LM4780 Datasheet, PDF (21/31 Pages) National Semiconductor (TI) – Audio Power Amplifier Series Stereo 60W, Mono 120W Audio Power Amplifier with Mute

LM4780, LM4780TABD
www.ti.com
SNAS193B – JULY 2003 – REVISED APRIL 2013
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 supplies. 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 resistance from the package case to the heat sink (θ_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 recommended
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 maximum pressure on the molded plastic should not exceed 150N/mm².
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 failure.
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 currents 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 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 problems 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 occurring 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, R₂, results in a voltage difference between the two points as
shown below.
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