TPA3003D2

SLOS406A − FEBRUARY 2003 − REVISED MARCH 2003

17

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APPLICATION INFORMATION

effects of applying a square wave into a speaker

Audio specialists have advised for years 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

frequencies beyond the audio band.

Damage may occur if the voice coil cannot handle the additional heat generated from the high-frequency

switching current. The amount of power dissipated in the speaker may be estimated by first considering the

dominant loss in the system, then the maximum theoretical efficiency for the TPA3003D2 with an 8-

Ω load is

as follows:

Efficiency (theoretical, %)

+ R

L

R

L )

r

ds(on)

100%

+ 8 (8 ) 1.4)

100%

+ 85.11%

The maximum measured output power is approximately 3 W with an 12-V power supply. The total theoretical

P

(total) +

P

O

Efficiency

+ 3W 0.8511 + 3.52 W

The efficiency measured in the lab using an 8-

Ω speaker was 75%. The power not accounted for as dissipated

Other losses

+ P

(total)

(measured)

* P

(total)

(theoretical)

+ 4 * 3.52 + 0.48 W

The quiescent supply current at 12 V is measured to be 28.5 mA. It can be assumed that the quiescent current

encapsulates all remaining losses in the device, i.e., biasing and switching losses. It may be assumed that any

remaining power is dissipated in the speaker and is calculated as follows:

P

(dis) +

0.48 W

* (12 V

28.5 mA)

+ 0.14 W

Note that these calculations are for the worst-case condition of 3 W delivered to the speaker. Since the 0.14 W

is only 5% of the power delivered to the speaker, it may be concluded that the amount of power actually

dissipated in the speaker is relatively insignificant. Furthermore, this power dissipated is well within the

specifications of most loudspeaker drivers in a system, as the power rating is typically selected to handle the

power generated from a clipping waveform.

when to use an output filter

Design the TPA3003D2 without the filter if the traces from amplifier to speaker are short (< 1 inch). Powered

speakers, where the speaker is in the same enclosure as the amplifier, is a typical application for class-D without

a filter.

Most applications require a ferrite bead filter. The ferrite filter reduces EMI around 1 MHz and higher (FCC and

CE only test radiated emissions greater than 30 MHz). When selecting a ferrite bead, choose one with high

impedance at high frequencies, but very 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 from

the amplifier to the speaker.

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