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Altera_Forum's avatar
Altera_Forum
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16 years ago

Vcca regulation requirements

I'm designing a board with a Cyclone III EP3C16E144, and I made the mistake of copying a similar board with a Cyclone I part on it. I've cleared up most of the errors, but I am having a bit of trouble with the Vcca pins. I can't easily run another plane without disturbing a fairly carefully done split-plane handling the 3.3V and 1.2V pins. I could potentially thread a 6-8mil trace between the split planes to get to the Vcca pins, but it would have a fairly high impedance.

The question I have is, how tight must the regulation between the four pins - could I drop a diode on all four pins fairly easily. I've already routed to via's outside the perimeter of the part. The actual voltage would differ slightly, particularly if one of the PLL's was being used or not, but would it cause problems?

Alternately, if I can manage to thread a high-impedance trace through the Vcc plane, would it cause problems? I understand the PLL's don't use much current, but the noise could be an issue. The Vcc plane is on the opposite side of the board, where there are fewer signals tracking across it, but most of the signals on the far side are clocks. How much noise can the Vcca pins tolerate before there are issues with the internal clocks?

I only plan to use one of the PLL's - the other three will probably remain idle. Would it be possible to use a clean LDO on the one I intend to use, and drop diodes on the others?

Thanks!

-Seth

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    16 years ago

    the vcca pins on the cyclone III BGA pcb here are tracks on the bottom layer in fact they are a polygon fill with a wide track to the LDO. This design is using a 4 layer pcb with cyclone III SDRAM and some other stuff like 2 chanel 12 bit adc 20MSPS the components are very densed and the plane distance is 50µ and it has passes alle approvals including R&TTE

    so no worry about vcca
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    I managed to get something that looks workable in. It's an L-shaped island between the 1.2v and 3.3v planes that connects three of the four Vcca pins. Width is about 50 mil min, and it's parked right under the pads. I moved the more solid ground plane up a layer, so that the Vcc plane is next to last - and rerouted the clocks to avoid crossing the new plane.

    The last Vcca pin is connected on the bottom of the board with a 25mil trace, which is probably overkill - since it has to neck down to 8mil to touch the pad. I may slim that track down, but I wanted to maintain a fairly decent amount of copper. All that is left is to add a couple of bypass caps to the new Vcc plane.