+5 to -5 charge pump thingie

The thing I'm doing is a little standalone optical-to-electrical converter, in a few cubic inches of metal box, powered by a wall wart. It's so simple it has no clocks or anything. I guess I could do a schmitt-trigger oscillator and a charge pump, but that's a lot of parts. I just need a negative rail for a couple of fast opamps.

I thought I'd ask the guys to see if anybody has a favorite part, and just to make conversation.

John

Yeah, that one should work with ceramic caps.

Your other problem, in the 1-amp range, is trickier. One of our products uses an LM2596 ($1.80 at 1K) in the weird upside-down config, where -Vout is the chip ground. We use a standard Talema inductor, 68 uH, 3A, another $1.33.

Thanks, everybody.

John

LM828 looks nice, not expensive.

How would you do +12 to -12 at 0.5 or maybe 1.5A with no custom magnetics? LM2576 is passable, but a bit pricey.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Somebody makes a something chip (not very specific, I admit) that has a 3 GHz on-chip oscillator used to invert the Vcc for some reason. Micrel or somebody with an M I think.

Isn't the classic bipolar process a p-type substrate? So if the substrate is V-, diode isolation works for all the sigs on the chip, but nothing can go negative without dire consequences.

Drams used to (still do?) pump the substrate negative.

John

I have +1.65V to +5.5V and need -1.5V

Device-level solution required

BUT, substrate is P-type, so the sneak paths get tricky ;-)

...Jim Thompson

-- | 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 | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Ah, that's a little beefier than my initial suggestion, and only a nickel more. (0.31 vs. 0.26 in 1K). Fairly high switching frequency too.

Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Bingo. I've done this, works great since op-amps are very noise-immune from the power feeds.

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He said 30mA. Which means you would have to source and sink >60mA (assuming 50% duty cycle). I doubt you'll get that much current from "any digital signal in the system". I've seen folks do things like parallel several buffers or inverters to get drive for stunts like this. And then there's the matter of the diode junction voltage loss. He didn't say how close his -5 needed to be...

Thanks, that's better than the 2576 and compatible- the output cap should be a lot smaller with the higher frequency. Good enuf.

Thank you, I didn't realize the low end ones were that cheap these days.. $260 for a reel of 1K of the low current ones. Nice if you need a taste of -V for a front end or whatever.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

I do want it to be possible to power a bunch of these from a grounded

+12 volt supply (if I can persuade anybody to buy a bunch of these) so that's my preferred way to do this. Sounds like an LM2664 will invert the +5 to give me enough to power the opamps.

John

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

John, your better than that. Take any digital signal in the system between 50KHz and 1MHz. Cap couple it with a 100nF cap to the anode of a Shockley diode to gnd and cathode of similar diode to the -4.65VDC output. Filter this output to gnd with 10uF. But you knew all of this, why am I preaching to the choir? You must have some other requirements. Regards, Harry

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

LM2664 cheap as chips and works a treat

***NOT SHORT CIRCUIT PROTECTED*** (bitter experience) M

In article , John Larkin wrote: [...]

How about a 5V zener between the ground and the return of the wallwart?

--
--
kensmith@rahul.net   forging knowledge

$2.07 each. Yikes! I don't need the regulation, so the LM2664 looks better at about $0.31 or something. Maxim and LTC are always way expensive.

John

If you already have a buck regulator in your system, you can steal some negative power with this circuit on the S.E.D/Schematics Page of my website...

OldStyleBuckSwitcherWithAddedNegativeOutput.pdf

...Jim Thompson

--
|  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  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

if the load is not that great have you throught about using a

5 V zener on the - side of the rectifier passing it through the diode to your command of common - side ? as long as you have an extra 5 volts to play with for + regulation this should work well. it will give you a steady - 5 volts. its a common pratice to do this in many integrated mini devices where power usage is low.

Provided you don't draw too much current from -V to ground (+V to -V current is okay). Of course you can put a resistor from +V to ground to get more available current.

If you just need a taste of -V, an LED can be used, perhaps doubling as a power-on indicator, but I'm always nervous about that sort of thing, as they tend to be more exposed to damage than other parts, and if the LED goes open the regulator goes balls-to-the-wall.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

LTC1983-5 ?

Bob

Peltier?

I want to develop a -1.5V (or more) source (relative to ground/substrate.

Voltage is really immaterial, I just need to turn on a P-channel that has micro-amps for load.

Substrate is P-type, so it becomes tricky.

Pump load current, is, however, trivial... just a P-channel gate capacitance.

...Jim Thompson

--
|  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  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.