LM3876 datasheet(14/19 Pages) NSC

14 / 19 page

Application Information (Continued)

LAYOUT, GROUND LOOPS AND STABILITY

The LM3876 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 LM3876 can be made to oscillate

under certain conditions. These usually involve printed cir-

cuit board layout or output/input coupling.

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 0.1 µF supply decoupling capacitors as

close as possible to the LM3876 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 re-

turn 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

2, results in a voltage difference between the

two points as shown below.

The load current I

L will be much larger than input bias current

I, thus V1 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 were only one device to worry about then the values of

1 and R2 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 instabili-

ties. Out of phase ground loops also are troublesome, caus-

ing 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 im-

practical. 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. Viola-

tions 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 in-

put, 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 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 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

Ω resistor in parallel with a 0.7 µH inductor. The

inductor-resistor combination as shown in the Typical Appli-

cation 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 combina-

tion 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-

DS011832-15

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