Author Topic: an evening with the ICL7660  (Read 22976 times)

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Offline cellularmitosis

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an evening with the ICL7660
« on: July 09, 2014, 11:31:53 pm »
The ICL7660 is a switched capacitor voltage converter for e.g. making a negative rail from a positive rail.

I decided to fool around with one, specifically to see how bad the switching noise was, and to try out some circuits to tame the noise.
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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #1 on: July 09, 2014, 11:34:32 pm »
Circuit A: straight from the datasheet.

Schematic:



Circuit construction:





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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #2 on: July 09, 2014, 11:38:42 pm »
Circuit A into a 10k load.

Input voltage: 9.26V



Output voltage: -8.39V



Probing technique:



Noise:











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Offline switcher

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Re: an evening with the ICL7660
« Reply #3 on: July 09, 2014, 11:41:08 pm »
The Fluke 8060A uses one. I've recently repaired two 8060As, and had to change to 7660 in each of them, both producing +0.5VDC (yes, positive half a volt! hahaha!)
 

Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #4 on: July 09, 2014, 11:41:58 pm »
and Circuit A into a 1k load:

Input voltage: 9.25V
Output voltage: -7.84V

Noise:














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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #5 on: July 09, 2014, 11:44:10 pm »
Circuit B is the simplest attempt possible at eliminating the high frequency switcher noise: a ferrite bead is added before the reservoir capacitor.

Schematic:



Construction:

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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #6 on: July 09, 2014, 11:48:41 pm »
Circuit B into a 10k load:

Input and output voltages were identical to Circuit A into a 10k load.

Noise:



















Surprisingly (to me), in this arrangement the ferrite bead basically did nothing.
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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #7 on: July 09, 2014, 11:51:41 pm »
and Circuit B into a 1k load:

again, input and output voltages were identical to Circuit A into a 1k load.

Noise:















Again, the ferrite bead does very little in this arrangement.

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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #8 on: July 09, 2014, 11:53:47 pm »
Next for Circuit C we try to improve the behavior of the reservoir capacitor by adding a 1uF polyester film cap ("box" cap).

Schematic:



Construction:

« Last Edit: July 10, 2014, 12:30:07 am by cellularmitosis »
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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #9 on: July 09, 2014, 11:57:23 pm »
Circuit C into a 10k load:

Input voltage: 9.25V
Output voltage: -8.39V

Noise:

















Now we are starting to see this noise budge a little!
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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #10 on: July 10, 2014, 12:00:18 am »
and Circuit C into a 1k load:

Input voltage: 9.25V
Output voltage: -7.85V

Noise:













Again, the box cap makes a very apparent difference here.
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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #11 on: July 10, 2014, 12:03:17 am »
for Circuit D we get a bit more serious with our output filter, by adding a 100uH inductor and a second pair of capacitors.  Additionally, the ferrite bead is moved in between the two capacitor stages.

Schematic:



Construction:



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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #12 on: July 10, 2014, 12:11:42 am »
Circuit D lowered the noise to the point that the noise floor of my measurement setup became problematic.

To overcome this, I moved the setup into my home-made faraday cage, which is an aluminum dutch oven which has been fitted with a BNC connector.





The lid is held down with 3 bags of lead weights, each weighing 5 lbs.  This ensures a good EMI seal.



The BNC cable used is 1.5 feet of Canare L-5CFB RG6 double-shielded coax.  I had to move to this cable as the cheap-o BNC cables allowed too much noise in.


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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #13 on: July 10, 2014, 12:14:14 am »
Scope noise floor with all BNC connections capped:





Scope + faraday cage + powered-off circuit noise floor:



As best I can tell, these are switching spikes from Rigol's internal power supply.
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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #14 on: July 10, 2014, 12:20:45 am »
Circuit D into a 10k load.

Input voltage: 9.26V
Output voltage: -8.39V

Noise:



















Here we see a HUGE change in the amplitude and waveform of the noise!
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Offline Vgkid

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Re: an evening with the ICL7660
« Reply #15 on: July 10, 2014, 12:21:52 am »
That is a really informative post, thanks.
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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #16 on: July 10, 2014, 12:24:13 am »
and Circuit D into a 1k load.

Input voltage: 9.25V
Output voltage: -7.84V

Noise:









Our noise is now mostly just a 24kHz sine wave, which should be relatively straight-forward to filter out with a regulator.
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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #17 on: July 10, 2014, 12:26:48 am »
So... that's where I'm at right now.  Tomorrow night I'll try tacking a regulator onto this circuit to see how much of the 24kHz ripple I can get rid of.

I think I'll try two approaches: a simple LM79XX, and also a shunt regulator (TL431), and see how they compare.  I suspect the PSRR of the LM79XX will be lower than the TL431, as I believe their performance starts to fall off above 1kHz or so.

