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