I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.

John

John Larkin wrote:
I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.

What does it do if the -12 volt output is overloaded?

On Sat, 07 Jul 2007 21:09:01 -0400, John Popelish
wrote:

John Larkin wrote:
I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.

What does it do if the -12 volt output is overloaded?

Just makes the +12 supply current-limit, optimistically. A
hard-shorted -12 could fry the fet or the inductor, I suppose.

John

Why does a UC3842, etc. suck, exactly? Wrong sign? (I don't remember if
the 14-pin version has + and - inputs at the pins.) Not cheap enough?

6W is a little much for a, say, cap switcher?...

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms

"John Larkin" wrote in message
...
I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.

John

On Sat, 7 Jul 2007 22:13:26 -0500, "Tim Williams"
wrote:

Why does a UC3842, etc. suck, exactly? Wrong sign? (I don't remember if
the 14-pin version has + and - inputs at the pins.) Not cheap enough?

Yup, wrong feedback and gate drive signs, and the current-limit thing
would be damned awkward with a p-fet switch.


6W is a little much for a, say, cap switcher?...

Probably, and it would lose some voltage, too. It would have to be
done with a *lot* of discretes, since the IC charge pump things
couldn't handle the net 24 volts across the chip.

A Cuk converter would be nice, even with the dual inductors, but I
haven't found one that will flip +12 to -12 at this current. I think
the math gets messy.

An unregulated p-p forward converter would be OK, but the transformer
would have to be 1ct to 1.1ct or something like that, custom
magnetics, always a huge hassle.

There are a bazillion buck regulators on the market, and very few
inverters.


John

John Larkin a écrit :
On Sat, 7 Jul 2007 22:13:26 -0500, "Tim Williams"
wrote:

Why does a UC3842, etc. suck, exactly? Wrong sign? (I don't remember if
the 14-pin version has + and - inputs at the pins.) Not cheap enough?

Yup, wrong feedback and gate drive signs, and the current-limit thing
would be damned awkward with a p-fet switch.

6W is a little much for a, say, cap switcher?...

Probably, and it would lose some voltage, too. It would have to be
done with a *lot* of discretes, since the IC charge pump things
couldn't handle the net 24 volts across the chip.

A Cuk converter would be nice, even with the dual inductors, but I
haven't found one that will flip +12 to -12 at this current. I think
the math gets messy.

An unregulated p-p forward converter would be OK, but the transformer
would have to be 1ct to 1.1ct or something like that, custom
magnetics, always a huge hassle.

There are a bazillion buck regulators on the market, and very few
inverters.



Use a buck IC in an inverter configuration with a common gate pfet to
extend the switch voltage compliance to negative voltages. Then twist
feedback adequately.
I did almost that once, with an LT1374, generating both +20V and -20V at
1A from 15V. In my case the feedback loop naturally came from the +20V
output.
Nice, simple and fully protected.


--
Thanks,
Fred.

On Sun, 08 Jul 2007 09:29:13 +0200, Fred Bartoli
r_AndThisToo wrote:

John Larkin a écrit :
On Sat, 7 Jul 2007 22:13:26 -0500, "Tim Williams"
wrote:

Why does a UC3842, etc. suck, exactly? Wrong sign? (I don't remember if
the 14-pin version has + and - inputs at the pins.) Not cheap enough?

Yup, wrong feedback and gate drive signs, and the current-limit thing
would be damned awkward with a p-fet switch.

6W is a little much for a, say, cap switcher?...

Probably, and it would lose some voltage, too. It would have to be
done with a *lot* of discretes, since the IC charge pump things
couldn't handle the net 24 volts across the chip.

A Cuk converter would be nice, even with the dual inductors, but I
haven't found one that will flip +12 to -12 at this current. I think
the math gets messy.

An unregulated p-p forward converter would be OK, but the transformer
would have to be 1ct to 1.1ct or something like that, custom
magnetics, always a huge hassle.

There are a bazillion buck regulators on the market, and very few
inverters.



Use a buck IC in an inverter configuration with a common gate pfet to
extend the switch voltage compliance to negative voltages. Then twist
feedback adequately.
I did almost that once, with an LT1374, generating both +20V and -20V at
1A from 15V. In my case the feedback loop naturally came from the +20V
output.
Nice, simple and fully protected.

I did this in one of the GenRad portable testers...

http://analog-innovations.com/SED/Ol...tiveOutput.pdf

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

America: Land of the Free, Because of the Brave

On Sat, 07 Jul 2007 21:15:31 -0700, John Larkin
wrote:

On Sat, 7 Jul 2007 22:13:26 -0500, "Tim Williams"
wrote:

Why does a UC3842, etc. suck, exactly? Wrong sign? (I don't remember if
the 14-pin version has + and - inputs at the pins.) Not cheap enough?

Yup, wrong feedback and gate drive signs, and the current-limit thing
would be damned awkward with a p-fet switch.


6W is a little much for a, say, cap switcher?...

Probably, and it would lose some voltage, too. It would have to be
done with a *lot* of discretes, since the IC charge pump things
couldn't handle the net 24 volts across the chip.

