Tr i path Technol ogy, I nc. - Techni cal I nfor m ati on
The OEM may benefit from some experimentation in the filter design, but the values provided in the
reference design, 11.3uH and 0.22uF (nominal resonant frequency of 101kHz), provide excellent results
for most loads between 4Ω and 8Ω.
As important as the values themselves, the material used in the core is important to the performance of
the filter. Core materials that saturates too easily will not provide acceptable distortion or efficiency
figures.
Tripath recommends a low-mu core, like type 2, iron powder cores.
Micrometals,
(www.micrometals.com), is a main supplier of iron powder cores. The core part number used on the RB-
TA3020 is T106-2 and is wound with 29 turns of 16AWG wire.
The Zobel circuits R117/C109 and R217/C209 are there in case an amplifier is powered up with no load
attached. The Q of the LC output filter, with no load attached, rises quickly out to 80kHz. Resonant
currents in the filter and ringing on the output could reduce the reliability of the amplifier. The Zobel
eliminates these problems by reducing the Q of the network significantly above 50kHz. Modifying the LC
output filter should not require a recalculation of the Zobel value, though depending on application, the
power capability of R117 and R217 may need to be increased to 5W from 2W. The components used on
the reference board should be quite adequate for almost all applications.
The bypass capacitors C105/C205 are critical to the reduction of ringing on the outputs of the FETs.
These parts are placed as closely as possible to the leads of the FETs, and the leads of the capacitors
themselves are as short as practical. Their values will not change with different output FETs.
Connection Diagram for Bridge Mode Operation
The amplifier is connected to the power supplies and load as shown in Figure 9. Note that an inverter
has been added in front of one of the channel inputs (i.e. Channel 2). The main reason for processing the
channels out of phase is to avoid potential problems with switching power supplies, but it also simplifies
the connections for bridged-mode operation. For bridged operation, simply connect the “-“ terminal to the
output of the inverted channel* (Channel 1 output, J101 pin 1) and the “+” terminal to the output of the
non-inverted channel with respect to the input signal (Channel 2 output, J201 pin 2).
The connection shown in Figure 9 is the easiest way to use the RB-TA3020 to test bridged mode
operation. If the evaluation setup does not provide two out of phase signals, there is another way to
evaluate bridge mode operation. This requires the RB-TA3020 to be modified.
Change R201 to 20KΩ from 49.9KΩ. Connect Pin 26 (OAOUT1) to IN2 on J200. The input signal is still
applied to (J100) and is inverted on chip thus providing the input signal for Channel 2 via J200. If the gain
of the system needs to be modified, R101 can be adjusted. R201 should be left at 20KΩ. If stereo
operation is again desired, then the value of R101 and R201 must be made the same to ensure nominal
gain for both channels. Additionally, the connection between Pin 26 and IN2 on J200 should be removed.
9
RB-TA3020, Rev. 3.0/03.02
TRIPATH [ TRIPATH TECHNOLOGY INC. ]
