Xilinx Virtex-6 FPGA User Manual

284 www.xilinx.com Virtex-6 FPGA GTX Transceivers User Guide
UG366 (v2.5) January 17, 2011
Chapter 5: Board Design Guidelines
Power Supply Pins, page 276 for a discussion of the internal power planes in the Virtex-6
FPGA packages.
Noise on the GTX analog power supplies can cause degradation in the performance of the
transceivers. The most likely form of degradation is an increase in jitter at the output of the
GTX transmitter and reduced jitter tolerance in the GTX receiver. Sources of power supply
noise are
• Power supply regulator noise
• Power distribution network
• Coupling from other circuits
Each of these noise sources must be considered in the design and implementation of the
GTX analog power supplies. The total peak-to-peak noise as measured at the input pin of
the FPGA should not exceed 10 mV
PK-PK
.
Although the nominal voltage for MGTAVCC and VCCINT are both 1.0V, a separate
power supply regulator should be used to power each of them. The reason for this
recommendation is that MGTAVCC is very sensitive to noise and VCCINT has the
potential to generate significant noise spikes well above the 10 mV
PK-PK
maximum noise
level recommended for the MGTAVCC power supply. Keeping the two power supply
networks separate minimizes noise coupling between them.
Power Supply Regulators
Normally, the GTX analog voltage supplies have local power supply regulators that
provide a final stage of voltage regulation. Preferably, these regulators are placed as close
as is feasible to the GTX power supply pins. Minimizing the distance between the analog
voltage regulators and the GTX power supply pins reduces the opportunity for noise
coupling into the supply after the regulator and for noise generated by current transients
caused by load dynamics.
Linear vs. Switching Regulators
The type of power supply regulator can have a significant impact on the complexity, cost,
and performance of the power supply circuit. A power supply regulator must provide
adequate power to the GTX transceiver with a minimum amount of noise while meeting
the overall system thermal and efficiency requirements. There are two major types of
power supply voltage regulators available for regulating the GTX analog voltage rails:
linear regulators and switching regulators. Each regulator type has advantages and
disadvantages. The optimal choice of regulator type depends on system requirements such
as:
•Physical size
•Thermal budget
• Power efficiency
•Cost
Linear Regulator
A linear regulator is usually the simplest means to provide voltage regulation for the GTX
analog supply rails. Inherently, a linear regulator does not inject significant noise into the
regulated output voltage. Some, not all, linear regulators provide noise rejection at the
output from noise present on the voltage input. The linear regulator usually requires a
minimal number of external components to realize a circuit on the PCB.
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Xilinx Virtex-6 FPGA Specifications

General

Brand	Xilinx
Model	Virtex-6 FPGA
Category	Transceiver
Language	English