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Micropower charge pump for 3-6v @ 20uA

I suspect this is a tall order.

But can anyone see a simple way to build a micropower charge pump for a hobby electronics PIC based device drawing approx 20uA at a voltage between 2.5 to 6v. The aim is to run it off a single cheap 1.5v battery rather than a pair of them or an expensive 3v cell. Efficiency of 20-50% would be OK - 40-100uA drain on the cell. Regulation need not be good - but its output mustn't go above 6v. Its for a PIC based sidereal clock for astronomy.

The linear LTC1751 looks like it might come close but the datasheet says it is only good for 2v and up input voltage. And if the solution is bigger than the battery then it may as well use a pair of cells. OTOH It draws so little current.

Thanks for any enlightenment.

Regards, Martin Brown

Reply to
Martin Brown
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You can use the MAX1724 (see e.g.

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how to buy it). I've used it for a product design some years ago, which is still produced and works without problems, so unlike some other Maxim chip looks like this is one of the more standard products and will be available in future, too.

According to the datasheet, efficiency is about 60% at 1V battery input,

100uA load and 3.3V output. It works down to 0.8V (but needs at least 0.91V for startup). 
--
Frank Buss, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de
Reply to
Frank Buss

Use a PIC port pin to drive an NPN that in turn drives the inductor of a boost converter. Let your software pulse it as needed. If you have a spare adc input, feed back on that, and you're now in the DSP power supply business!

You could also do a schottky charge pump/C-W multiplier directly off a port or clock pin, given the efficiency you're willing to accept. That's not bad... a 4-diode multiplier stack, two sot-23 dual schottkies and some caps, piggybacked on top the +1.5 rail.

John

Reply to
John Larkin

Did you check integrated versions such as the TPS61070? Ok, a bit overkill here and expensive. If it had to be really cheap I'd experiment with a JFET and tapped inductor (which could be made with a ferrite bead).

Basically you need something that will oscillate well at 1V or less and doesn't require more than 20-30uA or so of quiescent. No idea if some tiny Schmitt inverter would do but might be worth a look. 74AUP1G14 maybe. Those are also really cheap. Make an oscillator with it and then step that up with a ferrite bead transformer, LAN transformer or whatever you've got flying around, rectify, then regulate if needed.

Of course, if I was in a real rush I'd grab one of the Ge-transistors I've stashed away but I guess you want something that others can also build.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

But, as Jim would put it, how does it start?

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

The world runs on smoke and mirrors ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
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         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

I always thought it was pixie dust.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Oh. I was assuming the pic ran on 1.5, and the hv was for something else. My bad.

So, make a blocking-oscillator flyback converter, I guess. Or use a

3-volt lithium.

John

Reply to
John Larkin

Or just do the usual Schmitt inverter thing but instead of your LM5112 use a 71AUP1G14. Haven't tried that one but should work.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

OK, be pickey.

This version self-starts:

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The boost voltage isn't well defined, so if the pic can't control it maybe a shunt regulator, possibly a couple of led's, would help.

John

Reply to
John Larkin

How about this one?

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It's basically a blocking oscillator. Output is clipped to be 2x the input voltage. There are lots of small 1:1 inductor/transformer things in Mouser and Digikey.

John

Reply to
John Larkin

1.5V is too low for any PIC I've seen. There are some very low power micros which have an on-chip bootstrap charge pump that works pretty much as you describe, but the PIC isn't spec'd for it. It might work if you used an internal RC oscillator, OSCOUT and the oscillator actually started, but it wouldn't be guaranteed.

The SN74AUC1G14 is spec'd to operate with an 800mV power supply rail.

There are plenty of SOCs that work from single cells (eg. MP3 players) and IIRC some SMPS chips too.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
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Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

I don't know PICs but I would imagine they need something above 2V for a substantial amount of time (maybe msecs, don't know) to be able to start anything meaningful in there. Might be a wee problem if the battery is a well used 1.5V alkaline.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

This should boost to well over 2 volts at startup. It would peak at 3 volts except for the diode loss.

John

Reply to
John Larkin

Yes, either something like that or a JFET, tapped inductor at the drain, battery to tap, feedback from the top. As long as it all remains within Martin's power budget which probably doesn't allow more than 25uA or so of quiescent. I'd probably start with a 74AUP1G14 because they are surprisingly cheap.

I found inductors at Digikey, Mouser and other US sources a bit pricey. There ought to be a source where one can buy things like modem or LAN transformers for a quarter or so. Once I did a design and despite high production volume we could not find a source. So we had it custom made in Taiwan because that was cheaper.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

But it would first require the PIC to properly start. Worst case would be a PIC hanging with the port pin on high. Phssst ... pop ... crackle ... BADABANG.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

No, it doesn't. The LC just behaves that way at powerup. The pic just has to pump it now and then to keep the cap from discharging.

John

Reply to
John Larkin

...

Why not use one of Watson's LED flashlight circuits?

Cheers! Rich

Reply to
Rich Grise

Well, yes, but for that pumping to happen the PIC must be operational and well past its POR/BOR boot process.

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

I've said, three or four times now, that the cap voltage gets boosted at turnon, all by itself, because that's the way Ls and Cs behave. Given a reasonable C value, there should be plenty of energy available for the pic to start up, and begin pulsing the npn, before the voltage on the C discharges too much.

+1.5V----switch-------1mH-----|>|-----+--------->

diode | | | 1uF | | | | gnd

Simulate it!

John

Reply to
John Larkin

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