Vdds_ddr decoupling caps in am3358

[larbi....@gmail.com]

I see difference between last schematic of beaglebone black and am3358 starter kit board concerning vdds_ddr decoupling cap. Am3358 sk board has 21 decoupling cap and beaglebone black uses 9 decoupling caps. The am3358 datasheet specifies tu use min 20 decoupling caps. Why this difference?

[Gerald Coley]

Same answer as before.

BeagleBone came first. 

Smaller board size.

Lower cost

And it works.

Gerald

On Thu, May 26, 2016 at 10:46 AM, <larbi....@gmail.com> wrote:

I see difference between last schematic of beaglebone black and am3358 starter kit board concerning vdds_ddr  decoupling cap. Am3358 sk board has 21 decoupling cap and beaglebone black uses 9 decoupling caps. The am3358 datasheet specifies tu use min 20 decoupling caps. Why this difference?

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Gerald

 

ger...@beagleboard.org

http://beagleboard.org/

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[John Syne]

There are several other factors that affect decoupling:

PCB Layout (smaller is better)

Ground planes size, distance between ground planes and dielectric material used

Decoupling cap ESR

Proximity of the decoupling caps to the power pins

Number and size of vias connected to power pins (think of vias as a small inductor)

Copper pour profile used to connect power pins to power rail

Clearly the BBB layout is optimal and that is why they managed to make the board work with fewer decoupling caps.

Regards,

John

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[larbi....@gmail.com]

ok thank you for the reply.