Datasheet

APPLICATION INFORMATION
CLASS-D OPERATION
Traditional Class-D Modulation Scheme
+V
CC
0V
OutputCurrent
Supply Pumping
Gain Setting via GAIN0 and GAIN1 Inputs
TPA3125D2
www.ti.com ........................................................................................................................................................................................... SLOS611 – DECEMBER 2008
This section focuses on the class-D operation of the TPA3125D2.
The TPA3125D2 operates in AD mode. There are two main configurations that may be used. For stereo
operation, the TPA3125D2 should be configured in a single-ended (SE) half-bridge amplifier. For mono
applications, TPA3125D2 may be used as a bridge-tied-load (BTL) amplifier. The traditional class-D modulation
scheme, which is used in the TPA3125D2 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 prefiltered
output varies between positive and negative V
CC
, where filtered 50% duty cycle yields
0 V across the load. The class-D modulation scheme with voltage and current waveforms is shown in Figure 26 .
Figure 26. Class-D Modulation for TPA3125D2 SE Configuration
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 TPA3125D2 is set by two input terminals, GAIN0 and GAIN1.
The gains listed in Table 2 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.
For design purposes, the input network (discussed in the next section) should be designed assuming an input
impedance of 8 k Ω , which is the absolute minimum input impedance of the TPA3125D2. At the higher gain
settings, the input impedance could increase as high as 72 k Ω .
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