Honored Contributor
Honored ContributorIt is not only the value of the caps.
maybe i can illustrate the current peaks and what this means with the following ... imagine you through a stone into the middle of a sea. The stone is the current peak and the sea is your power supply system. the current peak is due to switching inside the fpga and other parts. if it summer, the sea is liquid. The stone will hit the surface and big waves will go out from the impact point. you will see min and max ... same with you power supply the voltage will over and undershot and raise some kind of ringing. your power supply is too weak if it is winter, the sea is frozen. the same stone wil hit the same place of the surface with the same energie. the stone will jump up and down ... now think of your power supply system ... not good either the best is if this sea is like a moor. the same stone will hit the surface and vanish away without resulting waves or a jumping stone. the best solution. of course you need the big caps. imagine a water supply system for a city. these water tanks will feed the waterpipes (your tracks) but if somebody opens a valve the preasure drops inside that pipe (the voltage on your track) your smal tracks have to carry the whole current that needs to be delivered to hold the voltage on each fpga pin. i guess you cant do that with traces, you need to do that with power planes. one for each voltage and gnd. these form a capacitor (made by the pcb). the smal caps with a few nF to uF are for haveing low impedance over a wider frequency range. the fpga 484 footprint needs 5 routing layers to gain access to the inner pins. how much routing layers are needed for the f256 cant say that out of my head. I have used the LM2677S-3.3 and a LT3412A for 1.2V a perfect combination here. a layerstack for the f484 could be top gnd 1v2 gnd 3v3 gnd s1 s2 gnd s3 s4 gnd s5 bot but with 6 layers i cant see a working solution that is stable and will pass all aproval tests. Michael
