14

LT6301

sn6301 6301f

APPLICATIO S I FOR ATIO

from the line is sensed across the back termination resis-

tors. With positive feedback, signals are present on both

Extra gain may be required in the receive channel to

compensate, or a completely separate receive path may be

implemented through a separate line coupling transformer.

Considerations for Fault Protection

The basic line driver design, shown on the front page of

this data sheet, presents a direct DC path between the

outputs of the two amplifiers. An imbalance in the DC

biasing potentials at the noninverting inputs through

either a fault condition or during turn-on of the system can

create a DC voltage differential between the two amplifier

outputs. This condition can force a considerable amount

of current to flow as it is limited only by the small valued

back-termination resistors and the DC resistance of the

transformer primary. This high current can possibly cause

the power supply voltage source to drop significantly

impacting overall system performance. If left unchecked,

the high DC current can heat the LT6301 to thermal

shutdown.

Using DC blocking capacitors, as shown in Figure 15, 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 LT6301 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

Figure 15. Protecting the Driver Against Load Faults and Line Transients

6301 F15

+

–

1/4

LT6301

–IN

–

+

1/4

LT6301

+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