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參數(shù)資料
| 型號(hào)： | L4782TA |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 3 Channel 25W Audio Power Amplifier with Mute and Standby |
| 中文描述： | 3頻道25瓦音頻功率放大器靜音和待機(jī) |
| 文件頁數(shù)： | 21/26頁 |
| 文件大?。?/td> | 1157K |
| 代理商： | L4782TA |
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Application Information
(Continued)
LAYOUT, GROUND LOOPS AND STABILITY
The LM4782 is designed to be stable when operated at a
closed-loop gain of 10 or greater, but as with any other
high-current amplifier, the LM4782 can be made to oscillate
under certain conditions. These oscillations usually involve
printed circuit board layout or output/input coupling issues.
When designing a layout, it is important to return the load
ground, the output compensation ground, and the low level
(feedback and input) grounds to the circuit board common
ground point through separate paths. Otherwise, large cur-
rents flowing along a ground conductor will generate volt-
ages on the conductor which can effectively act as signals at
the input, resulting in high frequency oscillation or excessive
distortion. It is advisable to keep the output compensation
components and the 0.1μF supply decoupling capacitors as
close as possible to the LM4782 to reduce the effects of PCB
trace resistance and inductance. For the same reason, the
ground return paths should be as short as possible.
In general, with fast, high-current circuitry, all sorts of prob-
lems can arise from improper grounding which again can be
avoided by returning all grounds separately to a common
point. Without isolating the ground signals and returning the
grounds to a common point, ground loops may occur.
“Ground Loop” is the term used to describe situations occur-
ring in ground systems where a difference in potential exists
between two ground points. Ideally a ground is a ground, but
unfortunately, in order for this to be true, ground conductors
with zero resistance are necessary. Since real world ground
leads possess finite resistance, currents running through
them will cause finite voltage drops to exist. If two ground
return lines tie into the same path at different points there will
be a voltage drop between them. The first figure below
shows a common ground example where the positive input
ground and the load ground are returned to the supply
ground point via the same wire. The addition of the finite wire
resistance, R
, results in a voltage difference between the
two points as shown below.
20081198
The load current I
will be much larger than input bias current
I
, thus V
will follow the output voltage directly, i.e. in phase.
Therefore the voltage appearing at the non-inverting input is
effectively positive feedback and the circuit may oscillate. If
there was only one device to worry about then the values of
R
and R
would probably be small enough to be ignored;
however, several devices normally comprise a total system.
Any ground return of a separate device, whose output is in
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 above 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
, (on the order of 50pF to 500pF) across
the LM4782 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
LM4782 is connected directly to a capacitor with no series
resistance, the square wave response will exhibit ringing if
the capacitance is greater than about 0.2μF. If highly capaci-
tive 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
resistor in parallel with a 0.7μH inductor. The
inductor-resistor combination as shown in the
Figure 5
iso-
lates the feedback amplifier from the load by providing high
output impedance at high frequencies thus allowing the 10
resistor to decouple the capacitive load and reduce the Q of
the series resonant circuit. The LR combination also pro-
vides low output impedance at low frequencies thus shorting
out the 10
resistor and allowing the amplifier to drive the
series RC load (large capacitive load due to long speaker
cables) directly.
INVERTING AMPLIFIER APPLICATION
The inverting amplifier configuration may be used instead of
the more common non-inverting amplifier configuration
shown in
Figure 1
. The inverting amplifier can have better
THD+N performance and eliminates the need for a large
capacitor (Ci) reducing cost and space requirements. The
values show in
Figure 7
are only one example of an amplifier
with a gain of 20V/V (Gain = -R
f
/R
i
). For different resistor
values, the value of R
B
should be eqaul to the parallel
combination of R
f
and Ri.
L
www.national.com
21
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