16

LT1794

APPLICATIO S I FOR ATIO

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

12.7

Ω

0.1

µF

12V –12V

24.9k

1:2

LINE

LOAD

110

Ω

1000pF

110

Ω

1k

1k

12.7

Ω

SHDNREF

0.1

µF

12V –12V

BAV99

BAV99

Figure 16. Protecting the Driver Against Load Faults and Line Transients