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Switched Cap Boost Converters

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col_implant

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I'm wondering is there such a thing as a switched cap boost converter that is not based on voltage doubling operation?

Im designing for a very small, low power implant (short bursts of up to 20mA, long periods of 'sleep' 100uA). Inductor based supplies are not so suitable as inductors are quite large, and inductor boost converters are bad at very low currents.

Every switched cap converter seems to be a doubler. I thought I had found one not based on this principal (no mention of the word doubler in the datasheet) however the efficiency is approximated as...

Eff % = Vout/2Vin >>> clearly a doubler!

Im working with a battery nominal 3V, operating range 2-2.85V stepping up to 3.3 so im getting shocking efficiency using switched cap converters!

Any insights anyone???
 
"Boost converters" use inductors by definition. You're looking for a charge pump then that doesn't double the voltage?
 
dknguyen said:
"Boost converters" use inductors by definition. You're looking for a charge pump then that doesn't double the voltage?

Ok sure... its all lingo! i mean a charge pump in 'boost mode' (quoting datasheet)
 
I suppose you could having something measure the voltage of the capacitor as it charges up and dump it prematurely so it gives less than double voltage. ANd then maybe put a bunch of these circuits in parallel and out of phase to smooth out the ripple (kind of like having more cylinders in a car engine). I know of no ASIC to do it though, and all of these parallel circuit might take more space than an inductor (or was it efficiency that you were after rather than space). You could breadboard a single circuit and see if it could do the job then you wouldn't need more of them in parallel and out of phase.
 
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well im designing for a implant... so size of the converter + associated circuitry must be a minimum. Also efficiency ultimately equals size (less efficient = larger battery required) so must be a maximum

Designing an ASIC also not an option power supply is a small part of a much larger project, neither is a breadboard circuit or indeed a large discrete circuit.

What i ment is: Is there an existing device, charge pump based that would be more efficient
 
I'm afraid you have been mislead... just like I was with our initial part.

If you refer the data sheet (page 6)>>>

Eff % = Vout/2Vin >>> clearly a doubler!

All the nice efficiency graphs aer using a Vin close to Vout/2
 
WHat's the problem? YOu want 3.3V from a 2-2.85V source without inductors that is small. THis gives 3.3V within 4% for a 2V-4V input (at 20mA) for 40mA you need a 2.5V-4V input. WHo care if it's a voltage doubler? It gives you the output you want for the input you want at the size you want.

"The LTC1754 uses a switched-capacitor charge pump to
boost VIN to a regulated output voltage. Regulation is
achieved by sensing the output voltage through an internal
resistor divider and enabling the charge pump when the
divided output drops below the lower trip point of COMP1."

THat's what I was describing with the discrete circuit- a charge pump voltage doubler whose voltage is limited by a threshold to give you a boostedthat is less than double voltage. THe equation for efficiency may be the same since it's almost the same circuit as a charge pump, just different voltage threshold. Why do you care so much if it's efficiency equation is the same a of a voltage doubler (even though it does't double the voltage and gives you what you want). In fact, why do you even care if it is a voltage doubler as long as it gives you the output you want for the input you want?

If none of this still works for you, then what you are looking for may not exist (or be possible).
 
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Inductor based supplies are not so suitable as inductors are quite large, and inductor boost converters are bad at very low currents.
You might want to look again at inductor boost converters.

Do you need the full 3.3V at the standby current? Lots of circuits don't. A boost converter can be almost 100% efficient when it's not boosting. As for size, the power levels you're looking at might need a very small inductor (perhaps 0603). [edit] The MAX856 family can work with 22-47 uH, which are even available in 0402. [/edit]
 
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