Xilinx 7 Series User Manual page 229

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 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 V , 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.
Switching Regulator
A switching regulator can provide a very efficient means to deliver a well regulated voltage for the
GTP 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 may require external switching
transistors and inductors. Besides the component count, these switching regulators require very
careful placement and routing on the printed circuit board in order to be effective.
Switching regulators generate significant noise and therefore usually require additional filtering
before the voltage is delivered to the GTP analog power supply input of the Artix-7 FPGA. As
mentioned previously, the amplitude of the noise should be limited to less than 10 mV
Therefore the power supply filter should be designed to attenuate the noise from the switching
regulator so that it will meet this requirement.
Power Supply Distribution Network
Staged Decoupling
Die
There is de-coupling capacitance on the die to filter the highest frequency noise components on the
power supplies. The source for this very high frequency noise will be the internal on-die circuits.
7 Series FPGAs GTP Transceivers User Guide
UG482 (v1.9) December 19, 2016
www.xilinx.com
Power Supply and Filtering
.
PK-PK
Send Feedback
229