Le87251
Passive Termination
16.5 V
P-P
into a 100
Ω
line
V
OUT DRIVER
= V
RBM
+ V
RLOAD
RBM = R
LOAD
V
RBM
= V
RLOAD
V
OUT DRIVER
= 33.52 V
V
SUPPLY
= 37.52 V
I
OUT
= 31.6 mA
P
OUT DRIVER
= V
SUPPLY
* I
OUT
= 1.185 W (plus
quiescent power)
Active Termination
16.5 V
P-P
into a 100
Ω
line
V
OUT DRIVER
= V
RBM
+ V
RLOAD
RBM = R
LOAD
/5
V
RBM
= V
RLOAD
/5
V
OUT DRIVER
= 20.11 V
V
SUPPLY
= 24.11 V
I
OUT
= 31.6 mA
P
OUT DRIVER
= V
SUPPLY
* I
OUT
= 0.714 W
(plus quiescent power)
Data Sheet
Table 2 - Passive and Active Termination Comparison
2.3
Line Driver Protection
High voltage transients such as lightning can appear on the telephone lines. Transient protection devices should be
used to absorb the transient energy and clamp the transient voltages. However, large transient voltages can still
couple to the primary side of the transformer. As shown in Reference Circuit 2, the outputs of the Le87251
incorporate on-chip circuitry that clamps the output voltage to no more than a diode drop beyond either rail. No
external diodes immediately at the output of the amplifiers are required. As shown in Figure 6, the series output
termination resistors limit the current going into the line driver and internal clamps, thus these termination resistors
should be specified at 0.5 W. The actual protection scheme may vary depending on the type of data transformer
used and the line protection components used in the front of the data transformer.
R
BM
0.5W
R
F
V
IN
+
2R
G
R
F
R
BM
V
IN
-
0.5W
Figure 6 - Line Driver Protection
Subscriber
Line
A large DC voltage can develop between the line driver outputs during system turn-on when the AFE has not been
reset or when the line driver is disabled. Figure 6 shows an AC coupling capacitor between the two line driver
outputs. This AC coupling capacitor prevents large DC current from flowing from one output of the line driver to
another.
8
Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
