7

LTC1487

APPLICATIO S I FOR ATIO

High Input Impedance

The LTC1487 is designed with a 96k

Ω (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 96k

Ω input impedance of the

LTC1487 will draw only 125

µA 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

P1

D1

OUTPUT

D2

N1

LTC1487 • F09

LOGIC

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.