Current Consumption of VCCA pin for CIII-25

[Altera_Forum]

Hello people, 

 On my project, i have 8 CIII-25 fpga's running parallel.  Fpgas will run over 100 MHz full performance with almost of the logic elements are used for switching.  I don't know Analog Electronics so much and need your assistance here.  fpga's have 1.2V core supply, 2.5V analog supply and 3.3V I/O supply.  I use ti's TPS5450 smps ic capable of supplying 5 Amps current.  I use one smps for two fpgas' core supply. (totally 4) I use one smps for 2.5V analog VCCA supply. (shared for all fpgas) I use one smps for 3.3V I/O supply. (shared for all fpgas)  totally 6 smps ic's are used.  1- I want to ask if this design apporach is good or not.  2- How much current does the Analog Supply Pins require? one TPS5450 (5 Amps) will be enough?  3- For this project, which ferrite bead for filtering do you advise me? and what should the shematic design look like? One 2.5V supply is filtered once and shared with all fpgas? or one 2.5V trace will be brought to all fpga's and ferrite filtering is made seperately for each fpga?  4- Do you have advises for me about pcb trace width, trace length, shape etc.  Thanks

[Altera_Forum]

VCCA is used only to supply device PLLs and it requires only a few mA. A 5A smps is wasted and I think it would introduce a lot of high frequency noise, unless you need 2.5V for other devices on your board.  

If you need it only for VCCA the best solution could be use a low power linear regulator which derives 2.5V supply from 3.3V. Current absorption from other rails will strongly depend from logic implemented in fpga (VCCint) and from number of used I/Os and their required switching speed (VCCio)

[Altera_Forum]

Read the design resources here; 

 http://www.ovro.caltech.edu/~dwh/carma_board/ (http://www.ovro.caltech.edu/%7edwh/carma_board/)  This board contains a 1.2@60A power supply for four Stratix II devices, a 2.5V@5A supply, and a bunch of others. Look at p3 and p4 of the schematic for a block diagram of the supplies  http://www.ovro.caltech.edu/~dwh/carma_board/gda06rb004_carma_v0.87_dec03.pdf (http://www.ovro.caltech.edu/%7edwh/carma_board/gda06rb004_carma_v0.87_dec03.pdf)  Then read the engineering specification and power supply design documents.  Cheers, Dave

[Altera_Forum]

Thanks Guys, 

 There will be 8 CIII-25 fpgas on the board.   - what can the total current needed for 2.5V line? i could not see any info on the device handbook.  - Can you advise me a LDO from 3.3V to 2.5V.  - We won't need ferrite beads if we use LDOs right? (although 8 fpgas will be supplied from the same LDO output?)  Thanks..

[Altera_Forum]

--- Quote Start ---   There will be 8 CIII-25 fpgas on the board.  - what can the total current needed for 2.5V line? i could not see any info on the device handbook.   --- Quote End ---   I found on CIII datasheet that current for VCCA is about 8mA. Then for 8 devices a 100mA supply could be ok.    --- Quote Start ---   - Can you advise me a LDO from 3.3V to 2.5V.   --- Quote End ---   There are a lot of devices good for this. Browse the selection guides from any vendor website (Texas Instruments, National or Linear Technology have plenty of LDOs) and then choose one based on availability from your local distributors.    --- Quote Start ---   - We won't need ferrite beads if we use LDOs right? (although 8 fpgas will be supplied from the same LDO output?)   --- Quote End ---   Probably ferrite beads are not strictly needed when you use a dedicated supply for analog circuitry. Anyway I think placing the ferrite + capacitor filter near the VCCA pins is still recommended; infact, even if 2.5V is not shared with any digital supply, connecting it to 8 fpgas will require rather long traces which could pick up high frequency noise, especially if you can't route them through a dedicated pcb plane.