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Effects of Applying a Square Wave Into a Speaker
Efficiency (theoretical, %) RL
RLrds(on)100% 8
(8 1.3) 100% 86%
(1)
P(total) PO
Efficiency 8.5 W
0.86 9.88 W
(2)
Other losses P(total)(measured) P(total) (theoretical) 10.49 9.88 0.61 W
(3)
P(dis) 0.61 W (12 V 22 mA) 0.35 W
(4)
When to Use an Output Filter for EMI Suppression
TPA3008D2
SLOS435A – MAY 2004 – REVISED JULY 2004
APPLICATION INFORMATION (continued)
Audio specialists have advised for years not to apply a square wave to speakers. If the amplitude of thewaveform is high enough and the frequency of the square wave is within the bandwidth of the speaker, thesquare wave could cause the voice coil to jump out of the air gap and/or scar the voice coil. A 250-kHz switchingfrequency, however, does not significantly move the voice coil, as the cone movement is proportional to 1/f
2
forfrequencies beyond the audio band.
Damage may occur if the voice coil cannot handle the additional heat generated from the high-frequencyswitching current. The amount of power dissipated in the speaker may be estimated by first considering theoverall efficiency of the system. If the on-resistance (rds(on)) of the output transistors is considered to cause thedominant loss in the system, then the maximum theoretical efficiency for the TPA3008D2 with an 8-Ωload is asfollows:
The maximum measured output power is approximately 8.5 W with an 12-V power supply. The total theoreticalpower supplied (P(total)) for this worst-case condition would therefore be as follows:
The efficiency measured in the lab using an 8-Ωspeaker was 81%. The power not accounted for as dissipatedacross the r
DS(on)
may be calculated by simply subtracting the theoretical power from the measured power:
The quiescent supply current at 12 V is measured to be 22 mA. It can be assumed that the quiescent currentencapsulates all remaining losses in the device, i.e., biasing and switching losses. It may be assumed that anyremaining power is dissipated in the speaker and is calculated as follows:
Note that these calculations are for the worst-case condition of 8.5 W delivered to the speaker. Because the 0.35W is only 4% of the power delivered to the speaker, it may be concluded that the amount of power actuallydissipated in the speaker is relatively insignificant. Furthermore, this power dissipated is well within thespecifications of most loudspeaker drivers in a system, as the power rating is typically selected to handle thepower generated from a clipping waveform.
Design the TPA3008D2 without the filter if the traces from amplifier to speaker are short (< 50 cm). Poweredspeakers, where the speaker is in the same enclosure as the amplifier, is a typical application for class-D withouta filter.
Most applications require a ferrite bead filter. The ferrite filter reduces EMI around 1 MHz and higher (FCC andCE only test radiated emissions greater than 30 MHz). When selecting a ferrite bead, choose one with highimpedance at high frequencies, but low impedance at low frequencies.
Use a LC output filter if there are low frequency (<1 MHz) EMI-sensitive circuits and/or there are long wires fromthe amplifier to the speaker.
When both an LC filter and a ferrite bead filter are used, the LC filter should be placed as close as possible tothe IC followed by the ferrite bead filter.
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