LTC486 Datasheet, PDF (5/8 Pages) Linear Technology – Quad Low Power RS485 Driver

LTC486
PI FU CTI S
DI1 (Pin 1): Driver 1 Input. If Driver 1 is enabled, then a low
on DI1 forces the driver outputs DO1A low and DO1B high.
A high on DI1 with the driver outputs enabled will force
DO1A high and DO1B low.
DO1A (Pin 2): Driver 1 Output.
DO1B (Pin 3): Driver 1 Output.
EN (Pin 4): Driver Outputs Enabled. See Function Table for
details.
DO2B (Pin 5): Driver 2 Output.
DO2A (Pin 6): Driver 2 Output.
DI2 (Pin 7): Driver 2 Input. Refer to DI1.
GND (Pin 8): Ground Connection.
DI3 (Pin 9): Driver 3 Input. Refer to DI1.
DO3A (Pin 10): Driver 3 Output.
DO3B (Pin 11): Driver 3 Output.
EN (Pin 12): Driver Outputs Disabled. See Function Table
for details.
DO4B (Pin 13): Driver 4 Output.
DO4A (Pin 14): Driver 4 Output.
DI4 (Pin 15): Driver 4 Input. Refer to DI1.
V_CC (Pin 16): Positive Supply; 4.75V < V_CC < 5.25V .
TEST CIRCUITS
A
V_OD
B
R
R
V_OC
LTC486 • TA02
Figure 1. Driver DC Test Load
EN
CI1
A
DI
DRIVER
R_DIFF
B
CI2
LTC486 • TA03
EN
Figure 2. Driver Timing Test Circuit
OUTPUT
UNDER TEST
500Ω
C_L
S1
V_CC
S2
LTC486 • TA04
Figure 3. Driver Timing Test Load #2
APPLICATI S I FOR ATIO
Typical Application
A typical connection of the LTC486 is shown in Figure 6.
A twisted pair of wires connect up to 32 drivers and
receivers for half duplex data transmission. There are no
restrictions on where the chips are connected to the
wires, and it isn’t necessary to have the chips connected
at the ends. However, the wires must be terminated only
at the ends with a resistor equal to their characteristic
impedance, typically 120Ω. The optional shields around
the twisted pair help reduce unwanted noise, and are
connected to GND at one end.
Thermal Shutdown
The LTC486 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 LTC486 drivers are
shorted directly, the driver outputs can not supply enough
current to activate the thermal shutdown. Thus, the ther-
mal shutdown circuit will not prevent contention faults
when two drivers are active on the bus at the same time.
Cable 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 120Ω
cables designed for RS485 applications.
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