LTC491
PPLICATI
O U
W
U
A
S I FOR ATIO
Thermal Shutdown
less flexible, more bulky, and more costly than twisted
pairs. Many cable manufacturers offer a broad range of
120Ω cables designed for RS485 applications.
The LTC491 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 LTC491
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.
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 over all loss (Figure 11).
When using low loss cables, Figure 12 can be used as a
guidelineforchoosingthemaximumlinelengthforagiven
datarate. WithlowerqualityPVCcables, thedielectricloss
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, theyareacceptableandmuchmoreeconomical.
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
10
10k
1k
1.0
100
10
0.1
0.1
1.0
10
100
10k
100k
1M 2.5M
10M
FREQUENCY (MH )
DATA RATE (bps)
Z
LTC491 • TA12
LTC491 • TA13
Figure 11. Attenuation vs Frequency for Belden 9481
Figure 12. Cable Length vs Data Rate
8
Linear [ Linear ]
