Cirrus Logic CRD44800-ST-FB - Manual (Page 10)

CRD44800-ST-FB
10
2.3 Electromagnetic Interference (EMI)
The EMI challenges that face a maker of Class-D amplifiers are largely the same challenges
that have been faced by the switch mode power supply industry for many years. The numer-
ous EMI consulting firms that have arisen and the many books that have been written on the
subject indicate the scope of potential problems and available solutions. They should be con-
sidered a resource - most makers of switch mode equipment would benefit from developing
a working relationship with a qualified EMI lab and from bringing their experience to bear on
design issues, preferably early in the design process.
This reference design from Cirrus Logic is a board level solution which is meant to control
emissions by minimizing and suppressing them at the source in contrast to containing them
in an enclosure.
The EMI requirements for an amplifier have added dimensions beyond those imposed on
power supplies. Audio amplifiers are usually located in close proximity to radio receivers, par-
ticularly AM receivers which are notoriously sensitive to interference. Amplifiers also need to
operate with speaker leads of unpredictable length and construction which make it possible
for any high frequency currents that appear on the outputs to generate nuisance emissions.
The criteria for judging successful EMI control is not as well defined for amplifier design as it
is for power supplies. While the techniques of measuring conducted and radiated emissions
are similar for both types of products, power supplies have a number of clearly defined (and
legally imposed) thresholds that are useful mainly as guidelines when testing amplifiers.
2.3.1 Suppression of EMI at the Source
Several techniques are used in the circuit design and board layout to minimize high fre-
quency fields in the immediate vicinity of the high power components. Specific techniques
include the following:
• As was mentioned in Section 2.2, effective power supply decoupling of high frequency
currents, and minimizing the loop area of the decoupling loop is one aspect of minimiz-
ing EMI.
• Each output of the STA500 includes “snubbing” components. For example, OUT1 of
U4 includes snubber components R10 (20
Ω), and C46 (330 pF). These components
serve to damp ringing on the switching outputs in the 30-50 MHz range. The snubbing
components should be as close as practical to the output pins to maximize their effec-
tiveness. Again, refer to Figure 11 for the preferred component layout.
• A separate ground plane with a solid electrical connection to the chassis and which
surrounds the speaker output connector should be implemented. This allows the
speaker outputs to be RF decoupled to the chassis just before they exit the chassis
from the speaker connector. Again, refer to Figure 11 for the preferred component lay-
out.
• Make use of source termination resistors on all digital signals whose traces are longer
than about 25 mm.