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APPLICATION INFORMATION
CLASS-D OPERATION
Traditional Class-D Modulation Scheme
0 V
0 V
-12 V
-12 V
+12 V
+12 V
Current
OUTP
Differential Voltage
Across Load
OUTN
0 V
Supply Pumping
Gain setting via GAIN0 and GAIN1 inputs
TPA3122D2
SLOS527A – DECEMBER 2007 – REVISED DECEMBER 2007
This section focuses on the class-D operation of the TPA3122D2.
The TPA3122D2 operates in AD mode. There are two main configurations that may be used. For stereo
operation, the TPA3122D2 should be configured in a single-ended (SE) half bridge amplifier. For mono
applications, TPA3122D2 may be used as a bridge tied load (BTL) amplifier. The traditional class-D modulation
scheme, which is used in the TPA3122D2 BTL configuration, 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 pre-filtered
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 25
.
Figure 25. Traditional Class-D Modulation Scheme's Output Voltage and Current Waveforms into an
Inductive Load With No Input
One issue encountered in single-ended (SE) class-D amplifier designs is supply pumping. Power-supply pumping
is a rise in the local supply voltage due to energy being driven back to the supply by operation of the class-D
amplifier. This phenomenon is most evident at low audio frequencies and when both channels are operating at
the same frequency and phase. At low levels, power-supply pumping results in distortion in the audio output due
to fluctuations in supply voltage. At higher levels, pumping can cause the overvoltage protection to operate,
which temporarily shuts down the audio output.
Several things can be done to relieve power-supply pumping. The lowest impact is to operate the two inputs out
of phase 180
°
and reverse the speaker connections. Because most audio is highly correlated, this causes the
supply pumping to be out of phase and not as severe. If this is not enough, the amount of bulk capacitance on
the supply must be increased. Also, improvement is realized by hooking other supplies to this node, thereby,
sinking some of the excess current. Power-supply pumping should be tested by operating the amplifier at low
frequencies and high output levels.
The gain of the TPA3122D2 is set by two input terminals, GAIN0 and GAIN1.
The gains listed in
Table 1
are realized by changing the taps on the input resistors and feedback resistors inside
the amplifier. This causes the input impedance (Z
I
) to be dependent on the gain setting. The actual gain settings
are controlled by ratios of resistors, so the gain variation from part-to-part is small. However, the input impedance
from part-to-part at the same gain may shift by ±20% due to shifts in the actual resistance of the input resistors.
10
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