tda2075a Tripath Technology Inc., tda2075a Datasheet - Page 22

• Manufacturer Part Number: tda2075a
• Description: Stereo Class-t Digital Audio Amplifier Driver Using Digital Power Processingtm
• Manufacturer: Tripath Technology Inc.

T r i p a t h T e c h n o l o g y , I n c . - T e c h n i c a l I n f o r m a t i o n
Low-frequency Power Supply Pumping
A potentially troublesome phenomenon in single-ended switching amplifiers is power supply pumping.
This phenomenon is caused by current from the output filter inductor flowing into the power supply output
filter capacitors in the opposite direction as a DC load would drain current from them. Under certain
conditions (usually low-frequency input signals), this current can cause the supply voltage to “pump”
(increase in magnitude) and eventually cause over-voltage/under-voltage shut down. Moreover, since
over/under-voltage are not “latched” shutdowns, the effect would be an amplifier that oscillates between
on and off states. If a DC offset on the order of 0.3V is allowed to develop on the output of the amplifier
(see “DC Offset Adjust”), the supplies can be boosted to the point where the amplifier’s over-voltage
protection triggers.
One solution to the pumping issue is to use large power supply capacitors to absorb the pumped supply
current without significant voltage boost. The low-frequency pole used at the input to the amplifier
determines the value of the capacitor required. This works for AC signals only.
A no-cost solution to the pumping problem uses the fact that music has low frequency information that is
correlated in both channels (it is in phase). This information can be used to eliminate boost by putting the
two channels of a TDA2075A amplifier out of phase with each other. This works because each channel is
pumping out of phase with the other, and the net effect is a cancellation of pumping currents in the power
supply. The phase of the audio signals needs to be corrected by connecting one of the speakers in the
opposite polarity as the other channel.
Performance Measurements of a TDA2075A Amplifier
Tripath amplifiers operate by modulating the input signal with a high-frequency switching pattern. This
signal is sent through a low-pass filter (external to the TDA2075A) that demodulates it to recover an
amplified version of the audio input. The frequency of the switching pattern is spread spectrum and
typically varies between 200kHz and 1.5MHz, which is well above the 20Hz – 22kHz audio band. The
pattern itself does not alter or distort the audio input signal but it does introduce some inaudible noise
components.
The measurements of certain performance parameters, particularly those that have anything to do with
noise, like THD+N, are significantly affected by the design of the low-pass filter used on the output of the
TDA2075A and also the bandwidth setting of the measurement instrument used. Unless the filter has a
very sharp roll-off just past the audio band or the bandwidth of the measurement instrument ends there,
some of the inaudible noise components introduced by the Tripath amplifier switching pattern will get
integrated into the measurement, degrading it.
Tripath amplifiers do not require large multi-pole filters to achieve excellent performance in listening tests,
usually a more critical factor than performance measurements. Though using a multi-pole filter may
remove high-frequency noise and improve THD+N type measurements (when they are made with wide-
bandwidth measuring equipment), these same filters can increase distortion due to inductor non-linearity.
Multi-pole filters require relatively large inductors, and inductor non-linearity increases with inductor value.
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