8

LTC1686/LTC1687

EQUIVALE T I PUT NETWORKS

Figure 9. Input Thevenin Equivalent

A

B

1686/87 F09

≥22k

3.3V

≥22k

3.3V

A

B

≥22k

≥22k

THEORY OF OPERATION

Unlike typical CMOS transceivers whose propagation

delay can vary by as much as 500% from package to

package and show significant temperature drift, the

LTC1686/LTC1687 employ a novel architecture that pro-

duces a tightly controlled and temperature compensated

propagation delay. The differential timing skew is also

minimized between rising and falling output edges of the

receiver output and the complementary driver outputs.

The precision timing features of the LTC1686/LTC1687

reduce overall system timing constraints by providing a

narrow

±3.5ns window during which valid data appears at

the receiver/driver output. The driver and receiver will

have propagation delays that typically match to within 1ns.

In clocked data systems, the low skew minimizes duty

cycle distortion of the clock signal. The LTC1686/LTC1687

can be used at data rates of 52Mbps with less than 5% duty

cycle distortion (depending on cable length). When a clock

signal is used to retime parallel data, the maximum recom-

mended data transmission rate is 26Mbps to avoid timing

errors due to clock distortion.

FAIL-SAFE FEATURES

The LTC1686/LTC1687 have a fail-safe feature that guar-

antees the receiver output to be in a logic HIGH state when

the inputs are either shorted or left open (note that when

inputs are left open, large external leakage currents might

override the fail-safe circuitry). In order to maintain good

high frequency performance, it is necessary to slow down

the transient response of the fail-safe feature. When a line

fault is detected, the output will go HIGH typically in 2

µs.

Note that the LTC1686/LTC1687 guarantee receiver fail-

safe performance over the

entire (– 7V to 12V) common

mode range!

When the inputs are accidentally shorted (by cutting

through a cable, for example), the short circuit fail-safe

feature will guarantee a high output logic level. Note also

that if the line driver is removed and the ground terminated

resistors are left in place, the receiver will see this as a

“short” and output a logic HIGH. Both of these fail-safe

features will keep the receiver from outputting false data

pulses under line fault conditions.

Thermal shutdown and short-circuit protection prevent

latchup damage to the LTC1686/LTC1687 during fault

conditions.

OUTPUT SHORT-CIRCUIT PROTECTION

The LTC1686/LTC1687 employ voltage sensing short-

circuit protection at the output terminals of both the driver

and receiver. For a given input polarity, this circuitry

determines what the correct output level should be. If the

output level is different from the expected, it shuts off the

big output devices. For example, if the driver input is >2V,

it expects the “A” output to be >3.25V and the “B” output

to be <1.75V. If the “A” output is subsequently shorted to

output devices and turns on a smaller device in its place