16
LT1794
APPLICATIO S I FOR ATIO
WUUU
Using DC blocking capacitors, as shown in Figure 16, to
AC couple the signal to the transformer eliminates the
possibility for DC current to flow under any conditions.
These capacitors should be sized large enough to not
impair the frequency response characteristics required for
the data transmission.
Another important fault related concern has to do with
very fast high voltage transients appearing on the tele-
phone line (lightning strikes for example). TransZorbs
®
,
varistors and other transient protection devices are often
used to absorb the transient energy, but in doing so also
create fast voltage transitions themselves that can be
coupled through the transformer to the outputs of the line
driver. Several hundred volt transient signals can appear
at the primary windings of the transformer with current
into the driver outputs limited only by the back termination
resistors. While the LT1794 has clamps to the supply rails
at the output pins, they may not be large enough to handle
the significant transient energy. External clamping diodes,
such as BAV99s, at each end of the transformer primary
help to shunt this destructive transient energy away from
the amplifier outputs.
TransZorb is a registered trademark of General Instruments, GSI
1794 F16
+
–
1/2
LT1794
–IN
–
+
1/2
LT1794
+IN
12V
SHDN
–12V
12.7Ω
0.1µF
12V –12V
24.9k
1:2
LINE
LOAD
110Ω
1000pF
110Ω
1k
1k
12.7Ω
SHDNREF
•
•
0.1µF
12V –12V
BAV99
BAV99
Figure 16. Protecting the Driver Against Load Faults and Line Transients
