LTC491

8

S

APPLICATI

I FOR ATIO

Thermal Shutdown

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.

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 over all loss (Figure 11).

When using low loss cables, Figure 12 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

LTC491 • TA13

2.5M

Figure 12. Cable Length vs Data Rate

Figure 11. Attenuation vs Frequency for Belden 9481

0.1

0.1

1.0

10

1.0

10

100

LTC491 • TA12