Datasheet

ELIMINATING THE OUTPUT FILTER WITH THE TPA2005D1
Effect on Audio
Traditional Class-D Modulation Scheme
0 V
–5 V
+5 V
Current
OUT+
Differential Voltage
Across Load
OUT–
TPA2005D1 Modulation Scheme
TPA2005D1
www.ti.com ............................................................................................................................................................... SLOS369F – JULY 2002 – REVISED JULY 2008
This section focuses on why the user can eliminate the output filter with the TPA2005D1.
The class-D amplifier outputs a pulse-width modulated (PWM) square wave, which is the sum of the switching
waveform and the amplified input audio signal. The human ear acts as a band-pass filter such that only the
frequencies between approximately 20 Hz and 20 kHz are passed. The switching frequency components are
much greater than 20 kHz, so the only signal heard is the amplified input audio signal.
The traditional class-D modulation scheme, which is used in the TPA005Dxx family, has a differential output
where each output is 180 degrees out of phase and changes from ground to the supply voltage, V
DD
. Therefore,
the differential pre-filtered output varies between positive and negative V
DD
, where filtered 50% duty cycle yields
0 volts across the load. The traditional class-D modulation scheme with voltage and current waveforms is shown
in Figure 39 . Note that even at an average of 0 volts across the load (50% duty cycle), the current to the load is
high causing a high loss and thus causing a high supply current.
Figure 39. Traditional Class-D Modulation Scheme's Output Voltage and Current Waveforms Into an
Inductive Load With no Input
The TPA2005D1 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.
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Product Folder Link(s): TPA2005D1