Xilinx Virtex UltraScale+ FPGAs User Manual page 128

Switching Regulator
A switching regulator can provide an efficient means to deliver a well-regulated voltage for the
GTM 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. It is impacted by the load
current in a much lesser degree than that of 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 to the switching regulator are 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 in switching regulators has recently been addressed by
several switching regulator component vendors. Normally, a switching power supply regulation
circuit requires a switching transistor element, an inductor, and a capacitor. Depending on the
required efficiency and load requirements, a switching regulator circuit might require external
switching transistors and inductors. Besides the component count, these switching regulators
require very careful placement and routing on the printed circuit board to be effective. Switching
regulators generate significant noise and therefore usually require additional filtering before the
voltage is delivered to the analog power supply input of the GTM transceiver. Because the
amplitude of the noise should be limited to less than 10 mVpp, the power supply filter should be
designed to attenuate the noise from the switching regulator to meet this requirement.
Power Supply Distribution Network
Die
The decoupling capacitance on the die filters the highest frequency noise components on the
power supplies. The source for this very high frequency noise is the internal on-die circuits.
Package
The UltraScale+ architecture package has additional decoupling. Decoupling capacitors in the
package provide attenuation for noise in the package power plane, thereby reducing the
interaction between GTM transceiver Duals. These capacitors in the package also aid in
maintaining a low-impedance, high-frequency path between the power supply, MGTAVCC,
MGTVCCAUX, VCCINT_GT or MGTAVTT, and GND.
UG581 (v1.0) January 4, 2019
Virtex UltraScale+ GTM Transceivers
Chapter 5: Board Design Guidelines
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