DS15MB200 Datasheet, PDF (9/10 Pages) National Semiconductor (TI) – Dual 1.5 Gbps 2:1/1:2 LVDS Mux/Buffer with Pre-Emphasis

Interfacing LVDS to LVPECL
An LVDS driver consists of a current source (nominal 3.5mA)
which drives a CMOS differential pair. It needs a differential
resistive load in the range of 70 to 130 ohms to generate
LVDS levels. In a system, the load should be selected to
match transmission line characteristic differential impedance
so that the line is properly terminated. The termination resis-
tor should be placed as close to the receiver inputs as
possible. When interfacing an LVDS driver with a non-LVDS
receiver, one only needs to bias the LVDS signal so that it is
within the common mode range of the receiver. This may be
done by using separate biasing voltage which demands
another power supply. Some receivers have required biasing
voltage available on-chip (V_T, V_TT or V_BB).
Packaging Information
The Leadless Leadframe Package (LLP) is a leadframe
based chip scale package (CSP) that may enhance chip
speed, reduce thermal impedance, and reduce the printed
circuit board area required for mounting. The small size and
very low profile make this package ideal for high density
PCBs used in small-scale electronic applications such as
cellular phones, pagers, and handheld PDAs. The LLP pack-
age is offered in the no Pullback configuration. In the no
Pullback configuration the standard solder pads extend and
terminate at the edge of the package. This feature offers a
visible solder fillet after board mounting.
The LLP has the following advantages:
• Low thermal resistance
• Reduced electrical parasitics
• Improved board space efficiency
• Reduced package height
• Reduced package mass
For more details about LLP packaging technology, refer to
applications note AN-1187, "Leadless Leadframe Package"
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FIGURE 5. DC Coupled LVDS to LVPECL Interface
Figure 5 illustrates interface between an LVDS driver and a
LVPECL with a V_T pin available. R1 and R2, if not present in
the receiver (Note 15), provide proper resistive load for the
driver and termination for the transmission line, and V_T sets
desired bias for the receiver.
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FIGURE 6. AC Coupled LVDS to LVPECL Interface
Figure 6 illustrates AC coupled interface between an LVDS
driver and LVPECL receiver without a V_T pin available. The
resistors R1, R2, R3, and R4, if not present in the receiver
(Note 15), provide a load for the driver, terminate the trans-
mission line, and bias the signal for the receiver.
Note 15: The bias networks shown above for LVPECL drivers and receivers
may or may not be present within the driver device. The LVPECL driver and
receiver specification must be reviewed closely to ensure compatibility be-
tween the driver and receiver terminations and common mode operating
ranges.
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