AN-5045 Datasheet, PDF (1/2 Pages) Fairchild Semiconductor – High Speed LVDS Repeaters

AN-5045
Fairchild Semiconductor
Application Note
August 2002
Revised March 2003
Using the V_BB Reference on High Speed LVDS Repeaters
Abstract
Fairchild’s LVDS 2, 4, and 8-port repeaters are designed
for high speed interconnects providing low Electromagnetic
Interference (EMI) and low power dissipation at data rates
up to 1.2Gb/s. The devices accept and generate LVDS lev-
els with a typical output voltage of 330 mV. The devices are
designed with a V_BB output reference voltage for AC cou-
pling on the inputs. This application brief will discuss when
to use AC coupling in an interconnect design and illustrate
how to use the V_BB reference feature for optimizing the
link.
Output Reference Voltage (V_BB)
The V_BB reference is a design feature of Fairchild’s LVDS
repeaters which is simply a DC voltage reference used in
applications employing AC coupling on the inputs. It is an
internally generated DC voltage bias output that sources
up to 275 µA of current. Test conditions used to
characterize this feature were with V_CC = 3.3V and an
I_BB = 0 to −275 µA to guarantee minimum and maximum
limits of 1.125V and 1.375V respectively. The V_BB pin does
not have to be used and can be left floating.
When would the
V_BB Reference Voltage be used?
Application specific requirements will determine whether
the V_BB reference is needed. Typically when AC coupling is
used to couple a transmitted differential signal to a
receiver, the coupling capacitor blocks DC and thus the off-
set is no longer present at the receiver input. By connecting
the V_BB output reference voltage between a split termina-
tion, it effectively provides a common mode voltage point
for the differential signals. Refer to Figure 1 for the V_BB
connection diagrams. The capacitor from V_BB pin to ground
is optional and is used to stabilize the common mode volt-
age point. The value of the capacitor will be dependent on
the specific application environment.
FIGURE 1. AC Coupling ECL to LVDS using the Output Reference Voltage (V_BB)
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