Datasheet

FCCClassB
f-Frequency-Hz
830M
LimitLevel-dB V/mm
30M
20
230M 430M 630M
0
40
10
60
30
70
50
TPA3110D2
www.ti.com SLOS528D –JULY 2009–REVISED JULY 2012
Also, it is important that the ferrite bead is large enough to maintain its impedance at the peak currents expected
for the amplifier. Some ferrite bead manufacturers specify the bead impedance at a variety of current levels. In
this case it is possible to make sure the ferrite bead maintains an adequate amount of impedance at the peak
current the amplifier will see. If these specifications are not available, it is also possible to estimate the bead
current handling capability by measuring the resonant frequency of the filter output at low power and at maximum
power. A change of resonant frequency of less than fifty percent under this condition is desirable. Examples of
ferrite beads which have been tested and work well with the TPA3110D2 include 28L0138-80R-10 and
HI1812V101R-10 from Steward and the 742792510 from Wurth Electronics.
A high quality ceramic capacitor is also needed for the ferrite bead filter. A low ESR capacitor with good
temperature and voltage characteristics will work best.
Additional EMC improvements may be obtained by adding snubber networks from each of the class D outputs to
ground. Suggested values for a simple RC series snubber network would be 10 Ω in series with a 330 pF
capacitor although design of the snubber network is specific to every application and must be designed taking
into account the parasitic reactance of the printed circuit board as well as the audio amp. Take care to evaluate
the stress on the component in the snubber network especially if the amp is running at high PVCC. Also, make
sure the layout of the snubber network is tight and returns directly to the PGND or the PowerPad™ beneath the
chip.
Figure 40. TPA3110D2 EMC spectrum with FCC Class B Limits
Efficiency: LC Filter Required With the Traditional Class-D Modulation Scheme
The main reason that the traditional class-D amplifier needs an output filter is that the switching waveform results
in maximum current flow. This causes more loss in the load, which causes lower efficiency. The ripple current is
large for the traditional modulation scheme, because the ripple current is proportional to voltage multiplied by the
time at that voltage. The differential voltage swing is 2 × V
CC
, and the time at each voltage is half the period for
the traditional modulation scheme. An ideal LC filter is needed to store the ripple current from each half cycle for
the next half cycle, while any resistance causes power dissipation. The speaker is both resistive and reactive,
whereas an LC filter is almost purely reactive.
The TPA3110D2 modulation scheme has little loss in the load without a filter because the pulses are short and
the change in voltage is V
CC
instead of 2 × V
CC
. As the output power increases, the pulses widen, making the
ripple current larger. Ripple current could be filtered with an LC filter for increased efficiency, but for most
applications the filter is not needed.
An LC filter with a cutoff frequency less than the class-D switching frequency allows the switching current to flow
through the filter instead of the load. The filter has less resistance but higher impedance at the switching
frequency than the speaker, which results in less power dissipation, therefore increasing efficiency.
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