Datasheet

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Fail Safe
R
t
100 Ω Typ
300 kΩ 300 kΩ
V
CC
V
IT
≈ 2.3 V
A
B
Y
SN65LVDS179-EP , , SN65LVDS180-EP
SN65LVDS050-EP , SN65LVDS051-EP
HIGH-SPEED DIFFERENTIAL LINE DRIVERS AND RECEIVERS
SGLS203B – SEPTEMBER 2003 – REVISED JANUARY 2007
APPLICATION INFORMATION (continued)
One of the most common problems with differential signaling applications is how the system responds when no
differential voltage is present on the signal pair. The LVDS receiver is like most differential line receivers, in that
its output logic state can be indeterminate when the differential input voltage is between –100 mV and 100 mV
and within its recommended input common-mode voltage range. However, TI's LVDS receiver is different in how
it handles the open-input circuit situation.
Open-circuit means that there is little or no input current to the receiver from the data line itself. This could be
when the driver is in a high-impedance state or the cable is disconnected. When this occurs, the LVDS receiver
pulls each line of the signal pair to near V
CC
through 300-k Ω resistors as shown in Figure 18 . The fail-safe
feature uses an AND gate with input voltage thresholds at about 2.3 V to detect this condition and force the
output to a high-level, regardless of the differential input voltage.
Figure 18. Open-Circuit Fail Safe of the LVDS Receiver
It is only under these conditions that the output of the receiver is valid with less than a 100-mV differential input
voltage magnitude. The presence of the termination resistor, R
t
, does not affect the fail-safe function as long as
it is connected as shown in the figure. Other termination circuits may allow a dc current to ground that could
defeat the pullup currents from the receiver and the fail-safe feature.
17
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