Datasheet

TPA2006D1 Modulation Scheme
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
TPA2006D1
SLOS498A – SEPTEMBER 2006 – REVISED JULY 2008 ................................................................................................................................................. www.ti.com
The TPA2006D1 uses a modulation scheme that still has each output switching from 0 to the supply voltage.
However, OUT+ and OUT- are now in phase with each other with no input. The duty cycle of OUT+ is greater
than 50% and OUT- is less than 50% for positive voltages. The duty cycle of OUT+ is less than 50% and OUT- is
greater than 50% for negative voltages. The voltage across the load sits at 0 volts throughout most of the
switching period greatly reducing the switching current, which reduces any I
2
R losses in the load.
Figure 34. The TPA2006D1 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 TPA2006D1 modulation scheme has little loss in the load without a filter because the pulses are 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 that results in less power
dissipated, which increases efficiency.
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