EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: hammil on March 31, 2013, 08:38:36 pm
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Hi all
I've been searching a while for a particular type of component.. A switching converter that uses capacitors only - no inductors - and includes some form of feedback regulation. A dual-supply output would be even better, because then I could supply almost any analog system using a coin cell and a single IC.
I have found devices that either have regulation OR don't use an inductor, but not both. Apart from the LM2760, which only supplies 3.3V - it's not adjustable, which is a great shame.
There is one beauty I found.. The MAX1748. It does dual-output regulation, up to 40V output from an input as low as 3V. /That's/ what I'm talking about... But a small hack is required to get the soft-start to work (shorting all the 'main converter' pins together, it amounts to), it's hard to get, and expensive. Is there another alternative, perhaps? 40V output isn't necessary, nor are high currents - 15V and tens of mA would do fine for most applications, I think.
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The big trouble with switched capacitor converters is that everything is a tradeoff, to get high step-up ratio you get ridiculous output ripple. I'd strongly urge you to consider a classic inductor based switcher, but if that's no good then...
The LT1054, whilst it is officially a negative output supply but it looks like it might fit the bill, so if you're using a battery just shove ground on the negative side and take the 0V as positive, there's also the LTC1263 which has the voltage but limited input range at basically 5v
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Yeah... Problem with inductors is that they generate a lot of EMI, and if you've got a lot of high-impedance nodes on your board...
Output ripple, I can deal with... to an extent, I guess. I'm often dealing with really low currents, like tens of microamps, so the effect is reduced.
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Sounds like very low power
A micro power micro wiggling some of it outputs?
If you can get the micro running it might even boost it own power.
if you need higher voltage then micro, some of the old cmos logic worked to 15 V
or wiggling a interface chip
You might even try wiggling the input of an op amp. The Op amp could build the higher supply for it;s self
C
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Ooh, that sounds interesting... Don't think it would suit my application, but I'm very curious. Could you post a schematic so I can see the concept?
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Not quickly,
The trick if there is one is to use the current paths and not to fight them.
for a + and - supply, you need two ic packages, not two op amps in one package.
1. Bat+
2. Bat-
3. Float+
4, Float-
depends on the macro what the macro can do directly, but most can make an op amp's output switch rails.
the + op amp uses #3 and #2 for supply and can pull something to #2 and lift it to almost #3
the - op amp uses #2 and #4 for supply and can pull something to #3 and push it down almost #4
the pull and push is limited to what the op amp's output current.
To get started you need some diodes to charge float to bat level.
+op output to boost cap neg which is charged from Bat+ and when lifted drains in to float+ cap
more op amps in package, use one for voltage monitor to micro's adc.
Pull the op amp's data sheet, look at the transistors in it an use them not fight them.
Edit Error corrected above
C
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You can even have your high impeadance traces perpendicular to the inductor. That will eliminate some of the EMI problems that you have. I'm not sure that it is necessary or that it would make a big difference :-//
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In close proximity you may very well find that the switched capacitor regulator may be no better than a shielded inductor based regulator. There are still strong spikes of voltage and current that'll couple quite nicely. The switched capacitor version will be better than the inductor solution at a distance because of the frequency characteristics of the noise.
Not saying anyone is right or wrong, but it would be well worth your while trying your luck with a 1W traco 5v to 15v converter and a bit of mu metal before opening up the can of worms that is switched capacitor regulators.