LM4882 National Semiconductor, LM4882 Datasheet - Page 9

• Manufacturer Part Number: LM4882
• Description: 250mW Audio Power Amplifier with Shutdown Mode
• Manufacturer: National Semiconductor

Application Information
vice or the shutdown function may cause the “click and pop”
circuitry to not operate fully, resulting in increased “click and
pop” noise.
The value of C
tain size for C
excessive attenuation. But in many cases, the speakers
used in portable systems have little ability to reproduce sig-
nals below 100 Hz to 150 Hz. In this case, using a large input
and output coupling capacitor may not increase system per-
formance. In most cases, choosing a small value of C
range of 0.1 µF to 0.33 µF, along with C
should produce a virtually clickless and popless turn-on. In
cases where C
geous to increase the value of C
stood that increasing the value of C
and pops” at the expense of a longer device turn-on time.
AUDIO POWER AMPLIFIER DESIGN
A designer must first determine the needed supply rail to ob-
tain the specified output power. Calculating the required sup-
ply rail involves knowing two parameters, V
the dropout voltage. The latter is typically 530mV and can be
found from the graphs in the Typical Performance Charac-
teristics. V
For 250 mW of output power into an 8
V
from adding V
voltage in most applications, it is chosen for the supply rail.
Design a 250 mW/8
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
OPEAK
is 2 volts. A minimum supply rail of 4.55V results
OPEAK
i
OPEAK
is needed to couple in low frequencies without
i
will also reflect turn-on pops. Clearly, a cer-
i
is larger than 0.33 µF, it may be advanta-
can be determined from Equation 3.
and V
Audio Amplifier
OD
. Since 5V is a standard supply
B
100 Hz–20 kHz
. Again, it should be under-
B
will reduce the “clicks
(Continued)
B
load, the required
equal to 1.0 µF
OPEAK
1 Vrms (max)
250 mWrms
±
0.50 dB
and also
20 k
i
in the
8
(3)
9
Extra supply voltage creates headroom that allows the
LM4882 to reproduce peaks in excess of 300 mW without
clipping the signal. At this time, the designer must make sure
that the power supply choice along with the output imped-
ance does not violate the conditions explained in the Power
Dissipation section.
Once the power dissipation equations have been addressed,
the required gain can be determined from Equation 4.
From Equation 4, the minimum gain is:
Since the desired input impedance was 20 k , and with a
gain of 1.4, a value of 28 k
5% tolerance resistors. This combination results in a nominal
gain of 1.4. The final design step is to address the bandwidth
requirements which must be stated as a pair of −3 dB fre-
quency points. Five times away from a −3 dB point is 0.17 dB
down from passband response assuming a single pole roll-
off. As stated in the External Components section, both R
in conjunction with C
pass filters. Thus to obtain the desired frequency low re-
sponse of 100 Hz within
into consideration. The combination of two single order filters
at the same frequency forms a second order response. This
results in a signal which is down 0.34 dB at five times away
from the single order filter −3 dB point. Thus, a frequency of
20 Hz is used in the following equations to ensure that the re-
sponse is better than 0.5 dB down at 100 Hz.
The high frequency pole is determined by the product of the
desired high frequency pole, f
V
ing GBWP = 140 kHz which is much smaller than the
LM4882 GBWP of 12.5Mhz. This figure displays that if a de-
signer has a need to design an amplifier with a higher gain,
the LM4882 can still be used without running into bandwidth
limitations.
. With a closed-loop gain of 1.4 and f
C
i
C
o
1 / (2 * 20 k
1 / (2 * 8
A
i
, and C
V
* 20 Hz) = 995 µF; use 1000 µF.
* 20 Hz) = 0.397 µF; use 0.39 µF.
= R
±
A
0.5 dB, both poles must be taken
V
f
o
/ R
= 1.4
is designated for R
with R
H
, and the closed-loop gain, A
i
L
(5)
, create first order high-
H
= 100 kHz, the result-
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f
, assuming
(4)
i