Clever ways to get +/- volts from MC34063

I've got a 24V incoming supply, and a need for a number of supplies (+12,

+5, -5, and +3.3). The +/- 5V supplies are for a bit of analog circuitry on a largely digital board, so they have low current requirements. My plan for the +5V supply is to just hang a 78xx05 off the +12V. +12V and +3.3V are going to come from switchers off of +24V.

Anyone have any suggestions for a clever way to get -(something) from one of the positive supplies? I'm thinking that if I can get some negative voltage for cheap from one of the existing switchers that I can then regulate it to -5V with a 79xx05. I expect I'll end up using a charge pump -- but suggestions are welcome.

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Reply to
Tim Wescott
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You can charge pump off the switcher node of one of the positive switchers to get negative volts, essentially a classic "voltage doubler" capacitor-diode thing. It will make roughly -24, which is a lot to regulate to -5. A small series resistor is a good idea... some switchers don't like this being done to them.

But if it's low current, you may as well buy a standard charge-pump voltage inverter chip and run that off +5.

The LTC "LTM" module series can be used to make -5 from +24. Not real cheap, but easy. Or build a Cuk switcher.

Another possibility: buy one of those cheap SIP dc/dc converters, $4 roughly.

John

Reply to
John Larkin

If you run a buck to make +5V you can use a dual inductor instead of a single, plus another diode -> There's your -5V. Dual inductors have become cheap catalog items these days.

As for the model, the LTSpice group on Yahoo has some. I think the best one was from Helmut.

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Regards, Joerg

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Joerg

I've used another winding on the inductor, but it worries me a bit.

Reply to
Spehro Pefhany

Here's how I did a +5V/-12V switcher at GenRad ~1980...

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I don't know if modern switcher chips will allow for such shenanigans :-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Why? Me no worry :-)

It does require the converter to keep running though. If the positive supply is not loaded much the negative can drop off. But with analog stuff that usually isn't a concern. On sync bucks one can force many chips to remain in sync-mode even under light loads and not let them go into burst mode.

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Regards, Joerg

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Reply to
Joerg

Tim Wescott a écrit :

I guess the +12V switcher is a buck. Just use a double winding coil in lieu of a simple inductor, and use the secondary so that it copies the buck output voltage at your negative output (energy taken back from the buck output bypass cap). If your main buck is *not* a synchronous rectification one, then you even have the buck free wheeling diode drop making for your negative branch diode drop.

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Thanks,
Fred.
Reply to
Fred Bartoli

NICE that seems a general and very useful technique.

I have used those dual inductors for flyback but never thought of them used like that.

[...]
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John Devereux
Reply to
John Devereux

Shhht! If was meant to be a secret. Oh wait ...

If you want it to be extra cool, there's lots of amps flowing or it has to be tracking more precisely in voltage then you can use a FET instead of the diode and control that from your sync buck chip. This cuts out the diode losses.

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Regards, Joerg

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Reply to
Joerg

ry

d

ttdesign.com

Unless you are looking for really low ripple then buck/boost is the modern way to go for everything. Far more efficient - up in the 95% odd.

Hardy

Reply to
HardySpicer

Software

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WRT ripple the MC34063 is absolutely horrid because of its regulating method. But sometimes it's good enough. I wouldn't use it in sensitive anlog apps.

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Regards, Joerg

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Reply to
Joerg

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

If there is board space a MAX232 can be used for it's +/- 10 v charge pump.

Cheers

Reply to
Martin Riddle

(+12,

circuitry

and

one

If the - (something) is low current and you are going to post regulate it, and you have a switcher already operating, you can pretty simply AC couple the switching off of the switcher and use 2 diodes to rectify it to a negative voltage = to the P-P of the switching waveform (less the diode drops). Its basically a charge pump put you already have the waveform.

Check the full range of loads and line though.

Mark

Reply to
MarkK

ry

d

ttdesign.com

The 34063 is false economy with its low frequency operation, internal heat dissipation, and primitive PWM architecture.

Reply to
Fred Bloggs

I've done that when I needed a voltage above the input voltage. The idea was implemented but was met with disgust, grunting and scoffing at the design review :-)

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Regards, Joerg

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

I hadn't realized just how primitive its PWM architecture is until I looked closely. Now I know...

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Reply to
Tim Wescott

Yeah, it's ghastly. However ... it is one of very few chips that is truly multi-sourced. And that counts for a lot. 15c a pop is also very hard to beat but in my cases the multi-source argument usually matters more.

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Regards, Joerg

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Reply to
Joerg

og

off

24V.

ing

re.

It's Stone Age stuff compared to the modern offerings. Even for a minimalist design, you have to use L's and C's 10x larger than what's needed switching at a relatively modest 500KHz- more expensive parts and more board space.

Reply to
Fred Bloggs

You should have spun it as being quasiresonant and cost-free. They would havw oohed and aawed.....

RL

Reply to
legg

True. However, since the advent of large numbers of Chinese inductor manufacturers that problem has largely gone away. And board space is rather cheap if you use ... gasp ... phenolic :-)

No kidding, I have samples of UHF stuff on phenolic here. With microstrip and all that on there.

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Regards, Joerg

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

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