Tr i path Technol ogy, I nc. - Techni cal I nfor m ati on
The resistors, R
OCR
in Figure 6 (labeled R111 and R211 in the schematic), set the overcurrent threshold
for the output devices. Note that these are NOT the sense resistors (the overcurrent sense resistors, R
S
,
are in the output stage). By adjusting the R
OCR
resistor values, the threshold at which the amplifier “trips”
can be changed. The range that the overcurrent trip point can be adjusted (by changing R
OCR
) is
determined by the value of the sense resistors.
R
OCR
on this reference board is pre-set to 20KΩ for a 4Ω single-ended application. For lower impedance
applications (i.e. 4Ω bridged), this board’s overcurrent may trip prematurely. This is indicated by the
amplifier going into mute; to clear, toggle the mute or cycle the power. To reduce overcurrent sensitivity,
decrease the value of R
OCR
until the sensitivity meets the desired level. R
OCR
can be reduced, though if
set too low of a value, this may result in an overcurrent threshold that is so high the amplifier will try to
drive a short circuit, possibly damaging the output FETs.
Finally, the Break-Before-Make (or “BBM”) lines are used to control the “dead time” of the output FETs.
The “dead time” is the period of time between the turn-off of one device and the turn-on of the opposite
device on the same channel. If the two devices are both on at the same time, current “shoots through”
from one supply to the other, bypassing the load altogether. Obviously, this will have a great impact on
the overall efficiency of the amplifier. However, if the dead time is too long, linearity suffers. The
optimum BBM setting will change with different output FETs, different operating voltages, different layouts
and different performance requirements. For this reason, Tripath has provided a means to adjust the
BBM0 setting, via jumper J2, on the 3-pin header (see Figure 6). Please note that BBM1 is hardwired to
“0” on the RB-TA3020 since operating the reference board with BBM delays of 40nS or less will result in
significant shoot through current and possible MOSFET destruction.
These settings should be verified over the full temperature and load range of the application to ensure
that any thermal rise of the output FETs and TA3020 does not impact the performance of the amplifier.
This amplifier board is set to 80nS, and the table below shows the BBM values for various settings of the
jumpers (Figure 7).
BM1
BM0
Delay
0
1
0
1
120nS
80nS
40nS
0nS
J3
0
1)
2)
3)
4)
0
0
1
1
J2
0
1
1
BB
delay
BBM1
Note: The default
M0
jumper setting is 80nS.
Figure 7
7
RB-TA3020, Rev. 3.0/03.02
TRIPATH [ TRIPATH TECHNOLOGY INC. ]
