LM4780TABD National Semiconductor, LM4780TABD Datasheet - Page 19

• Manufacturer Part Number: LM4780TABD
• Description: BOARD EVALUATION LM4780TA
• Manufacturer: National Semiconductor
• Series: Overture™r

Specifications of LM4780TABD

• Amplifier Type: Class AB
• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 60W x 2 @ 8 Ohm
• Voltage - Supply: 20 V ~ 84 V, ±10 V ~ 42 V
• Operating Temperature: -20°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4780
• Lead Free Status / RoHS Status: Not applicable / Not applicable

Integrated circuits have additional open loop gain allowing
additional feedback loop gain in order to lower harmonic dis-
tortion and improve frequency response. It is this additional
bandwidth that can lead to erroneous signal-to-noise mea-
surements if not considered during the measurement pro-
cess. 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 theoretical
difference in the signal-to-noise ratio (white noise is propor-
tional 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
quency 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 different
noise readings. Meter responses include:
1.
2.
3.
4.
Although theoretical noise analysis is derived using true RMS
based calculations, most actual measurements are taken with
ARM (Average Responding Meter) test equipment.
Typical signal-to-noise figures are listed for an A-weighted fil-
ter 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 re-
duced 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
RMS reading,
average responding,
peak reading, and
quasi peak reading.
(Note
16). A “weighting” filter alters the fre-
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19
with the heat sink will increase the thermal resistance from
the package case to the heat sink (θ
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 recommended mounting
screw size is M3 with a maximum torque of 50 N-cm. Addi-
tionally, 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 in-
clude a spring clip. If the package is secured with pressure on
the front of the package the maximum 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 voltages
on the conductor which can effectively act as signals at the
input, resulting in high frequency oscillation or excessive dis-
tortion. It is advisable to keep the output compensation com-
ponents 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 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
shown below.
2
, results in a voltage difference between the two points as
2
.
CS
) resulting in higher
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