Until next time!
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Offline tautech

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Re: an evening with the ICL7660
« Reply #18 on: July 10, 2014, 12:28:36 am »
Nice thread and a good amount of work you have done.  :-+
But for these result to have any meaning you must list some characteristics of the components used.
Namely the Electrolytics, low ESR or GP????????? 
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Offline cellularmitosis

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Re: an evening with the ICL7660
« Reply #19 on: July 10, 2014, 12:43:07 am »
Good point.

The 10uF electrolytics are basically bottom of the barrel (cheap, general purpose).  They either came with my Jameco capacitor assortment, or were ordered from e.g. Tayda / ebay (they've become mixed together at this point).

The film cap is a Kemet ($0.56): http://www.digikey.com/product-detail/en/R82DC4100DQ60J/399-5447-1-ND/1930840



The LM7809 is also ebay / tayda (cheapest I could find).

The inductor is also from ebay.

The ferrite bead is (surprisingly, the only) through-hole axial 0-turn available on digikey ($0.16): http://www.digikey.com/product-detail/en/28L0138-40R-10/240-2439-1-ND/806799



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Offline tautech

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Re: an evening with the ICL7660
« Reply #20 on: July 10, 2014, 12:52:03 am »
If you did some tests with ML ceramic X7R caps as are often specified with switchers we would then have real results we can use.  :-+
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Offline LukeW

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Re: an evening with the ICL7660
« Reply #21 on: July 10, 2014, 06:11:10 am »
This thread is worth it just for the DIY Faraday cage alone!  :-+

But I would use a 50-ohm coax, which seems more "standard" as what you'd expect in test/instrumentation applications, as opposed to 75-ohm RG6 TV antenna cable.

Great post.
 

Offline AndyC_772

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Re: an evening with the ICL7660
« Reply #22 on: July 10, 2014, 06:18:09 am »
If you did some tests with ML ceramic X7R caps as are often specified with switchers we would then have real results we can use.  :-+

That was exactly my thought. Electrolytic caps have essentially no ability to filter high frequency noise and transients (for some definition of "high"), so there was no way the circuit was ever going to be quiet with just an electrolytic output cap. Stretching out the circuit over quite so much board area isn't helping either, even though the dead-bug style of construction is otherwise pretty good.

I've used the 7660 and generally found it does a good job, but I'd always use it in surface mount form with X7R (or maybe X5R) ceramic caps located right next to the device.

I'd suggest starting again, but instead of leaded caps, solder some SMT caps directly between the pins of the 7660 and the ground plane. No leads == minimal series inductance. Something like this should work well and shouldn't be too fiddly to work with:

http://uk.farnell.com/murata/grm31cr71e106ka12l/cap-mlcc-x7r-10uf-25v-1206/dp/1828837

I'd expect the result to be not too far from the multi-stage filtered method in terms of performance, and about 1/4 the size.

Offline Andreas

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Re: an evening with the ICL7660
« Reply #23 on: July 10, 2014, 06:19:10 am »
Hello,

nice test: never thought that a ICL7660 is capable to produce such spikes.
Some annotations:

From the first zoomed picture you can see that the rise time is around 100ns.
This is a equivalent frequency of around 3 MHz.
So you should select your components that they have maximum filtering at the 3 MHz point.

A ferrite bead has its best filtering around 100 MHz or above. -> you will not need it.
When I calculate the XL = j*2*PI*f*L = around 2k Ohms for 3 MHz and 100uH.
So I would expect a dampening in the range of 2K divided by the ESR of the capacitors.

If you select the capacitors for optimum filtering you will have to use either a (4 wire kelvin connected) X7R with 1uF for the 3 MHz together with a low ESR 100uF electrolytic. Or a 100 - 220nF foil (higher inductance) together with a around 10-22uF + a larger electrolytic.
But be aware that the component values are only valid for short leads (every half mm counts). Otherwise the self resonance will be shifted.

Edit: if you do not need the full output current you could try a small resistor 10-100 Ohms in series to the floating capacitor C3.

With best regards

Andreas



« Last Edit: July 10, 2014, 07:05:35 am by Andreas »
 

Offline tautech

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Re: an evening with the ICL7660
« Reply #24 on: July 10, 2014, 07:33:39 am »
To his credit cellularmitosis stated in the first post "as per datasheet"or words to this effect.
However "switchers" are noisy and one must always follow the minimum guidelines.
Low ESR caps are a must and even a bead tantalum (as much as I dislike them) would have been a far better choice than a GP electrolytic.
I had to use a 100 uF SMD Tant before I was happy with the ripple for a MCP1640 in SOT23/6 drawing 3 mA and 20 mA peaks for 70 mS.
It can be done as I'm sure cellularmitosis will show.
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