Application Information
(Continued)
phase, can feedback in a similar manner and cause insta-
bilities. Out of phase ground loops also are troublesome,
causing unexpected gain and phase errors.
The solution to most ground loop problems is to always use
a single-point ground system, although this is sometimes
impractical. The third figure below is an example of a single-
point ground system.
The single-point ground concept should be applied rigor-
ously to all components and all circuits when possible. Vio-
lations of single-point grounding are most common among
printed circuit board designs, since the circuit is surrounded
by large ground areas which invite the temptation to run a
device to the closest ground spot. As a final rule, make all
ground returns low resistance and low inductance by using
large wire and wide traces.
Occasionally, current in the output leads (which function as
antennas) can be coupled through the air to the amplifier
input, resulting in high-frequency oscillation. This normally
happens when the source impedance is high or the input
leads are long. The problem can be eliminated by placing a
small capacitor, C
C
, (on the order of 50 pF to 500 pF) across
the LM3886 input terminals. Refer to the
External Compo-
nents Description
section relating to component interaction
with C
f
.
REACTIVE LOADING
It is hard for most power amplifiers to drive highly capacitive
loads very effectively and normally results in oscillations or
ringing on the square wave response. If the output of the
LM3886 is connected directly to a capacitor with no series
resistance, the square wave response will exhibit ringing if
the capacitance is greater than about 0.2 µF. If highly ca-
pacitive loads are expected due to long speaker cables, a
method commonly employed to protect amplifiers from low
impedances at high frequencies is to couple to the load
through a 10
Ω
resistor in parallel with a 0.7 µH inductor. The
inductor-resistor combination as shown in the
Typical Ap-
plication Circuit
isolates the feedback amplifier from the
load by providing high output impedance at high frequencies
thus allowing the 10
Ω
resistor to decouple the capacitive
load and reduce the Q of the series resonant circuit. The LR
combination also provides low output impedance at low
frequencies thus shorting out the 10
Ω
resistor and allowing
the amplifier to drive the series RC load (large capacitive
load due to long speaker cables) directly.
GENERALIZED AUDIO POWER AMPLIFIER DESIGN
The system designer usually knows some of the following
parameters when starting an audio amplifier design:
Desired Power Output
Input Level
Input Impedance
Load Impedance
Maximum Supply Voltage
Bandwidth
The power output and load impedance determine the power
supply requirements, however, depending upon the applica-
tion some system designers may be limited to certain maxi-
mum supply voltages. If the designer does have a power
supply limitation, he should choose a practical load imped-
ance which would allow the amplifier to provide the desired
output power, keeping in mind the current limiting capabili-
ties of the device. In any case, the output signal swing and
current are found from (where P
O
is the average output
power):
(5)
(6)
To determine the maximum supply voltage the following
parameters must be considered. Add the dropout voltage
(4V for LM3886) to the peak output swing, V
opeak
, to get the
supply rail value (i.e.
±
(V
opeak
+ Vod) at a current of I
opeak
).
The regulation of the supply determines the unloaded volt-
age, usually about 15% higher. Supply voltage will also rise
10% during high line conditions. Therefore, the maximum
supply voltage is obtained from the following equation:
Max. supplies
)
±
(V
opeak
+ Vod)(1 + regulation)(1.1)(7)
The input sensitivity and the output power specs determine
the minimum required gain as depicted below:
(8)
Normally the gain is set between 20 and 200; for a 40W, 8
Ω
audio amplifier this results in a sensitivity of 894 mV and
89 mV, respectively. Although higher gain amplifiers provide
greater output power and dynamic headroom capabilities,
there are certain shortcomings that go along with the so
called “gain.” The input referred noise floor is increased and
hence the SNR is worse. With the increase in gain, there is
also a reduction of the power bandwidth which results in a
decrease in feedback thus not allowing the amplifier to re-
spond quickly enough to nonlinearities. This decreased abil-
ity to respond to nonlinearities increases the THD + N speci-
fication.
The desired input impedance is set by R
IN
. Very high values
can cause board layout problems and DC offsets at the
output. The value for the feedback resistance, R
f1
, should be
chosen to be a relatively large value (10 k
Ω
–100 k
Ω
), and
the other feedback resistance, Ri, is calculated using stan-
dard op amp configuration gain equations. Most audio am-
plifiers are designed from the non-inverting amplifier configu-
ration.
DESIGN A 40W/4
Ω
AUDIO AMPLIFIER
Given:
Power Output
40W
Load Impedance
4
Ω
Input Level
1V(max)
Input Impedance
100 k
Ω
Bandwidth
20 Hz–20 kHz
±
0.25 dB
Equations (5), (6)
give:
40W/4
Ω
V
opeak
= 17.9V
I
opeak
= 4.5A
Therefore the supply required is:
±
21.0V
@
4.5A
With 15% regulation and high line the final supply voltage is
±
26.6V using
Equation (7)
. At this point it is a good idea to
check the Power Output vs Supply Voltage to ensure that the
required output power is obtainable from the device while
maintaining low THD + N. It is also good to check the Power
Dissipation vs Supply Voltage to ensure that the device can
handle the internal power dissipation. At the same time
designing in a relatively practical sized heat sink with a low
thermal resistance is also important. Refer to
Typical Per-
formance Characteristics
graphs and the
Thermal Con-
siderations
section for more information.
LM3886
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