LM4733 datasheet(17/23 Pages) NSC

17 / 23 page

Application Information (Continued)

ments. This is often the case when comparing integrated

circuit designs to discrete amplifier designs. Discrete transis-

tor amps often “run out of gain” at high frequencies and

therefore have small bandwidths to noise as indicated below.

20079499

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 17). 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.

Note 17: CCIR/ARM: A Practical Noise Measurement Method; by Ray

Dolby, David Robinson and Kenneth Gundry, AES Preprint No. 1353 (F-3).

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.

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