Texas Instruments TMS320C6722 Power Supplies, Power-Supply Sequencing, Power-Supply Decoupling

TMS320C6727, TMS320C6726, TMS320C6722

Floating-Point Digital Signal Processors

www.ti.com

SPRS268E –MAY 2005 –REVISED JANUARY 2007

4.7 Power Supplies

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4.7.1Power-Supply Sequencing

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.

4.7.2Power-Supply Decoupling

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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