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LTC490 Datasheet(Fiches technique) 6 Page - Linear Technology

Numéro de pièce LTC490
Description  Differential Driver and Receiver Pair
Télécharger  8 Pages
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Fabricant  LINER [Linear Technology]
Site Internet  http://www.linear.com
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LTC490
6
Figure 8. RS485 Cable Length Specification. Applies for 24
Gauge, Polyethylene Dielectric Twisted Pair.
S
APPLICATI
I FOR ATIO
one driver can be connected on the bus, the cable can be
terminated only at the receiving end. The optional shields
around the twisted pair help reduce unwanted noise, and
are connected to GND at one end.
The LTC490 can also be used as a line repeater as shown
in Figure 6. If the cable length is longer than 4000 feet, the
LTC490 is inserted in the middle of the cable with the
receiver output connected back to the driver input.
LTC490 • TA07
120
LTC490
DRIVER
DX
RX
2
3
5
6
7
8
RECEIVER
DATA IN
DATA OUT
Figure 6. Line Repeater
Thermal Shutdown
The LTC490 has a thermal shutdown feature which pro-
tects the part from excessive power dissipation. If the
outputs of the driver are accidently shorted to a power
supply or low impedance, source, up to 250mA can flow
through the part. The thermal shutdown circuit disables
the driver outputs when the internal temperature reaches
150
°C and turns them back on when the temperature
cools to 130
°C. If the outputs of two or more LTC490
drivers are shorted directly, the driver outputs can not
supply enough current to activate the thermal shutdown.
Thus, the thermal shutdown circuit will not prevent con-
tention faults when two drivers are active on the bus at the
same time.
Cables and Data Rate
The transmission line of choice for RS485 applications is
a twisted pair. There are coaxial cables (twinaxial) made
for this purpose that contain straight pairs, but these are
less flexible, more bulky, and more costly than twisted
pairs. Many cable manufacturers offer a broad range of
120
Ω cables designed for RS485 applications.
Losses in a transmission line are a complex combination
of DC conductor loss, AC losses (skin effect), leakage and
AC losses in the dielectric. In good polyethylene cables
such as the Belden 9841, the conductor losses and
dielectric losses are of the same order of magnitude,
leading to relatively low overall loss (Figure 7).
FREQUENCY (MHz)
0.1
0.1
1.0
10
1.0
10
100
LTC490 • TA08
Figure 7. Attenuation vs Frequency for Belden 9841
When using low loss cables, Figure 8 can be used as a
guideline for choosing the maximum line length for a given
data rate. With lower quality PVC cables, the dielectric loss
factor can be 1000 times worse. PVC twisted pairs have
terrible losses at high data rates (>100kbs), and greatly
reduce the maximum cable length. At low data rates
however, they are acceptable and much more economical.
DATA RATE (bps)
10k
10
100
1k
10k
100k
1M
10M
LTC490 • TA09
2.5M




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