Texas Instruments TPA3008D2 - Manual (Page 13)

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CLASS-D OPERATION
Traditional Class-D Modulation Scheme
0 V
−12 V
+12 V
Current
OUTP
Differential Voltage
Across Load
OUTN
TPA3008D2 Modulation Scheme
TPA3008D2
SLOS435A – MAY 2004 – REVISED JULY 2004
APPLICATION INFORMATION (continued)
This section focuses on the class-D operation of the TPA3008D2.
The traditional class-D modulation scheme, which is used in the TPA032D0x family, has a differential output
where each output is 180 degrees out of phase and changes from ground to the supply voltage, V
CC
. Therefore,
the differential prefiltered output varies between positive and negative V
CC
, where filtered 50% duty cycle yields
0 V across the load. The traditional class-D modulation scheme with voltage and current waveforms is shown in
Figure 17. Note that even at an average of 0 V across the load (50% duty cycle), the current to the load is high,
causing high loss and thus causing a high supply current.
Figure 17. Traditional Class-D Modulation Scheme's Output Voltage and Current Waveforms Into an
Inductive Load With No Input
The TPA3008D2 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.
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