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Josey Angili

Aug 12, 2015

tpa3140d2pwpevm

audioportal

Class-D
audio power amplifier

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Related

Improve Class-D EMI to downsize BOM cost without compromising
audio performance

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Designers frequently choose Class-D audio amplifiers to drive the speakers in
a variety of mid-power applications like TVs, Bluetooth® speakers and
laptops. After all, when compared to conventional Class-AB, Class-D has lower
heat dissipation and relatively high efficiency (for increased battery life).
Class-D is also beneficial if compact board space is important.

The biggest challenge associated with Class-D is electromagnetic interference
(EMI). External inductor-capacitor filtering is traditionally used to mitigate
EMI, but it adds cost, area and complexity to end equipment.

TI has developed several closed-loop amplifiers including the TPA3110 (released in 2010), which made significant
improvements to EMI by using advanced closed-loop power stages. TI has also just
released the TPA3140 Class-D audio power amplifier, which includes
several innovations that help provide true inductor-free performance even for
speaker cables up to 1m in length. This inductor-free device is already in
production in LCD TVs, where long speaker cables make meeting EMI requirements a
challenge.

Edge-rate control

One method used to reduce EMI radiation is to reduce the slew rate of the
amplifier output transitions. Since the TPA3140 uses a proprietary high-performance feedback
topology, a reduction in slew rate will not degrade total harmonic distortion
(THD) or audio quality. The fast Fourier transform (FFT) image in Figure 1 shows
a reduction in high-frequency content with slower edges.

Figure 1: EMI plots without edge rate (red) and with edge
rate (yellow)

Spread-spectrum clocking

While edge-rate control is an effective means of attenuating EMI when it
arises in frequency ranges greater than 30MHz, it does not address the
fundamental carrier frequency of the Class-D amplifier’s switching output and
its related harmonics, which fall in the range below 30MHz.

The TPA3140 includes a proprietary algorithm that adds a small
amount of frequency modulation to the amplifier’s clock circuitry. This
algorithm doesn’t affect the amplified audio quality, but significantly reduces
peak energy of the switching frequency.

EMI results

Figure 2 represents EMI test results from a TV with a close to 1m speaker
cable length. The red line is the quasi-peak limit, and the green line is the
average limit.

Figure 2: EMI plot:  blue quasi-peak and green is
average curve

Audio performance:

<0.05% THD+N at 1 W/4 Ω/1 kHz

<65-µV A-wgt output noise

In conclusion, the TPA3140 Class-D audio power amplifier provides significant
improvement in EMI that allows inductor-free operation providing major BOM cost
savings without compromising audio quality.

Additional resources

Work with the TPA3140 evaluation module (EVM).

Learn more about our other Class-D audio power amplifiers.

Visit the TI E2E™ CommunityAudio Amplifiers forum, where you can search for
solutions, get help, share knowledge and solve problems with fellow engineers
and TI experts.

Rocky Chen

Great innovation! My customer loves it.

Aug 13, 2015

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