Altera made a spreadsheet named “Power Delivery Network” designed to determine an optimum number, type and value of decoupling capacitors to achieve the target impedance. I select the VRM type (Switcher), the supply voltage (3.3V), maximum current (2.58A), transient current (I typed 30%, but it seems extremely complicated to obtain a value without having the programming of the FPGA already done) and ripple tolerance (5% given the specifications of the FPGA and knowing that we don’t have transceivers for PCIe, which is the only bank requiring at most 3% ripple). For the spreading inductance, BGA Via inductance and plane capacitance profile I left the default values because I don’t know how to estimate that.

Auto decoupling

The tool looks up the capacitors it has in its database and tries to find the optimal combination for our target. With the preceding values it outputs 2x 0.01µF, 2x 0.047µF, 1x 1µF and 1x 10µF capacitors, with their respective ESR, ESL and mounting inductance (Lmnt).

Development board decoupling

If I take the configuration of the development board of our SoM, they are using 4x 10µF and 9x 100nF capacitors. If I use the capacitors pre-configured in the Altera tool, it shows that the resulting impedance is higher than the one I got with my inputs, but given that the development board has to be designed for any configuration of the FPGA, their decoupling capacitors are certainly adapted to more extreme power draws than what we will be experiencing. So, I think it would be a good idea to keep what Aries did with their development board.