Datasheet

15
LT1794
Note that the overall gain is increased:
V
V
RRR
nRRRRR
O
I
PPP
FG P P P
=
+
()
+
()
+
()
[]
−+
()
[]
221
12 1
11 1
/
// / /
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 –R
F
/R
P
from –V
O
to V
A
V
A
= V
O
(R
F
/R
P
)
and assuming R
P
>> R
L
, we require
V
A
= V
O
(1 – 1/n)
solving
R
F
/R
P
= 1 – 1/n
So to reduce the back-termination by a factor of 3 choose
R
F
/R
P
= 2/3. Note that the overall gain is increased to:
V
O
/V
I
= (1 + R
F
/R
G
+ R
F
/R
P
)/[2(1 – R
F
/R
P
)]
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 the100Ω 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 R
BT
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
WUUU
–
+
R
BT
R
F
R
G
R
P
R
P
R
G
R
L
R
L
–V
I
V
A
–V
A
V
I
–V
O
V
O
–
+
R
BT
1794 F15
R
F
R
L
n
=
V
O
V
I
n =
1 –2
FOR R
BT
=
R
F
R
P
R
F
R
P
+
R
F
R
G
1 +
1 –
R
F
R
P
1
()
Figure 15. Back Termination Using Differential Postive Feedback