LM4780 datasheet(18/24 Pages) NSC

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Application Information (Continued)

20058699

Integrated circuits have additional open loop gain allowing

additional feedback loop gain in order to lower harmonic

distortion and improve frequency response. It is this addi-

tional bandwidth that can lead to erroneous signal-to-noise

measurements if not considered during the measurement

process. In the typical example above, the difference in

bandwidth appears small on a log scale but the factor of 10in

bandwidth, (200kHz to 2MHz) can result in a 10dB theoreti-

cal difference in the signal-to-noise ratio (white noise is

proportional to the square root of the bandwidth in a system).

In comparing audio amplifiers it is necessary to measure the

magnitude of noise in the audible bandwidth by using a

“weighting” filter (Note 16). A “weighting” filter alters the

frequency response in order to compensate for the average

human ear’s sensitivity to the frequency spectra. The weight-

ing filters at the same time provide the bandwidth limiting as

discussed in the previous paragraph.

In addition to noise filtering, differing meter types give differ-

ent noise readings. Meter responses include:

RMS reading,

average responding,

peak reading, and

quasi peak reading.

Although theoretical noise analysis is derived using true

RMS based calculations, most actual measurements are

taken with ARM (Average Responding Meter) test equip-

ment.

Typical signal-to-noise figures are listed for an A-weighted

filter which is commonly used in the measurement of noise.

The shape of all weighting filters is similar, with the peak of

the curve usually occurring in the 3kHz–7kHz region.

LEAD INDUCTANCE

Power op amps are sensitive to inductance in the output

leads, particularly with heavy capacitive loading. Feedback

to the input should be taken directly from the output terminal,

minimizing common inductance with the load.

Lead inductance can also cause voltage surges on the sup-

plies. With long leads to the power supply, energy is stored in

the lead inductance when the output is shorted. This energy

can be dumped back into the supply bypass capacitors when

the short is removed. The magnitude of this transient is

reduced by increasing the size of the bypass capacitor near

the IC. With at least a 20µF local bypass, these voltage

surges are important only if the lead length exceeds a couple

feet (>1µH lead inductance). Twisting together the supply

and ground leads minimizes the effect.

PHYSICAL IC MOUNTING CONSIDERATIONS

Mounting of the package to a heat sink must be done such

that there is sufficient pressure from the mounting screws to

insure good contact with the heat sink for efficient heat flow.

Over tightening the mounting screws will cause the package

to warp reducing contact area with the heat sink. Less

contact with the heat sink will increase the thermal resis-

tance from the package case to the heat sink (

CS) resulting

in higher operating die temperatures and possible unwanted

thermal shut down activation. Extreme over tightening of the

mounting screws will cause severe physical stress resulting

in cracked die and catastrophic IC failure. The recom-

mended mounting screw size is M3 with a maximum torque

of 50 N-cm. Additionally, it is best to use washers under the

screws to distribute the force over a wider area or a screw

with a wide flat head. To further distribute the mounting force

a solid mounting bar in front of the package and secured in

place with the two mounting screws may be used. Other

mounting options include a spring clip. If the package is

secured with pressure on the front of the package the maxi-

mum pressure on the molded plastic should not exceed

150N/mm

Additionally, if the mounting screws are used to force the

package into correct alignment with the heat sink, package

stress will be increased. This increase in package stress will

result in reduced contact area with the heat sink increasing

die operating temperature and possible catastrophic IC fail-

ure.

LAYOUT, GROUND LOOPS AND STABILITY

The LM4780 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 LM4780 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 LM4780 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

2, results in a voltage difference between the

two points as shown below.

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