LM4681SQ NSC [National Semiconductor], LM4681SQ Datasheet - Page 17

• Manufacturer Part Number: LM4681SQ
• Description: 10 Watt Stereo CLASS D Audio Power Amplifier with Stereo Headphone Amplifier and I2C/SPI Volume Control
• Manufacturer: NSC [National Semiconductor]

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
power V
parallel combination of the low value ceramic (0.1µF) and
high value tantalum (10µF) should be used to bypass the
PV
used to bypass the SV
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
DD
pin. A small value (0.1µF) ceramic or tantalum can be
DD
(PV
DD
) and a set for signal V
DD
pin.
DD
DD
pins: a set for
(SV
DD
). The
17
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
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.
DD
and GND allows the LM4681 to operate at
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