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EFFECTS OF APPLYING A SQUARE WAVE INTO A SPEAKER

(1)

(2)

(3)

WHEN TO USE AN OUTPUT FILTER

C2

C1

L1

L2

OUT–

OUT+

TPA2000D1

SLOS328F – JUNE 2000 – REVISED MARCH 2004

APPLICATION INFORMATION (continued)

Audio specialists advise not to apply a square wave to speakers. If the amplitude of the waveform is high enough

and the frequency of the square wave is within the bandwidth of the speaker, the square wave could cause the

voice coil to jump out of the air gap and/or scar the voice coil. A 250-kHz switching frequency, however, is not

Therefore, the amount of cone movement at the switching frequency is very small. However, damage could

occur to the speaker if the voice coil is not designed to handle the additional power. To size the speaker for

added power, the ripple current dissipated in the load needs to be calculated by subtracting the theoretical

switching power dissipated in the speaker is the inverse of the measured efficiency (

η

theoretical efficiency (

η

The maximum efficiency of the TPA2000D1 with an 8-

Ω load is 85%. Using Equation 3 with the efficiency at

maximum power (78%), we see that there is an additional 106 mW dissipated in the speaker. The added power

dissipated in the speaker is not an issue as long as it is taken into account when choosing the speaker.

Design the TPA2000D1 without the filter if the traces from amplifier to speaker are short. The TPA2000D1

passed FCC and CE radiated emissions with no shielding with speaker wires eight inches long or less. Notebook

PCs and powered speakers where the speaker is in the same enclosure as the amplifier are good applications

for class-D without a filter.

A ferrite bead filter (shown in Figure 11) can often be used if the design is failing radiated emissions without a

filter, and the frequency sensitive circuit is greater than 1 MHz. This is good for circuits that just have to pass

FCC and CE because FCC and CE only test radiated emissions greater than 30 MHz. If choosing a ferrite bead,

choose one with high impedance at high frequencies, but low impedance at low frequencies.

Use an LC output filter if there are low frequency (<1 MHz) EMI sensitive circuits and/or there are long leads

from amplifier to speaker.

The LC output filter is shown in Figure 11.

•

L1 = L2 = 22 µH (DCR = 110 m

Ω, part number = SCD0703T-220 M-S, manufacturer = GCI)

•

C1 = C2 = 1 µF

The ferrite filter is shown in Figure 11, where L is a ferrite bead.

•

L1 = L2 = ferrite bead (part number = MPZ1608S221, manufacturer = TDK)

•

C1 = C2 = 1 nF

Figure 11. Class-D Output Filter

11