FCCClassB

f-Frequency-Hz

830M

30M

20

230M

430M

630M

0

40

10

60

30

70

50

TPA3110D2

www.ti.com

SLOS528E – JULY 2009 – REVISED NOVEMBER 2015

Feature Description (continued)

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 36. TPA3110D2 EMC Spectrum With FCC Class B Limits

9.3.1.2 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

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

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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