8

LTC1346A

Theory of Operation

The transmitter outputs consist of complementary

switched-current sources as shown in Figure 7.

With a logic zero at the transmitter input, the inverting

output Z sources 11mA and the noninverting output Y

sinks 11mA. The differential transmitter output voltage is

then set by the termination resistors. With two differential

50

Ω resistors at each end of the cable, the voltage is set to

(50

Ω)(11mA) = 0.55V. With a logic 1 at the transmitter

input, output Z sinks 11mA and Y sources 11mA. The

common mode voltage of Y and Z is 0V when both current

sources are matched and there is no ground potential

difference between the cable terminations. The transmitter

current sources have a common mode range of

±2V,which

allows for a ground difference between cable terminations

of

±4V.

Each receiver input has a 30k resistance to ground and

requires external termination to meet the V.35 input imped-

ance specification. The receivers have an input hysteresis

of 50mV to improve noise immunity.

Three Select pins, S0, S1 and S2, configure the chip as

described in Function Tables. When the transmitters and

Figure 7. Simplified Transmitter Schematic

11mA

11mA

LTC1346A • F07

Y

Z

T

50

Ω

125

Ω

50

Ω

CHIP

BOUNDARY

receivers are OFF, all outputs are forced into high imped-

ance. The S0 pin can be used as receiver output enable.

and receivers OFF. When the LTC1346A is enabled from

shutdown the transmitters and receivers require 2

µs to

stabilize.

Complete V.35 Port

Figure 8 shows the schematic of a complete surface

mounted,

±5V DTE and DCE V.35 port using only three ICs

and six capacitors per port. The LTC1346A is used to

transmit the clock and data signals and the LT1134A to

transmit the control signals. If test signals 140, 141 and

142 are not used, the transmitter inputs should be tied

RS422/RS485 Applications

The receivers on the LTC1346A can be used for RS422

and RS485 applications. Using the test circuit in Figure 9,

the LTC1346A receivers are able to successfully extract

the data stream from the common mode voltage, meeting

RS422 and RS485 requirements as shown in Figures 10

and 11.