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?

 

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.

 

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.

 

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).

 

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.

 

They talk about electrolytic capacitors with an ESR of 10 ohms. I have never seen one higher than 0.1 ohms. 100 times less.

 

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.

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.

 

As with all filter caps - the bigger the better.

 

As with all filter caps - the bigger the better.

filter caps??. C1 and C2 in figure 20 http://www.jameco.com/Jameco/Products/ProdDS/1139469.pdf
couplers i presume. C3 and C4 are the "reservoirs" ?

 

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.

 

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.

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

 

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

John

 

You can't find 10 uF ceramics that will fit inside a blimp hanger.

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.

 

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

John

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....

 

The ESR will also depend on the physical size of the cap and the applied frequency. You should be OK with electrolytics. There are some newer X5R ceramics that are in the 10uF range and 25V:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=PCC2326CT-ND

 

i went with ceramic smt's #587-1352-1-ND