Datasheet

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APPLICATION INFORMATION
FAIL SAFE
Rt = 100 Ω (Typ)
300 kΩ 300 kΩ
V
CC
V
IT
≈ 2.3 V
A
B
Y
SN65LVDS048A
SLLS451B – SEPTEMBER 2000 – REVISED SEPTEMBER 2002
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. TI's LVDS receiver is different in how it handles
the open-input circuit situation, however.
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
will pull each line of the signal pair to near V
CC
through 300-k Ω resistors as shown in Figure 10. 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 12. Open-Circuit Fail Safe of the LVDS Receiver
It is only under these conditions that the output of the receiver will be valid with less than a 100-mV differential
input voltage magnitude. The presence of the termination resistor, Rt, 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.
9
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