TMS320C6424: DRDR_VDDDLL

I have done some digging and found an early version of the DDR2 PCB layout application note (TI SPRAAL6) for this device, where a similar filter was shown on DDR_VDDDLL the pin. The recommendation was removed from next revision of this document. Therefore, it appears the original design team found an issue with filtering this supply rail and purposefully removed the requirement. The original design team is no longer available to ask, so I would suggest you do not filter this pin.

I suspect they found the natural resonance frequency of the filter and the transient profile of the DDR_VDDDLL pin introduced a power supply resonance, and it was worse that the noise normally found on the shared power source. They may have been able to resolve this by including a bulk capacitor between the filter and the DDR_VDDDLL pin, but decided it was also acceptable to simply remove the filter.

Regards,
Paul

Are you proposing a separate filter circuit like the one being used for PLLpwr18 as shown in the snapshot inserted above for DDR_VDDDLL?

If so, the filter you are proposing has bulk capacitance between the series inductance and the processor pin which should help resolve any power supply resonance. 

I do not know why the filter was removed. Maybe it was removed to eliminate DC voltage drop.

I have not been able to find anyone at TI that knows why the filter was removed from DDR_VDDDLL, so I'm not able to answer your question with any certainty. I suppose you can always replace the Ferrite in the filter with a 0 ohm resistor if it causes any problems.

Regards,
Paul

Yes, it should be fine. Their filter implementation shown above is better than the one shown in the datasheet.

Regards,
Paul