SN75LVDS179, SN75LVDS180, SN75LVDS050, SN75LVDS051
HIGH-SPEED DIFFERENTIAL LINE DRIVERS AND RECEIVERS
SLLS361A – JUNE 1999 – REVISED MARCH 2000
APPLICATION INFORMATION
fail safe
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
through 300-kΩ resistors as shown in Figure 14. The fail-safe
CC
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.
V
CC
300 kΩ
300 kΩ
A
R
t
100 Ω Typ
Y
B
V
IT
≈ 2.3 V
Figure 14. 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.
16
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