Vccaux on Spartan 3


I'm wondering if there is any advantage on using a separate regulator for the Vccaux.

I already have 2.5V generated on the board by a switching regulator for the multiple DDR chips and for some Vccio of the spartan 3. Now, with the new TI regulator that has 3 regulators in a chip, I can have a separate LDO to power just the Vccaux portion. Is it useful to have it separate ?



Reply to
Sylvain Munaut
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Do not use a switching reg for Vccaux, it's noisy and not recomended to power your DCM (DCM is powered by VCCaux)

Yes it is. Aurash

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Reply to
Aurelian Lazarut


Thanks for the information, just what I needed


Reply to
Sylvain Munaut

Vccaux powers the DCMs so having a separate power supply should help to reduce clock jitter. If you have a separate supply available I'd use it.

Reply to
General Schvantzkoph


A separate power supply for Vccaux which powers the DCM delay lines, IO predrivers, and some bias circuits is a good idea to reduce system jitter.

Switching regulators are just fine to use in this application. As long as there is no more than a 10 mV droop in a 1 ms period (that would be a really slow switcher -- 1 KHz????). Variations of hundreds of mV at a fast rate are smoothed out by the DCM's filtering (typical of a switcher). It is a sudden droop followed by a slow recovery that affects the DCM, as it can not track fast enough to correct for the push out in delay of the delay lines. This primarily affects the falling edge of the clock coming out, and the Clock 2X output, and Clock DV output (which all use falling edges to synthesize their signals).

Aust> Hello,

Reply to
Austin Lesea

thanks for the information, just planning on making a pcb with a spartan3 on it (my first own pcb) ...

kind regards,





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