LM3876 Overture Audio Power Amplifier Series National Semiconductor, LM3876 Overture Audio Power Amplifier Series Datasheet - Page 18

• Manufacturer Part Number: LM3876 Overture Audio Power Amplifier Series
• Description: LM3876 - High-Performance 56W Audio Power Amplifier with Mute
• Manufacturer: National Semiconductor

www.national.com
Application Information
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
the LM3876 input terminals. Refer to the External Compo-
nents Description section relating to component interaction
with C
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
LM3876 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 10Ω 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 10Ω 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 10Ω 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:
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
power):
Desired Power Output
Input Impedance
Maximum Supply Voltage
f
.
C
, (on the order of 50 pF to 500 pF) across
O
Input Level
Load Impedance
Bandwidth
is the average output
(Continued)
18
To determine the maximum supply voltage the following
parameters must be considered. Add the dropout voltage
(5V for LM3876) to the peak output swing, V
supply rail value (i.e.
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:
The input sensitivity and the output power specs determine
the minimum required gain as depicted below:
Normally the gain is set between 20 and 200; for a 40W, 8Ω
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
can cause board layout problems and DC offsets at the
output. The value for the feedback resistance, R
chosen to be a relatively large value (10 kΩ–100 kΩ), 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/8Ω AUDIO AMPLIFIER
Given:
Equations (5), (6) give:
Therefore the supply required is:
With 15% regulation and high line the final supply voltage is
±
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.
38.3V using Equation (7). At this point it is a good idea to
40W/8Ω
Max. supplies ≈
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
V
opeak
±
±
= 25.3V
(V
(V
opeak
opeak
+ Vod)(1 + regulation)(1.1) (7)
+ Vod) at a current of I
20 Hz–20 kHz
±
I
opeak
30.3V
IN
= 3.16A
@
. Very high values
opeak
3.16A
f1
±
, should be
, to get the
1V(max)
0.25 dB
100 kΩ
opeak
40W
8Ω
(5)
(6)
(8)
).