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Visitor
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Registered: ‎06-03-2014

Recommended decoupling caps for multiple Vcc-Pins

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Hi everybody

 

I'm designing a board with a Spartan-6 LX9, TQG144 package. I followed the guidelines in UG393 (table 2-1) to select the decoupling capacitors. However I'm a bit confused how the proposed caps should be placed in an optimal way.

 

E.g. there are three Vcco1 pins, which are quite widespread along the bank, the guide recommends  tu use 2x 0.47uF and 1x 4.7uF caps. How can I place the two high-freq caps as close as possible to three Vcc pins? Same goes for all the other Vcc's, the number of recommended caps seems not to correlate with the number of pins. Or am I totally misunderstanding the whole decoupling concept?

 

In an other design (Papilio Pro Board) and according to some answers in this forum, another way is to place a 0.1uF close to each Vcc pin and add some additionial bulk caps. Would that be a better solution than the one proposed in the UG393?

 

Thanks for any help or advice!

tankser

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Instructor
Instructor
12,419 Views
Registered: ‎07-21-2009

there are three Vcco1 pins, which are quite widespread along the bank, the guide recommends  tu use 2x 0.47uF and 1x 4.7uF caps. How can I place the two high-freq caps as close as possible to three Vcc pins?

 

The answer to your question is clear:  the recommendation is neither perfect nor reasonable.

 

Almost every PCB capacitor guideline or requirement document needs to be viewed with a bit of skepticism.  There is more than one correct solution (or implementation) for board layout and power rail decoupling.

 

Let's clear up a misunderstanding in your post, one that is held and taught by far too many.

 

Yes, the 0.47uF caps are the 'high-freq' caps and the 4.7uF is not a 'high-freq cap.  But this is true not by virtue of the capacitance (value) of the caps, but because of their size.  The 0.47uF caps are 0402 size, and the 4.7uF cap is 0805 size.  Their differences in package size are the reasons for their distinctions with respect to performance at high frequencies.


In the old days of leaded capacitors, the small value caps had much better high-frequency characteristics than the big, bulky higher value capacitors.  In the age of multi-level ceramic capacitors (MLCCs) this is no longer true.

 

If you compare the frequency characteristics (impedance and inductance vs. frequency) of a 0.01uF 0402 capacitor to a 4.7uF 0402 capacitor, you will see that their high-frequency characteristics are identical.  They are distinguished only by their low-frequency characteristics, in which the higher value capacitor provides lower (better) impedance than the low-value capacitor.

 

In other words, a single 4.7uF 0402 capacitor will do everything you would expect of a 4.7uF 0805 cap and a 0.47uF 0402 cap.

 

In sum:  the best per-pin decoupling capacitor (all else being equal) is the highest value capacitor in the smallest size.  Start with the capacitor size (I generally use 0402 size for per-pin supply decoupling), and find the highest value in that size (keeping in mind reasonable cost, availability, temperature grade, and voltage rating).

 

NOTE:  If you read UG393 Figure 2-6 and the text below it, Xilinx is saying much the same as I am -- that high-frequency performance is determined by the device package size, not capacitor value.  Unfortunately, Xilinx does not faithfully apply this reality in their choce of recommended capacitors!

 

The 'sweet spot' for decoupling caps used to be 1uF in 0402 packages, but 4.7uF 0402-size 6.3V X5R capacitors are now readily available for less than $0.02 in high volume (compared with $0.013 for 4.7uF 0805-size caps).  Using high value per-pin capacitors allows you to reduce the number of 'bulk' decoupling capacitors and reduce power supply ripple.  The board area saved with this implementation approach can be quite valuable in some designs!

There are 10uF 0402 6.3V X5R capacitors available, but their high cost ($0.08 or more) is too high for per-pin usage for most customers and most designs.

 

By comparison, 0.1uF and 1.0uF 0402 X5R capacitors are selling for about $.0014 and $.0029, respectively.  When you factor in the per-component cost of pick-and-place, the in-circuit costs of 0.1uF, 1.0uF, and 4.7uF 0402 6.3V X5R capacitors are roughly equivalent.

 

My advice to you is to use your best judgment, even if it seems at odds with the PCB Design Guide.

 

For my most recent designs, I have been using 4.7uF 6.3V X5R 0402 caps for per-pin decoupling of low voltage (3.3V and lower) supplies, and add additional 0805-size bulk decoupling caps only if necessary.

