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TPA2036D1 Datasheet(PDF) 19 Page - Texas Instruments

Part No. TPA2036D1
Description  2.5-W MONO CLASS-D AUDIO AMPLIFIER WITH AUTO-RECOVERY
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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TPA2036D1 Datasheet(HTML) 19 Page - Texas Instruments

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0 V
-5 V
+5 V
Current
OUT+
OUT-
Differential
Voltage
Across
Load
0 V
-5 V
+5 V
Current
OUT+
OUT-
Differential
Voltage
Across
Load
Output = 0 V
Output > 0 V
Efficiency: Why You Must Use a Filter With the Traditional Class-D Modulation Scheme
Effects of Applying a Square Wave Into a Speaker
TPA2036D1
www.ti.com.............................................................................................................................................................................................. SLOS589 – OCTOBER 2008
Figure 36. The TPA2036D1 Output Voltage and Current Waveforms Into an Inductive Load
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 × VDD 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 TPA2036D1 modulation scheme has very little loss in the load without a filter because the pulses are very
short and the change in voltage is VDD instead of 2 × VDD. 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 than the speaker that results in less power
dissipated, which increases efficiency.
If the amplitude of a square wave is high enough and the frequency of the square wave is within the bandwidth
of the speaker, a 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, however, is not significant because the speaker cone movement is proportional to
1/f2 for frequencies beyond 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 theoretical supplied power, PSUP THEORETICAL, from the actual supply power, PSUP, at
maximum output power, POUT. The switching power dissipated in the speaker is the inverse of the measured
efficiency,
η
MEASURED, minus the theoretical efficiency, ηTHEORETICAL.
Copyright © 2008, Texas Instruments Incorporated
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