LT1886CS8 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1886CS8' PDF : 16 Pages View PDF
LT1886
TYPICAL APPLICATIO
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). TransZorbsTM,
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.
TransZorb is a registered trademark of General Instruments, GSI
Split Supply ±5V ADSL CPE Line Driver
130Ω
5V
3+ 8
1/2 LT1886
2–
5V
0.47µF**
1 6.19Ω
BAV99**
–5V
100pF
1k
1:2*
+
866Ω
2k
VIN
–
866Ω
1k
2k
100pF
130Ω
6–
1/2 LT1886
5+
4
7 6.19Ω
0.47µF**
5V
BAV99**
–5V
–5V 1886 TA02
+
100Ω VL
–
VL = 5
VIN
(ASSUME 0.5dB TRANSFORMER POWER LOSS)
REFLECTED LINE IMPEDANCE = 100Ω / 22 = 25Ω
EFFECTIVE TERMINATION = 2 • 6.19 • 2kΩ = 24.8Ω
1kΩ
EACH AMPLIFIER: 0.56VRMS, 29.9mARMS
±3V PEAK, ±160mA PEAK
*COILCRAFT X8390-A OR EQUIVALENT
**SEE TEXT REGARDING FAULT PROTECTION
RELATED PARTS
PART NUMBER
LT1207
LT1361
LT1396
LT1497
LT1795
LT1813
DESCRIPTION
Dual 250mA, 60MHz Current Feedback Amplifier
Dual 50MHz, 800V/µs Op Amp
Dual 400MHz, 800V/µs Current Feedback Amplifier
Dual 125mA, 50MHz Current Feedback Amplifier
Dual 500mA, 50MHz Current Feedback Amplifier
Dual 100MHz, 750V/µs, 8nV/√Hz Op Amp
16 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX:(408)434-0507 q www.linear-tech.com
COMMENTS
Shutdown/Current Set Function
±15V Operation, 1mV VOS, 1µA IB
4.6mA Supply Current Set, 80mA IOUT
900V/µs Slew Rate
Shutdown/Current Set Function, ADSL CO Driver
Low Noise, Low Power Differential Receiver
1886fa LT/LCG 0301 2K REV A • PRINTED IN USA
© LINEAR TECHNOLOGY CORPORATION 1999
Share Link: 
