LT1794 datasheet(15/20 Pages) LINER

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LT1794

Note that the overall gain is increased:

nR R

()

() +

()

[]−+

()

[]

A simpler method of using positive feedback to reduce the

back-termination is shown in Figure 15. In this case, the

drivers are driven differentially and provide complemen-

tary outputs. Grounding the inputs, we see there is invert-

ing gain of –RF/RP from –VO to VA

VA = VO (RF/RP)

and assuming RP >> RL, we require

VA = VO (1 – 1/n)

solving

RF/RP = 1 – 1/n

So to reduce the back-termination by a factor of 3 choose

RF/RP = 2/3. Note that the overall gain is increased to:

VO/VI = (1 + RF/RG + RF/RP)/[2(1 – RF/RP)]

Using positive feedback is often referred to as active

termination.

Figure 17 shows a full-rate ADSL line driver incorporating

positive feedback to reduce the power lost in the back

termination resistors by 40% yet still maintains the proper

impedance match to the100

Ω characteristic line imped-

ance. This circuit also reduces the transformer turns ratio

over the standard line driving approach resulting in lower

peak current requirements. With lower current and less

power loss in the back termination resistors, this driver

dissipates only 1W of power, a 30% reduction.

While the power savings of positive feedback are attractive

there is one important system consideration to be ad-

dressed, received signal sensitivity. The signal received

from the line is sensed across the back termination resis-

tors. With positive feedback, signals are present on both

ends of the RBT resistors, reducing the sensed amplitude.

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.

A demo board, DC306A, is available for the LT1794. This

demo board is a complete line driver with an LT1361

receiver included. It allows the evaluation of both standard

and active termination approaches. It also has circuitry

built in to evaluate the effects of operating with reduced

supply current.

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 LT1794 to thermal

shutdown.

APPLICATIO S I FOR ATIO

–

RBT

–VI

–VA

–VO

RBT

1794 F15

n =

1 –

FOR RBT =

1 +

1 –

()

Figure 15. Back Termination Using Differential Postive Feedback

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