set for
DD
DD
16
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 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 8Ω speak-
ers (or load resistors). Apply 12V and ground to board’s V
and GND terminals respectively. And apply 5V to the
VOLV
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.
DD
(refer to power supply sequencing for details). Con-
DD
and GND allows the LM4682 to operate at
DD