TMX320C6720RFP300 datasheet(37/118 Pages) TI

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4.7 Power Supplies

4.7.1

Power-Supply Sequencing

4.7.2

Power-Supply Decoupling

TMS320C6727B, TMS320C6726B, TMS320C6722B, TMS320C6720

Floating-Point Digital Signal Processors

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SPRS370E – SEPTEMBER 2006 – REVISED JULY 2008

For more information regarding TI’s power management products and suggested devices to power TI

DSPs, visit www.ti.com/dsppower.

This device does not require specific power-up sequencing between the DVDD and CVDD voltage rails;

however, there are some considerations that the system designer should take into account:

1. Neither supply should be powered up for an extended period of time (>1 second) while the other

supply is powered down.

2. The I/O buffers powered from the DVDD rail also require the CVDD rail to be powered up in order to be

controlled; therefore, an I/O pin that is supposed to be 3-stated by default may actually drive

momentarily until the CVDD rail has powered up. Systems should be evaluated to determine if there is

a possibility for contention that needs to be addressed. In most systems where both the DVDD and

CVDD supplies ramp together, as long as CVDD tracks DVDD closely, any contention is also mitigated by

the fact that the CVDD rail would reach its specified operating range well before the DVDD rail has fully

ramped.

In order to properly decouple the supply planes from system noise, place as many capacitors (caps) as

possible close to the DSP. The core supply caps can be placed in the interior space of the package and

the I/O supply caps can be placed around the exterior space of the package. For the BGA package, it is

recommended that both the core and I/O supply caps be placed on the underside of the PCB. For the

TQFP package, it is recommended that the core supply caps be placed on the underside of the PCB and

the I/O supply caps be placed on the top side of the PCB.

Both core and I/O decoupling can be accomplished by alternating small (0.1

µF) low ESR ceramic bypass

caps with medium (0.220

µF) low ESR ceramic bypass caps close to the DSP power pins and adding

large tantalum or ceramic caps (ranging from 10

µF to 100 µF) further away. Assuming 0603 caps, it is

recommended that at least 6 small, 6 medium, and 4 large caps be used for the core supply and 12 small,

12 medium, and 4 large caps be used for the I/O supply.

Any cap selection needs to be evaluated from an electromagnetic radiation (EMI) point-of-view; EMI varies

from one system design to another so it is expected that engineers alter the decoupling capacitors to

minimize radiation. Refer to the High-Speed DSP Systems Design Reference Guide (literature number

SPRU889) for more detailed design information on decoupling techniques.

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