T E C H N I C A L I N F O R M A T I O N
TA3020
OAOUT1
V5
RF
CI
RI
-
+
INV1
INPUT
BIASCAP
AGND
V5
20k
20k
+
-
INV2
AGND
OAOUT2
Figure 6: Input Stage Setup for Bridging
The switching outputs, OUT1 and OUT2, are not synchronized, so a common inductor may not be
used with a bridged TA3020. For this same reason, individual zobel networks must be applied to
each output to load each output and lower the Q of each common mode differential LC filter.
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 it 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 TA3020 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.
26
TA3020, Rev 2.1, 01.01
TRIPATH [ TRIPATH TECHNOLOGY INC. ]
