Which cap should i use?

Discussion in 'General Electronics Chat' started by DC_Kid, May 22, 2008.

  1. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    i'm building simple dc-dc converter using ICL7660. a basic design uses 3 caps and two diodes. i've been reading some about dc-dc converters like this ICL and it seems that cap choice is important for the integrated oscillator to work properly. seems that the device is sensitive to cap ESR. tantalum seems to be the right type but i have a few questions.

    1. my input to the ICL is +5v, so the output from ICL will get me -5v and +9v. i can buy 16v or 25v rated caps, which one to buy (same price)?
    2. the reason i ask about cap voltage rating is for a couple of reasons. ESR goes up as cap dialectric goes up (how much i am unsure of, no ESR specs from the Jameco site). and, it is not clear to me if during power-on the ICL might generate output voltage more than 16v.

    any suggestions?
     
  2. Caveman

    Active Member

    Apr 15, 2008
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    The problem with ESR is not with the integrated oscillator, but rather with the efficiency. Just try to go low. If you really want to control this, buy a cap with proper specs. the formula for the efficiency is in the ICL7660 datasheet.

    As far the voltage range, the maximum voltage that you should get across the charge cap is the input voltage. The maximum on the output cap is double that.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    The oscillator and timing capacitors are inside the ICL7660. The external capacitors are just a low current charge pump. The output resistance of the ICL7660 is so high (60 ohms) that the ESR of your capacitors doesn't matter.
    Tantalum capacitors are not reliable. Use electrolytic capacitors instead. Use 25V or 35V ones.
     
  4. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    datasheet states:
    "Since the ESRs of the capacitors are reflected in the output
    impedance multiplied by a factor of 5, a high value could
    potentially swamp out a low 1/(fPUMP • C1) term, rendering an
    increase in switching frequency or filter capacitance ineffective.
    Typical electrolytic capacitors may have ESRs as high as 10Ω"

    i would think lower ESR's are better, so why not use ceramics here?

    also, why do they choose 10uF for C1 and C2? the datasheet says that if you drop the freq down by factor of 10 then cap size has to go up by same factor. why is this (trying to understand the relationship, etc).
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    You can't find 10 uF ceramics that will fit inside a blimp hanger. Why worry about effective series resistance? The frequency isn't that high, and a .1 uF ceramic in parallel with the tantalum can be a low impedance path for noise. The purpose of the tantalum cap is to filter the negative output from the 7660. I have used several 7660's with the 10 uF tantalum alone with no problem.

    The capacity of a capacitor has a lot to do with the amount of energy storage it can provide. Using a lower oscillator frequency means the cap gets a recharge of energy as less frequent intervals. The cap has to supply the current for more time without excessive droop. That is a lot more significant than an ESR rating.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    They talk about electrolytic capacitors with an ESR of 10 ohms. I have never seen one higher than 0.1 ohms. 100 times less.
     
  7. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    huh? i'm assuming you just mean "big as heck" (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=445-2898-ND). perhaps bigger than a 25v/10uF electrolytic..... but ceramic will have much lower ESR.

    so do the caps really matter (size and type)? can i simply use some 22uF/25v electrolytics i have for C1 and C2? i'm building figure 20 in the datasheet.
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    As with all filter caps - the bigger the better.
     
  9. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    Last edited: May 22, 2008
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    They are called pump and reservoir caps, but they just hold a voltage or source a load, so I imprecisely call them filters. I find myself back in technician mode due to external circumstances, so my correct terminology has suffered.
     
  11. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    ceramics are probably better than others, but come with a price and need more board space. from the Linear LTC1046 datasheet (a direct pin replacement for ICL7660). i read it as "the lower ESR the better". so perhaps a small ceramic with small ESR paralled with a electrolytic...? 10uF/16v ceramic is only 40cents each.

    Code ( (Unknown Language)):
    1.  
    2. Capacitor Selection:
    3. While the exact values of CIN and COUT are noncritical,
    4. good quality, low ESR capacitors such as solid tantalum
    5. are necessary to minimize voltage losses at high currents.
    6. For CIN the effect of the ESR of the capacitor will be
    7. multiplied by four, due to the fact that switch currents are
    8. approximately two times higher than output current, and
    9. losses will occur on both the charge and discharge cycle.
    10. This means that using a capacitor with 1W of ESR for CIN
    11. will have the same effect as increasing the output impedance
    12. of the LTC1046 by 4W. This represents a significant
    13. increase in the voltage losses. For COUT the effect of ESR
    14. is less dramatic. COUT is alternately charged and discharged
    15. at a current approximately equal to the output
    16. current, and the ESR of the capacitor will cause a step
    17. function to occur, in the output ripple, at the switch
    18. transitions. This step function will degrade the output
    19. regulation for changes in output load current, and should
    20. be avoided. Realizing that large value tantalum capacitors
    21. can be expensive, a technique that can be used is to
    22. parallel a smaller tantalum capacitor with a large aluminum
    23. electrolytic capacitor to gain both low ESR and
    24. reasonable cost. Where physical size is a concern some
    25. of the newer chip type surface mount tantalum capacitors
    26. can be used. These capacitors are normally rated at
    27. working voltages in the 10V to 20V range and exhibit very
    28. low ESR (in the range of 0.1W).
     
    Last edited: May 22, 2008
  12. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I have used the ICL7660 and got some small, 1206 size, 10uF/35V ceramics to use. Digikey #587-1352-1-ND

    John
     
  13. Caveman

    Active Member

    Apr 15, 2008
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    Need to bone up on your current technology. The cap guys have been busy. They are called MLCC (Multilayer Ceramic Chip). I have designed in 22uF 1206 smt ceramics. They're expensive, but are available.
     
  14. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    i'm looking at #445-2881-ND ($0.21 ea qty100). i only need to make ~30 dc-dc converters to fix my mistake. the smt you quote is $0.208 ea qty100. hmmm, which one? smt take up less space, but a tad harder to solder in by hand....
     
    Last edited: May 22, 2008
  15. nanovate

    Distinguished Member

    May 7, 2007
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  16. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    i went with ceramic smt's #587-1352-1-ND
     
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