www.ti.com
ꢀ ꢁꢂ ꢃꢄꢄ ꢅꢆ ꢇ
SLOS407D − FEBRUARY 2003 − REVISED AUGUST 2004
OUTP
OUTN
Output = 0 V
Differential
+12 V
0 V
Voltage
Across
Load
−12 V
Current
OUTP
OUTN
Output > 0 V
Differential
Voltage
Across
Load
+12 V
0 V
−12 V
Current
Figure 38. The TPA3004D2 Output Voltage and Current Waveforms Into an Inductive Load
Efficiency: LC Filter Required With the Traditional Class-D Modulation Scheme
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 , and the time at each voltage
CC
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 TPA3004D2 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
instead of 2 × V . As the output power increases, the pulses widen,
CC
CC
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.
Effects of Applying a Square Wave Into a Speaker
Audio specialists have advised for years not to apply a square wave to speakers. If the amplitude of the
waveform is high enough and the frequency of the square wave is within the bandwidth of the speaker, the
square wave could cause the voice coil to jump out of the air gap and/or scar the voice coil. A 250-kHz switching
2
frequency, however, does not significantly move the voice coil, as the cone movement is proportional to 1/f
for frequencies beyond the audio band.
23
TI [ TEXAS INSTRUMENTS ]
