參數(shù)資料
型號(hào)： LM4682SQ
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 音頻／視頻放大
英文描述： 10 Watt Stereo CLASS D Audio Power Amplifier with Stereo Headphone Amplifier and DC Volume Control
中文描述： 10 W, 2 CHANNEL, AUDIO AMPLIFIER, QCC48
封裝： LLP-48
文件頁(yè)數(shù)： 8/22頁(yè)
文件大?。?/td> 1033K
代理商： LM4682SQ
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 fre-
quencies 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 ca-
pacitor 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. 10
μF tantalum capacitors
and 4.7
μF ceramic capacitors are needed for this kind of de-
coupling of LM4682 switching operation. This results an im-
provement 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 capacitors (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
(4.7
μF) 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
DD _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 22H is recom-
mended. As shown in Figure 1, using the values of the com-
ponents 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 im-
portant parameter by which all audio amplifiers are measured.
Often it is shown as a graph where either the output power or
frequency is changed over the operating range. A very im-
portant variable in the measurement of THD+N is the band-
width-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 voltage between
V
DD and GND allows the LM4682 to operate at much higher
efficiency than that achieved by traditional Class AB ampli-
fiers. Switching the outputs at high frequency also increases
the out-of-band noise. Under normal circumstances the out-
put lowpass filter significantly reduces this out-of-band noise.
If the low pass filter is not optimized for a given switching fre-
quency, there can be significant increase in out-of-band
noise. THD+N measurements can be significantly 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
speak-
ers (or load resistors). Apply 12V and ground to board’s V
DD
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 de-
signing the layout of the PCB layout, please pay attention to
the output terminals of LM4682.
www.national.com
16
LM4682
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