 

I remember a board design guide published by a very large FPGA manufacturer which recommended a capacitor from 4 decade ranges of values -- per pin!  Yes, it required a .0001, .001, .01,  and .1uF capacitor on each and every power supply pin.  This is the 'truth' of an era far in the past.  MLCC surface mount capacitors have changed the implementation rules, while the fundamental concepts remain the same.  If you follow the concepts (but not necessarily the implementation rules), you will be on solid ground.

 

I hope this helps...

 

-- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.

View solution in original post

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Highlighted
Instructor
Instructor
12,420 Views
Registered: ‎07-21-2009

there are three Vcco1 pins, which are quite widespread along the bank, the guide recommends  tu use 2x 0.47uF and 1x 4.7uF caps. How can I place the two high-freq caps as close as possible to three Vcc pins?

 

The answer to your question is clear:  the recommendation is neither perfect nor reasonable.

 

Almost every PCB capacitor guideline or requirement document needs to be viewed with a bit of skepticism.  There is more than one correct solution (or implementation) for board layout and power rail decoupling.

 

Let's clear up a misunderstanding in your post, one that is held and taught by far too many.

 

Yes, the 0.47uF caps are the 'high-freq' caps and the 4.7uF is not a 'high-freq cap.  But this is true not by virtue of the capacitance (value) of the caps, but because of their size.  The 0.47uF caps are 0402 size, and the 4.7uF cap is 0805 size.  Their differences in package size are the reasons for their distinctions with respect to performance at high frequencies.


In the old days of leaded capacitors, the small value caps had much better high-frequency characteristics than the big, bulky higher value capacitors.  In the age of multi-level ceramic capacitors (MLCCs) this is no longer true.

 

If you compare the frequency characteristics (impedance and inductance vs. frequency) of a 0.01uF 0402 capacitor to a 4.7uF 0402 capacitor, you will see that their high-frequency characteristics are identical.  They are distinguished only by their low-frequency characteristics, in which the higher value capacitor provides lower (better) impedance than the low-value capacitor.

 

In other words, a single 4.7uF 0402 capacitor will do everything you would expect of a 4.7uF 0805 cap and a 0.47uF 0402 cap.

 

In sum:  the best per-pin decoupling capacitor (all else being equal) is the highest value capacitor in the smallest size.  Start with the capacitor size (I generally use 0402 size for per-pin supply decoupling), and find the highest value in that size (keeping in mind reasonable cost, availability, temperature grade, and voltage rating).

 

NOTE:  If you read UG393 Figure 2-6 and the text below it, Xilinx is saying much the same as I am -- that high-frequency performance is determined by the device package size, not capacitor value.  Unfortunately, Xilinx does not faithfully apply this reality in their choce of recommended capacitors!

 

The 'sweet spot' for decoupling caps used to be 1uF in 0402 packages, but 4.7uF 0402-size 6.3V X5R capacitors are now readily available for less than $0.02 in high volume (compared with $0.013 for 4.7uF 0805-size caps).  Using high value per-pin capacitors allows you to reduce the number of 'bulk' decoupling capacitors and reduce power supply ripple.  The board area saved with this implementation approach can be quite valuable in some designs!

There are 10uF 0402 6.3V X5R capacitors available, but their high cost ($0.08 or more) is too high for per-pin usage for most customers and most designs.

 

By comparison, 0.1uF and 1.0uF 0402 X5R capacitors are selling for about $.0014 and $.0029, respectively.  When you factor in the per-component cost of pick-and-place, the in-circuit costs of 0.1uF, 1.0uF, and 4.7uF 0402 6.3V X5R capacitors are roughly equivalent.

 

My advice to you is to use your best judgment, even if it seems at odds with the PCB Design Guide.

 

For my most recent designs, I have been using 4.7uF 6.3V X5R 0402 caps for per-pin decoupling of low voltage (3.3V and lower) supplies, and add additional 0805-size bulk decoupling caps only if necessary.

 

I remember a board design guide published by a very large FPGA manufacturer which recommended a capacitor from 4 decade ranges of values -- per pin!  Yes, it required a .0001, .001, .01,  and .1uF capacitor on each and every power supply pin.  This is the 'truth' of an era far in the past.  MLCC surface mount capacitors have changed the implementation rules, while the fundamental concepts remain the same.  If you follow the concepts (but not necessarily the implementation rules), you will be on solid ground.

 

I hope this helps...

 

-- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.

View solution in original post

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Highlighted
Visitor
Visitor
7,850 Views
Registered: ‎06-03-2014

Thank you very much for that superb answer! That helped a lot

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