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Complex FPGA boards nowadays can have a large number of voltage rails like 5V, 3.3V, 2.5V, 1.8V, 1.2V and seprate analogue and digital supplies and then separate voltage rails for specific components like DDR memories.

From what I know, we need to use power planes in this case to distribute the power. In this case, the square/rectangle shape of a copper plane will be split into sections akin to floorplan of a house where different regions will be dedicated to different voltage rails.

The confusion I have is, how does it all connect together. The voltage regulators will be in a specific region/section of the board. From there we need to connect them to power planes on the other side of the board. We need to decide how many power planes we need as well and the voltage regulators will have to connect to planes on different layers. And we need to make sure that the noise on the rails is not so much that the component fails.

How is the "floorplan" and quantity of power planes decided? Do we need to distribute the voltage regulators everywhere on the board like SMPS on one side (or on bottom) and then lot of linear regulators connected to tiny power planes in middle of the PCB. Or all SMPS and linear regulators on one side of the board and then connected to planes everywhere somehow.

I am an electronic engineer so you do not need to write too much. If you could guide me to some application note, that would be great. All the search I have done on google so far, just leads me to topic of ground planes and whether they should be split for analogue-digital grounds or not. I am not looking for that.

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    \$\begingroup\$ They're called point-of-load regulators for a reason. \$\endgroup\$
    – Hearth
    Commented Oct 10, 2023 at 2:03
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    \$\begingroup\$ A lot depends on how much current is needed off of each supply rail. We typically use one or two outputs of a SMPS to provide the highest current voltages. Then we use LDO's off of one the higher voltage rails to supply the other, lower current voltages. \$\endgroup\$
    – SteveSh
    Commented Oct 10, 2023 at 2:49
  • \$\begingroup\$ I am talking about maybe a few Amps at most. Definitely not something like 110A specified for Core™ i7-800 and i5-700 processors in their datasheet Table 7-5 of the "Document Number: 322164-004 Intel® Core™ i7-800 and i5-700 Desktop Processor Series Datasheet – Volume 1" \$\endgroup\$
    – quantum231
    Commented Oct 10, 2023 at 15:28
  • \$\begingroup\$ Also, since complex boards need 5V, 3.3V, 2.5V, 1.8V, 1.2V and separate analogue and digital supplies and then separate voltage rails for specific components like DDR memories; I don't see how we can design such a board without dedicated power planes. \$\endgroup\$
    – quantum231
    Commented Oct 10, 2023 at 15:28
  • \$\begingroup\$ We've done mixed signal boards (digital, RF, DAC & ADC) with over 30 different voltage rails. All power plane layers had 6 or more different voltages on them, so each plane was split into approximately 6 or more sections. So I would think you could do your board (with maybe 10 voltages) on 2 power planes. \$\endgroup\$
    – SteveSh
    Commented Oct 10, 2023 at 16:27

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You can get by without planes, unless you have specific stringent requirements for low power supply impedance above ~100 MHz.

FPGA chips (like other ASICs) do include on-chip and/or on-carrier decoupling capacitors. So the mere fact that the chip works at high frequencies is not enough to tell whether you have such requirements. Check the datasheet, it should specify the maximum power supply impedance over frequency.

If such requirements do not exist, it is good enough to place a voltage regulator in the general vicinity of the FPGA, route power to the regulator, and use a short patch to connect the regulator output, the FPGA pins and a bank of decoupling capacitors for low impedance between approx. 10 kHz - 100 MHz.

Further reading: Altera/Intel guide about the influence of various decoupling schemes on power supply impedance over frequency

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  • \$\begingroup\$ Thanks Tobalt. With 5V, 3.3V, 2.5V, 1.8V, 1.2V and seprate analogue and digital supplies and then separate voltage rails for specific components like DDR memories, I don't see how we can get away with no having dedicated power supply planes. \$\endgroup\$
    – quantum231
    Commented Oct 10, 2023 at 15:24

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