FCCClassB

f-Frequency-Hz

830M

30M

20

230M

430M

630M

0

40

10

60

30

70

50

TPA3110D2-Q1

www.ti.com

SLOS794B – SEPTEMBER 2012 – REVISED SEPTEMBER 2015

Typical Application (continued)

Figure 38. TPA3110D2-Q1 EMC Spectrum With FCC Class-B Limits

8.2.2.3 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 because 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

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-Q1 modulation scheme has little loss in the load without a filter because the pulses are short

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.

8.2.2.4 When to Use an Output Filter for EMI Suppression

The TPA3110D2-Q1 has been tested with a simple ferrite bead filter for a variety of applications including long

speaker wires up to 125 cm and high power. The TPA3110D2-Q1 EVM passes FCC Class-B specifications

under these conditions using twisted speaker wires. The size and type of ferrite bead can be selected to meet

application requirements. Also, the filter capacitor can be increased if necessary with some impact on efficiency.

There may be a few circuit instances where it is necessary to add a complete LC reconstruction filter. These

circumstances might occur if there are nearby circuits which are sensitive to noise. In these cases a classic

second order Butterworth filter similar to those shown in the figures below can be used.

Some systems have little power supply decoupling from the AC line but are also subject to line conducted

interference (LCI) regulations. These include systems powered by wall warts and power bricks. In these cases,

the LC reconstruction filters can be the lowest cost means to pass LCI tests. Common mode chokes using low

frequency ferrite material can also be effective at preventing line conducted interference.

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