LM4756 datasheet(13/17 Pages) NSC

13 / 17 page

Application Information (Continued)

and power op amps should have their supply leads by-

passed with low-inductance capacitors having short leads.

All high frequency bypass capacitors should be located as

close to the package terminals as possible and have a clear

unobstructed current return path to ground. It is typical to use

capacitor values that are a factor of 100 different from each

other to minimize interaction with each other. The LM4756

should be bypassed with 0.1µF ceramic and 100µF tantalum

capacitors for optimum performance. The 100µF tantalum

can be replaced with an electrolytic, but the bypassing per-

formance of the tantalum will be better. There should also be

large supply reservoir capacitors of about 4700µF on each

supply rail. A larger reservoir capacitor will reduce the supply

ripple and will supply larger current burst requirements in-

stead of requiring those large currents to come from the

main power supply transformer.

If adequate bypassing is not provided, the current in the

supply leads, which is a rectified component of the load

current, may be fed back into internal circuitry. This signal

may cause signal distortion to increase.

Layout and Ground Loops

When designing a printed circuit board layout, it is important

to return the load ground, any output compensation ground,

and the low-level (feedback and input) grounds to the circuit

board common ground point through separate paths. Large

currents flowing along a ground conductor will generate

voltages which effectively act as signals to the input ground

reference. This can result in high frequency oscillation or

excessive distortion. Output compensation components and

the high frequency supply bypass capacitors should be

placed as close as possible to the IC to reduce the effects of

PCB trace resistance and inductance. For cases where long

traces must exist, widen the traces to minimize their induc-

tance.

References

International Electronic Research Corporation

P.O. Box 7704,

Burbank, California 91510-7704,

(818) 842-7277

Thermalloy Inc.

P.O. Box 810839,

Dallas, Tx 75381-0839,

(214) 243-4321,

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