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EFFECTS OF APPLYING A SQUARE WAVE INTO A SPEAKER
PSPKR = PSUP – PSUP THEORETICAL (at max output power)
(1)
PSPKR = POUT(PSUP / POUT – PSUP THEORETICAL / POUT) (at max output power)
(2)
PSPKR = POUT(1/ηMEASURED – 1/ηTHEORETICAL) (at max output power)
(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
significant because the speaker cone movement is proportional to 1/f2 for frequencies beyond the audio band.
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
supplied power (PSUP THEORETICAL) from the actual supply power (PSUP) at maximum output power (POUT). The
switching power dissipated in the speaker is the inverse of the measured efficiency (
η
MEASURED) minus the
theoretical efficiency (
η
THEORETICAL) all multiplied by POUT.
The maximum efficiency of the TPA2000D1 with an 8-
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.
L1 = L2 = 22 H (DCR = 110 m
, part number = SCD0703T-220 M-S, manufacturer = GCI)
C1 = C2 = 1 F
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