TPA2000D4

STEREO 2W CLASSD AUDIO POWER AMPLIFIER

WITH STEREO HP AMPLIFIER

SLOS337B − DECEMBER 2000 − REVISED NOVEMBER 2005

17

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APPLICATION INFORMATION

effects of applying a square wave into a speaker

Audio specialists have said 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 frequency,

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

minus the theoretical efficiency,

(1)

(2)

(3)

The maximum efficiency of the TPA2000D4 with an 8-

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

maximum power from Figure 2 (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.

when to use an output filter

Design the TPA2000D4 without the filter if the traces from amplifier to speaker are short. The TPA2000D4

passed FCC and CE radiated emissions with no shielding with speaker wires 8 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 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 very low impedance at low frequencies.

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

amplifier to speaker.

gain setting via GAIN0 and GAIN1 inputs

The gain of the TPA2000D4 is set by two input terminals, GAIN0 and GAIN1.

The gains listed in Table 1 are realized by changing the taps on the input resistors inside the amplifier. This

by ratios of resistors, so the actual gain distribution from part-to-part is quite good. However, the input

impedance may shift by 30% due to shifts in the actual resistance of the input resistors.

For design purposes, the input network (discussed in the next section) should be designed assuming an input

impedance of 20 k

Ω, which is the absolute minimum input impedance of the TPA2000D4. At the higher gain

settings, the input impedance could increase as high as 115 k

Ω.