Datasheet

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TPA2008D2 MODULATION SCHEME
0 V
−5 V
+5 V
Current
OUTP
OUTN
Differential
Voltage
Across
Load
0 V
−5 V
+5 V
Current
OUTP
OUTN
Differential
Voltage
Across
Load
Output = 0 V
Output > 0 V
EFFICIENCY: LC FILTER REQUIRED WITH THE TRADITIONAL CLASS-D MODULATION SCHEME
TPA2008D2
SLOS413C – JULY 2003 – REVISED MAY 2004
APPLICATION INFORMATION (continued)
The TPA2008D2 uses a modulation scheme that still has each output switching from 0 to the supply voltage.
However, OUTP and OUTN are now in phase with each other with no input. The duty cycle of OUTP is greater
than 50% and OUTN is less than 50% for positive output voltages. The duty cycle of OUTP is less than 50% and
OUTN is greater than 50% for negative output voltages. The voltage across the load sits at 0 V throughout most
of the switching period, greatly reducing the switching current, which reduces any I
2
R losses in the load.
Figure 15. The TPA2008D2 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 × V
DD
, 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 TPA2008D2 modulation scheme has very little loss in the load without a filter because the pulses are very
short and the change in voltage is V
DD
instead of 2 × V
DD
. 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, which results in less power
dissipation, therefore increasing efficiency.
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