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ACTIVE FAILSAFE FEATURE
_
+
Main Receiver
_
+
_
+
A > B + 80 mV
B > A + 80 mV
Failsafe
Timer
Failsafe
Output
Buffer
Reset
Window Comparator
A
B
R
ECL/PECL-to-LVTTL CONVERSION WITH TI's LVDS RECEIVER
SN65LVDS348 , SN65LVDT348
SN65LVDS352 , SN65LVDT352
SLLS523E – FEBRUARY 2002 – REVISED MAY 2004
APPLICATION INFORMATION (continued)
A differential line receiver commonly has a failsafe circuit to prevent it from switching on input noise. Current
LVDS failsafe solutions require either external components with subsequent reductions in signal quality or
integrated solutions with limited application. This family of receivers has a new integrated failsafe that solves the
limitations seen in present solutions. A detailed theory of operation is presented in application note The Active
Fail-Safe in TI's LVDS Receivers, literature number SLLA082B.
The following figure shows one receiver channel with active failsafe. It consists of a main receiver that can
respond to a high-speed input differential signal. Also connected to the input pair are two failsafe receivers that
form a window comparator. The window comparator has a much slower response than the main receiver and it
detects when the input differential falls below 80 mV. A 600-ns failsafe timer filters the window comparator
outputs. When failsafe is asserted, the failsafe logic drives the main receiver output to logic high.
Figure 16. Receiver With Active Failsafe
The various versions of emitter-coupled logic (i.e., ECL, PECL and LVPECL) are often the physical layer of
choice for system designers. Designers know that established technology is capable of high-speed data
transmission. In the past, system requirements often forced the selection of ECL. Now technologies like LVDS
provide designers with another alternative. While the total exchange of ECL for LVDS may not be a design
option, designers have been able to take advantage of LVDS by implementing a small resistor divider network at
the input of the LVDS receiver. TI has taken the next step by introducing a wide common-mode LVDS receiver
(no divider network required) which can be connected directly to an ECL driver with only the termination bias
voltage required for ECL termination (V
CC
- 2 V).
Figure 17 shows the use of an LV/PECL driver driving 5 meters of CAT-5 cable and being received by TI's wide
common-mode receiver and the resulting eye-pattern. The values for R3 are required in order to provide a
resistor path to ground for the LV/PECL driver. With no resistor divider, R1 simply needs to match the
characteristic load impedance of 50 Ω . The R2 resistor is a small value intended to minimize common-mode
reflections.
15
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