TPA3100D2-Q1_15 Datasheet, PDF (18/38 Pages) Texas Instruments – 20-W STEREO CLASS-D AUDIO POWER AMPLIFIER

TPA3100D2-Q1
SLOS557 – SEPTEMBER 2008 ......................................................................................................................................................................................... www.ti.com
OUTP
OUTN
Differential
Voltage
Across
Load
+12 V
0V
-12 V
Current
Output = 0 V
OUTP
OUTN
Differential
Voltage
Across
Load
+12 V
0V
-12 V
Output > 0 V
Current
Figure 24. The TPA3100D2 Output Voltage and Current Waveforms Into an Inductive Load
Efficiency: LC Filter Required With Traditional Class-D Modulation Scheme
The main reason that the traditional class-D amplifier needs an output filter is that the switching waveform results
in maximum current flow. This causes more loss in the load, which causes lower efficiency. The ripple current is
large for the traditional modulation scheme, because the ripple current is proportional to voltage multiplied by the
time at that voltage. The differential voltage swing is 2 × V_CC, and the time at each voltage is half the period for
the traditional modulation scheme. An ideal LC filter is needed to store the ripple current from each half cycle for
the next half cycle, while any resistance causes power dissipation. The speaker is both resistive and reactive,
whereas an LC filter is almost purely reactive.
The TPA3100D2 modulation scheme has little loss in the load without a filter because the pulses are short and
the change in voltage is V_CC instead of 2 × V_CC. As the output power increases, the pulses widen, making the
ripple current larger. Ripple current could be filtered with an LC filter for increased efficiency, but for most
applications the filter is not needed.
An LC filter with a cutoff frequency less than the class-D switching frequency allows the switching current to flow
through the filter instead of the load. The filter has less resistance but higher impedance at the switching
frequency than the speaker, which results in less power dissipation, therefore increasing efficiency.
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