16
LT1886
PART NUMBER DESCRIPTION COMMENTS
LT1207 Dual 250mA, 60MHz Current Feedback Amplifier Shutdown/Current Set Function
LT1361 Dual 50MHz, 800V/µs Op Amp ±15V Operation, 1mV V
OS
, 1µA I
B
LT1396 Dual 400MHz, 800V/µs Current Feedback Amplifier 4.6mA Supply Current Set, 80mA I
OUT
LT1497 Dual 125mA, 50MHz Current Feedback Amplifier 900V/µs Slew Rate
LT1795 Dual 500mA, 50MHz Current Feedback Amplifier Shutdown/Current Set Function, ADSL CO Driver
LT1813 Dual 100MHz, 750V/µs, 8nV/√Hz Op Amp Low Noise, Low Power Differential Receiver
LINEAR TECHNOLOGY CORPORATION 1999
1886fa LT/LCG 0301 2K REV A • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear-tech.com
RELATED PARTS
TYPICAL APPLICATIO
U
Considerations for Fault Protection
The basic line driver design 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 consid-
erable amount of current, 500mA or more, to flow as it is
limited only by the small valued back-termination resis-
tors 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 cur-
rent can heat the LT1886 to destruction.
Using DC blocking capacitors 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
TM
,
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 LT1886 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.
–
+
6.19Ω
5V
–5V
–5V
5V
1
0.47µF**
BAV99**
8
5V
4
–5V
1k
866Ω
2k
2k
866Ω
–
+
V
IN
–
+
V
L
*COILCRAFT X8390-A OR EQUIVALENT
**SEE TEXT REGARDING FAULT PROTECTION
–
+
6.19Ω
1/2 LT1886
1/2 LT1886
1886 TA02
1k
100pF
1:2*
100Ω
130Ω
3
2
7
6
5
0.47µF**
100pF
130Ω
= 5 (ASSUME 0.5dB TRANSFORMER POWER LOSS)
REFLECTED LINE IMPEDANCE = 100Ω / 2
2
= 25Ω
EFFECTIVE TERMINATION = 2 • 6.19 •
EACH AMPLIFIER: 0.56V
RMS
, 29.9mA
RMS
±3V PEAK, ±160mA PEAK
= 24.8Ω
V
L
V
IN
2kΩ
1kΩ
BAV99**
Split Supply ±5V ADSL CPE Line Driver
TransZorb is a registered trademark of General Instruments, GSI
