Application Information
(Continued)
AUDIO POWER AMPLIFIER DESIGN
Design a 300 mW/8Ω Audio Amplifier
Given:
Power Output
Load Impedance
Input Level
Input Impedance
300 mWrms
8Ω
1 Vrms
20 kΩ
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4864 is tolerant to a
variety of external component combinations, consideration
to component values must be used to maximize overall sys-
tem quality.
The LM4864 is unity-gain stable, giving a designer maximum
system flexibility. The LM4864 should be used in low gain
configurations to minimize THD+N values, and maximize the
signal to noise ratio. Low gain configurations require large in-
put signals to obtain a given output power. Input signals
equal to or greater than 1 Vrms are available from sources
such as audio codecs. Please refer to the section,
Audio
Power Amplifier Design,
for a more complete explanation
of proper gain selection.
Besides gain, one of the major considerations is the
closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components
shown in
Figure 1.
The input coupling capacitor, C
i
, forms a
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed fre-
quency response for a few distinct reasons.
Selection of Input Capacitor Size
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenua-
tion. But in many cases the speakers used in portable sys-
tems, whether internal or external, have little ability to repro-
duce signals below 150 Hz. In this case using a large input
capacitor may not increase system performance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
C
i
. A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally
1
⁄
2
V
DD
). This
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the ca-
pacitor size based on necessary low frequency response,
turn-on pops can be minimized.
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
B
, is the most critical component to minimize
turn-on pops since it determines how fast the LM4864 turns
on. The slower the LM4864’s outputs ramp to their quiescent
DC voltage (nominally
1
⁄
2
V
DD
), the smaller the turn-on pop.
Choosing C
B
equal to 1.0 µF along with a small value of C
i
(in the range of 0.1 µF to 0.39 µF), should produce a click-
less and popless shutdown function. While the device will
function properly, (no oscillations or motorboating), with C
B
equal to 0.1 µF, the device will be much more susceptible to
turn-on clicks and pops. Thus, a value of C
B
equal to 1.0 µF
or larger is recommended in all but the most cost sensitive
designs.
Bandwidth
100 Hz–20 kHz
±
0.25 dB
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the
Typical Per-
formance Characteristics
section, the supply rail can be
easily found. A second way to determine the minimum sup-
ply rail is to calculate the required V
opeak
using Equation 4
and add the dropout voltage. Using this method, the mini-
mum supply voltage would be (V
opeak
+ (2
*
V
OD
)), where
V
OD
is extrapolated from the Dropout Voltage vs Supply Volt-
age curve in the
Typical Performance Characteristics
sec-
tion.
(4)
Using the Output Power vs Supply Voltage graph for an 8Ω
load, the minimum supply rail is 3.5V. But since 5V is a stan-
dard supply voltage in most applications, it is chosen for the
supply rail. Extra supply voltage creates headroom that al-
lows the LM4864 to reproduce peaks in excess of 500 mW
without producing audible distortion. At this time, the de-
signer must make sure that the power supply choice along
with the output impedance does not violate the conditions
explained in the
Power Dissipation
section.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 5.
(5)
(6)
R
F
/R
i
= A
VD
/2
From Equation 5, the minimum A
VD
is 1.55; use A
VD
= 2.
Since the desired input impedance was 20 kΩ, and with a
A
VD
of 2, a ratio of 1:1 of R
F
to R
i
results in an allocation of
R
i
= R
F
= 20 kΩ. The final design step is to address the
bandwidth requirements which must be stated as a pair of
−3 dB frequency points. Five times away from a pole gives
0.17 dB down from passband response which is better than
the required
±
0.25 dB specified.
f
L
= 100 Hz/5 = 20 Hz
f
H
= 20 kHz x 5 = 100 kHz
As stated in the
External Components
section, R
i
in con-
junction with C
i
create a highpass filter.
C
i
≥
1/(2π
*
20 kΩ
*
20 Hz) = 0.397 µF; use 0.39 µF
The high frequency pole is determined by the product of the
desired high frequency pole, f
H
, and the differential gain,
A
VD
. With a A
VD
= 2 and f
H
= 100 kHz, the resulting GBWP
= 100 kHz which is much smaller than the LM4864 GBWP of
18 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4864 can still be used without running into bandwidth
problems.
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