Re: Bypass Caps : XAPP623 vs Spartan-3 Starter Kit Board

First, please note that the Spartan-3 starter kit was not designed by Xilinx. Digilentinc.com is where you should find the trail ending. Just because a manufacturer recommends something doesn't mean board developers are going to follow it.

Glancing through XAPP623, it's surprising how many points we've *just* been talking about this last week here on this forum are touched upon. As that discussion went, there's more than one approach to a solution that can work for your needs.

One suggestion I'd give regarding your cap numbers: round up.

The numbers you have may work very well. There's no guarantee that it's the "best" or most appropriate solution for the board you need. In the XAPP623 example, there's no adjustment to capacitor count for plane spacing demonstrating where "all" the information might not be there.

One suggestion: buy a book from a guy named Ritchey. Just search on this group for his name using google or similar search tool and you'll find many posts from this past week. More ideas are presented in those posts.


jidan1@xxxxxxxxxxx wrote:
Hi,

I want to design a development board containing Xilinx Spartan-3
XC3S400. My problem is regarding the number and values of bypass
capacitors that I should use for the power supplies.
I use 3 power supplies: VCCINT=1.2V; VCCAUX=2.5V; VCCO=3.3V.
I took the Spartan-3 Starter Kit Board User Guide
(http://direct.xilinx.com/bvdocs/userguides/ug130.pdf) as a refernce,
but I fond it contradicts XAPP623 - Power Distribution System (PDS)
Design: Using Bypass/Decoupling Capacitors
(http://direct.xilinx.com/bvdocs/appnotes/xapp623.pdf).

XAPP623 recommends for every power supply:
=Capacitor= =Value Quantity Percentage=
470 µF to 1000 µF 4%
1.0 to 4.7 µF 14%
0.1 to 0.47 µF 27%
0.01 to 0.047 µF 55%

Therefore...

Bypass caps for VCCINT/VCCAUX (8 pins) according to XAPP623
=Value range= =No. of Caps=
470 µF to 1000 µF 8 pins x 4%= 0.32(0)
1.0 to 4.7 µF 8 pins x14%= 1.12(1)
0.1 to 0.47 µF 8 pins x27%= 2.16(2)
0.01 to 0.047 µF 8 pins x55%= 4.4(5)

Bypass caps for VCCO (24 pins) according to XAPP623
=Value range= =No. of Caps=
470 µF to 1000 µF 24 pins x 4%= 0.96(1)
1.0 to 4.7 µF 24 pins x14%= 3.36(3)
0.1 to 0.47 µF 24 pins x27%= 6.48(7)
0.01 to 0.047 µF 24 pins x55%= 13.2(13)

_______________
But Spartan-3 Starter Kit Board uses:
For VCCINT
=No. of Caps=
2 x 10uF
6 x 0.01uF
10 x 0.047uF

For VCCAUX
=No. of Caps=
1 x 10uF
16 x 0.01uF

For VCCO
=No. of Caps=
3 x 10uF
32 x 0.047uF

I would like to know how the Spartan-3 Starter Kit Board got those
calculations and why it didn't use the XAPP623 recommendations and
which one is better for me to choose?
Thank you very much
JJ

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