8.2.2.2 Efficiency: LC Filter Required with the Traditional Class-D Modulation Scheme

The main reason that the traditional class-D amplifier requires 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 × VCC, and the time at each voltage is

half the period for the traditional modulation scheme. An ideal LC filter is required 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 TPA3139D2 modulation scheme has little loss in the load without a filter because the pulses are short and

the change in voltage is VCC instead of 2 × VCC. 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 required.

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.

8.2.2.3 When to Use an Output Filter for EMI Suppression

The TPA3139D2 device has been tested with a simple ferrite bead filter for a variety of applications including

long speaker wires up to 100 cm and high power. The TPA3139D2 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, 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.

1nF

Ferrite

ChipBead

OUTP

OUTN

Ferrite

ChipBead

1nF

Figure 8-3. Typical Ferrite Chip Bead Filter (Chip Bead Example: NFZ2MSM series from Murata)

1 mF

1 mF

33

H

m

33 mH

OUTP

OUTN

L1

L2

C2

C3

Figure 8-4. Typical LC Output Filter, Cutoff Frequency of 27 kHz, Speaker Impedance = 8 Ω

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TPA3139D2

SLOS810A – OCTOBER 2019 – REVISED AUGUST 2020

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