7
LTC1487
sn1487 1487fs
APPLICATIO S I FOR ATIO
UU W U
High Input Impedance
The LTC1487 is designed with a 96kΩ (typ) input imped-
ance to allow up to 256 transceivers to share a single
RS485 differential data bus. The RS485 specification
requires that a transceiver be able to drive as many as 32
“unit loads.” One unit load (UL) is defined as an imped-
ance that draws a maximum of 1mA with up to 12V across
it. Typical RS485 transceivers present between 0.5 and 1
unit load at their inputs. The 96kΩ input impedance of the
LTC1487 will draw only 125µA under the same 12V
condition, presenting only 0.125UL to the bus. As a result,
256 LTC1487 transceivers (32UL/0.125UL = 256) can be
connected to a single RS485 data bus without exceeding
the RS485 driver load specification. The LTC1487 meets
all other RS485 specifications, allowing it to operate
equally well with standard RS485 transceiver devices or
high impedance transceivers.
CMOS Output Driver
The RS485 specification requires that a transceiver with-
stand common-mode voltages of up to 12V or –7V at the
RS485 line connections. Additionally, the transceiver must
be immune to both ESD and latch-up. This rules out
traditional CMOS drivers, which include parasitic diodes
from their driver outputs to each supply rail (Figure 9). The
LTC1487 uses a proprietary process enhancement which
adds a pair of Schottky diodes to the output stage (Figure
10), preventing current from flowing when the common-
mode voltage exceeds the supply rails. Latch-up at the
output drivers is virtually eliminated and the driver is
prevented from loading the line under RS485 specified
fault conditions. A proprietary output protection structure
protects the transceiver line terminals against ESD strikes
(Human Body Model) of up to ±10kV.
Figure 9. Conventional CMOS Output Stage
LOGIC
V
CC
P1
D1
OUTPUT
D2
N1
LTC1487 • F09
LOGIC
V
CC
SD3
P1
D1
OUTPUT
SD4
D2
N1
LTC1487 • F10
Figure 10. LTC1487 Output Stage
When two or more drivers are connected to the same
transmission line, a potential condition exists whereby
more than two drivers are simultaneously active. If one or
more drivers is sourcing current while another driver is
sinking current, excessive power dissipation may occur
within either the sourcing or sinking element. This condi-
tion is defined as driver contention, since multiple drivers
are competing for one transmission line. The LTC1487
provides a current limiting scheme to prevent driver
contention failure. When driver contention occurs, the
current drawn is limited to about 70mA, preventing exces-
sive power dissipation within the drivers.
The LTC1487 has a thermal shutdown feature which
protects the part from excessive power dissipation. Under
extreme fault conditions, up to 250mA can flow through
the part, causing rapid internal temperature rise. The
thermal shutdown circuit will disable the driver outputs
when the internal temperature reaches 150°C and turns
them back on when the temperature cools to 130°C. This
cycle will repeat as necessary until the fault condition is
removed.
Receiver Inputs
The LTC1487 receiver features an input common-mode
range covering the entire RS485 specified range of –7V to
12V. Internal 96k input resistors from each line terminal to
ground provide the 0.125UL load to the RS485 bus.
Differential signals of greater than ±200mV within the
specified input common-mode range will be converted to
a TTL-compatible signal at the receiver output. A small
amount of input hysteresis is included to minimize the
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of circuits as described herein will not infringe on existing patent rights.
