Xilinx Virtex UltraScale+ FPGAs User Manual page 127

Linear versus 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 GTM 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 GTM transceiver analog voltage rails, linear regulators, and
switching regulators. Each of these types of regulator 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 GTM
transceiver analog supply rails. Inherently, a linear regulator does not inject significant noise into
the regulated output voltage. In fact, some, 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 printed
circuit board.
There are potentially two major disadvantages to linear regulators: 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 consider 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 VDC and
the output voltage of the regulator is 1.2 VDC, the voltage difference is 1.3 VDC. Assuming that
the current into the regulator is essentially equal to the current out of the regulator, the
maximum efficiency of the regulator is 48%. This means that 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. So even though from a
simple component count and complexity cost the linear regulator appears to have an advantage
over the switching regulator, if the overall system cost is considered, including power
consumption and heat dissipation, in high current applications the linear regulator can actually be
at a disadvantage.
UG581 (v1.0) January 4, 2019
Virtex UltraScale+ GTM Transceivers
Chapter 5: Board Design Guidelines
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