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# Become a Decoupling Capacitor Network Guru, Part 1

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Re: Doubt about the target impedance equation
12/16/2013 8:17:12 AM
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Thank you for the clarification. Units analysis indicates that the original equation is not correct and that one of the numerators must be unitless.

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Re: Doubt about the target impedance equation
12/13/2013 3:27:53 AM
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Sorry I should have been a little clearer in the article the Max voltage ripple is not a voltage but actually a scaled percentage e.g.0.05 for 5% ripple

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Doubt about the target impedance equation
12/12/2013 8:00:34 PM
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Hi! I liked your article a lot. I have learned a lot from your articles. This time I wanted to ask you about the equation for the target impedance: since it is (voltage) * (maxVoltageRipple) / (current), I guess there is a problem with the units for impedance, which instead of being "ohms", the would be ( volts * ohm)? right,  {V}*{V}/{A} = V*Ω . Please correct me if I am wrong, I am a complete newbie. Thanks.

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Re: what's the ESR?
12/12/2013 6:51:03 PM
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Or Min Max and Sigma over operating params! It's even worse with optics and electroptics.

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Re: Cost vs Characteristics?
12/12/2013 3:22:49 PM
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Faraday <=> friend, Henry <=> horror.
SMT any day, minimum layer spacing, lots of planes.
4 via routing or 6 if you can afford feed-thru caps or posh X2Y caps. (prefer feed-thru's, but Scot's are canny with the cash), don't accept more than a few hundred pH for each cap on the layout. Cross sectional area of the power/return loop is proportional to inductance. Arrays are also useful, just alternate power and return. Sub 100nm CMOS have rise/fall times of a few 100ps, so the harmonics are well into the GHz.
Biot-Savart law, James Clerk Maxwell etc.
Dielectrics, most exhibit a capacitance/voltage/temperature drop off and ageing.
What do you call a Welshman that makes deionized water for car batteries? Di Electric.
X7R and especially X5R are ferroelectric as well, so we have a non-linear TCC and VCC with piezoelectric microphony chucked for bad measure!
Despite the last paragraph, the ferroelectric dielectrics have a lower Q, fewer layers and less metal in comparison to low permittivity NP0/COG, therefore higher resistance and lower Q, less PSD oscillation.
0402 2 terminal 4 via, or 0603 3 terminal feed thru's with 6 via, X7R 1uF and lot's of them. It usually costs the same price of the cap to get them stuffed onto the PCB. Random geometry proffered. And watch out for the unintended EBG!
For SMT and ROHS, forget poly-anything. Unless necessary in analogue or RF.
For bulk, Lytics have a very limited life unless de-rated (a few thousand hours). Tants fare better but 1/3 of voltage is a safe de-rate. (Tants do blow the copper and laminate off PCB's if production bungs them the wrong way round!).

Overstayed my time, but hope the thrupence worth is not too boring!
I live in a market town in Schottland. Started learning German and met a German tractor driver today - "Kann ich kompostieren Ihren Traktor".

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Re: Are SMT Decoupling Capacitors Ceramic
12/12/2013 3:07:58 PM
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You can get surface mount versions of a lot of flavors! A quick look on Digikey shows: ceramic, Al electrolytic, Al poly, tantalumn, thin film, niobium dioxide, silicon(?), supercaps, mica, and PTFE.

Poly caps are nice for high ripple current applicationsthat due to a much lower ESR than regular Al electrolytics but can be expensive and take up a large amount of volume.

Ceramics can also have the ferruls on the long edge or in the X2Y configuration where there are two more pads between the normal terminals. You can even use a single X2Y to implement two caps!

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what's the ESR?
12/12/2013 2:01:18 PM
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For caps on the outputs of voltage regulators the ESR value has a significant effect on stability, but getting min/typ/max numbers from a cap manufacturer is like pulling teeth.

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Re: Are SMT Decoupling Capacitors Ceramic
12/12/2013 12:37:29 PM
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Most capacitor types are available as either SMT or TH.  Oftentimes ceramic caps are used int SMT as decoupling due to low cost and low ESR.  As others have noted, temperature and voltage operating conditions need to be accounted for.

(Aluminum) electrolytics are commonly available, as are tantalum.  The Al electrolytics tend to be relatively large depending on voltage and capacitance.  This can lead to mechanical shock and vibration problems, as the only thing holding them on the board is the glue between the pads and the PCB material.

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Re: Don't Forget ALL Those Other Important Characteristics!
12/12/2013 3:03:40 AM
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Hi Howard,

Thank you for the comment, there is limited space for a blog so a number of your points will be addressed in part 2 which looks at how and where we mount the devices and what the driving requirements are.

Hopefully max will be posting it soon

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Don't Forget ALL Those Other Important Characteristics!
12/11/2013 7:09:16 PM
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Your article has started out with good information but please don't forget to talk about the following:

1) change in capacitance of ceramic caps versus bias voltage or the DC voltage across them

2) connection impedances to the ground plane and power plane and to the device you're trying to bypass

3) performance of ceramic caps versus electrolytics as bypass elements

4) location of bypassing caps versus location of the part you're trying to bypass

5) change in capacitance versus bias voltage for other types of capacitors other than ceramic caps

6) amount of charge storage or capacitance required for various current transients

7) package resonances when using several caps to get low impedance over wide frequency range

8) when to use broad frequency range decoupling versus narrow frequency range decoupling

There are more but I suspect you don't have the time or space available to talk about all of them.

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