Xilinx Virtex-6 FPGA User Manual page 145

of power supply voltage regulators available for regulating the GTH transceiver analog
voltage rails: linear regulators and switching regulators. Both have advantages and
disadvantages. The optimal choice of regulator type depends on system requirements such
as:
•
•
•
•
Linear Regulator
A linear regulator is usually the simplest means to provide voltage regulation for the GTH
transceiver analog supply rails. Inherently, a linear regulator does not inject significant
noise into the regulated output voltage. In fact some, but not all, linear regulators provide
noise rejection at the output from noise present on the voltage input. Another advantage of
the linear regulator is that it usually requires a minimal number of external components to
realize a circuit on the PCB.
There are potentially two major disadvantages to linear regulators: the minimum dropout
voltage and limited efficiency. Linear regulators require an input voltage that is higher
than the output voltage. This minimum dropout voltage often is dependent on the load
current. Even low dropout linear regulators require a minimum difference between the
input voltage and the output voltage of the regulator. The system power supply design
must meet the minimum dropout voltage requirements of the linear regulators.
The efficiency of a linear regulator is dependent on the voltage difference between the
input and output of the linear regulator. For instance, if the input voltage of the regulator
is 2.5 V
1.3 V
of the regulator, the maximum efficiency of the regulator is 48%. For every watt delivered
to the load, the system must consume an additional watt for regulation. This power
consumed by the regulator generates heat that must be dissipated by the system.
Providing a means to dissipate the heat generated by the linear regulator can drive up the
system cost. Therefore, even though linear regulators are less complex and use fewer
external components, if the overall system cost is considered (including power
consumption and heat dissipation), linear regulators can be at a disadvantage in high
current applications.
Switching Regulator
A switching regulator can provide a very efficient means to deliver a well regulated
voltage for the GTH transceiver analog power supply. Unlike the linear regulator, the
switching regulator does not depend on the voltage drop between the input voltage of the
regulator and the output voltage to provide regulation. Therefore, the switching regulator
can supply large amounts of current to the load while maintaining high power efficiency. It
is not uncommon for a switching regulator to maintain efficiencies of 95% or greater. This
efficiency is not severely impacted by the voltage drop between the input of the regulator
and the output. Additionally, it is impacted by the load current to a much lesser degree
than the linear regulator. Because of the efficiency of the switching regulator, the system
does not need to supply as much power to the circuit, and it does not need to provide a
means to dissipate power consumed by the regulator.
The disadvantages of the switching regulator are the complexity of the circuit and noise
generated by the regulator switching function. Switching regulator circuits are usually
more complex than linear regulator circuits. This shortcoming has recently been addressed
Virtex-6 FPGA GTH Transceivers User Guide
UG371 (v2.0) February 16, 2010
Physical size
Thermal budget
Power efficiency
Cost
, and the output voltage of the regulator is 1.2 V
DC
. Assuming that the current into the regulator is essentially equal to the current out
DC
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Power Supplies and Filtering
, the voltage difference is
DC
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