Arria II Power Supplies - Switcher or Linear?

[Altera_Forum]

Upon reading through the Arria II Device Family Pin Connection Guideline document (PCG-01007-1.5), we noticed that only switcher power supplies are used in the power tree examples for the VCC (0.9V) supply (on page 10). Though the document does not explicitly state a switching regulators should be used for VCC, we have yet to see suggestions or examples for other kinds of supplies. We were wondering if that is the case and whether a linear regulator may be used in place of a switching supply. We are currently designing a board where we would like to minimize the amount of noise introduced to the system and we would like to use all linear regulators for our FPGA supplies.  

 Assuming the power load is low enough for a linear to supply, what are the repercussions of using a linear supply to derive our VCC (0.9V) supply?   Thank you

[Altera_Forum]

--- Quote Start ---   Assuming the power load is low enough for a linear to supply, what are the repercussions of using a linear supply to derive our VCC (0.9V) supply?    --- Quote End ---    There is no problem using linear supplies for all power rails.  The main reason you see switchers suggested is that the core current requirements often exceed what is possible with a linear regulator, eg., currents higher than 1 to 2A.  The main design constraint with a linear regulator is the power dissipation. Even if you have a linear regulator that is rated for a maximum output current of 5A or 6A, the input-to-output voltage drop and power dissipation usually limits you to much less.  The nice thing about the PCGs telling you which supplies can use switching regulators, is that it helps tell you that those supplies are tolerant to switching noise.  Why are you interested in minimizing noise? Do you have analog circuits on the same PCB? If your board is fully digital, then using switchers for the design is fine. To reduce the switching noise on some power rails, you can use ferrite beads or linear regulators on the switcher outputs.  Cheers, Dave

[Altera_Forum]

Previous FPGA families (upto Cyclone II/Stratix II) had quite noise sensitive 1.2V analog PLL supplies. Since Cyclone III/Stratix III, an on-chip regulator reduces the PLL noise sensitivity considerably. Some Altera development kits for the older FPGAs came with separate linear regulators for the PLL supply. I believe that it had been possible to get sufficient power supply quality for these devices with switched supplies and passive filters, but linear regulators have been at least reasonable in this special case. 

 For newer FPGAs, linear reguator might be still meaningful for low load supply nodes, e.g. 2.5V with only a few LVDS IOs, or even VCCINT of small FPGAs with low core clock frequency and moderate switching activity. But we also have small switching regulators with low external part count.

[Altera_Forum]

Yes, there are analog circuits on the PCB board as well, and we are really concerned about noise in this application because of the analog circuits associated with the application. 

 Thank you for your thoughts.

[Altera_Forum]

--- Quote Start ---  Yes, there are analog circuits on the PCB board as well, and we are really concerned about noise in this application because of the analog circuits associated with the application.  --- Quote End ---    Ok. Then analyze the FPGA power requirements and see if they can be met by linear supplies. If the FPGA needs to use a switcher, then you will need to pay attention to layout.  If your analog section can use differential signalling, then that will improve the common-mode rejection of any switching signals.  Micrel have a range of switcher noise suppression linear regulators; basically active filters acting as low-dropout regulators. You might be able to use those to cut down the noise from the digital section of the board.  Cheers, Dave