A Cuk converter would be nice, even with the dual inductors, but I
haven't found one that will flip +12 to -12 at this current. I think
the math gets messy.

An unregulated p-p forward converter would be OK, but the transformer
would have to be 1ct to 1.1ct or something like that, custom
magnetics, always a huge hassle.

There are a bazillion buck regulators on the market, and very few
inverters.


John

LT1372 will do the job in a Cuk configuration. LT1372 is a bit pricy
at $4.30 in unit quantities, but you do get a built in switch. You can
get dual inductors from Coilcraft and Coiltronics (Digikey). You do
get some sort of overload protection with the LT1372.

---
Mark

"John Larkin" wrote in message
...
I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.

John

have you considered a MC34063A ?

might just do it wothout an external switch.
input range 40v, for voltage inverter just connect ic ground to neg op.

Colin =^.^=

On Sun, 08 Jul 2007 19:26:32 GMT, "colin"
wrote:


"John Larkin" wrote in message
.. .
I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.

John

have you considered a MC34063A ?

might just do it wothout an external switch.
input range 40v, for voltage inverter just connect ic ground to neg op.

Colin =^.^=


Yeah, I looked at that. It would probably work, but it's an old, slow
bipolar part with a darlington switch, so it would be pretty
inefficient and would run at maybe 100 khz, so it would need a big
inductor. But it is cheap, 35 cents or something.

Thanks.

John

On Sun, 08 Jul 2007 19:18:30 GMT, qrk wrote:

On Sat, 07 Jul 2007 21:15:31 -0700, John Larkin
wrote:

On Sat, 7 Jul 2007 22:13:26 -0500, "Tim Williams"
wrote:

Why does a UC3842, etc. suck, exactly? Wrong sign? (I don't remember if
the 14-pin version has + and - inputs at the pins.) Not cheap enough?

Yup, wrong feedback and gate drive signs, and the current-limit thing
would be damned awkward with a p-fet switch.


6W is a little much for a, say, cap switcher?...

Probably, and it would lose some voltage, too. It would have to be
done with a *lot* of discretes, since the IC charge pump things
couldn't handle the net 24 volts across the chip.

A Cuk converter would be nice, even with the dual inductors, but I
haven't found one that will flip +12 to -12 at this current. I think
the math gets messy.

An unregulated p-p forward converter would be OK, but the transformer
would have to be 1ct to 1.1ct or something like that, custom
magnetics, always a huge hassle.

There are a bazillion buck regulators on the market, and very few
inverters.


John

LT1372 will do the job in a Cuk configuration. LT1372 is a bit pricy
at $4.30 in unit quantities, but you do get a built in switch. You can
get dual inductors from Coilcraft and Coiltronics (Digikey). You do
get some sort of overload protection with the LT1372.

---
Mark

What's the output current in your cukfun sim? National has a similar
part, LM2611, with a 1.2 amp current limit switch. In the +12 to -12
config, it's only good for 170 mA out.

All the LTC stuff is "a bit pricey."

John

"John Larkin" wrote in message
...
On Sun, 08 Jul 2007 19:26:32 GMT, "colin"
wrote:


"John Larkin" wrote in
message
. ..
I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.

John

have you considered a MC34063A ?

might just do it wothout an external switch.
input range 40v, for voltage inverter just connect ic ground to neg op.

Colin =^.^=


Yeah, I looked at that. It would probably work, but it's an old, slow
bipolar part with a darlington switch, so it would be pretty
inefficient and would run at maybe 100 khz, so it would need a big
inductor. But it is cheap, 35 cents or something.

Yep it is realy cheap, and as you say old too.
but ive used it many times now.

it works realy well for lowish power if you have a fairly constant load,
other wise it teneds to cycle skip wich can make some audible noise.

ive used it from all sorts of combinations, from +30v to 3.3v to 3.3v to
250v lol
The size of the inductor would probably not be any bigger than say the extra
space for an external switch.

I realy wish there was a more up to date replacement that had the wide input
voltage.
many nice new ones with integral FET seem to be for low voltage.

there is also the lm259x series wich are much the same,
they run quieter but are quite expensive.

Colin =^.^=

On Sun, 08 Jul 2007 15:29:59 -0700, John Larkin
wrote:

On Sun, 08 Jul 2007 19:18:30 GMT, qrk wrote:

On Sat, 07 Jul 2007 21:15:31 -0700, John Larkin
wrote:

On Sat, 7 Jul 2007 22:13:26 -0500, "Tim Williams"
wrote:

Why does a UC3842, etc. suck, exactly? Wrong sign? (I don't remember if
the 14-pin version has + and - inputs at the pins.) Not cheap enough?

Yup, wrong feedback and gate drive signs, and the current-limit thing
would be damned awkward with a p-fet switch.


6W is a little much for a, say, cap switcher?...

Probably, and it would lose some voltage, too. It would have to be
done with a *lot* of discretes, since the IC charge pump things
couldn't handle the net 24 volts across the chip.

A Cuk converter would be nice, even with the dual inductors, but I
haven't found one that will flip +12 to -12 at this current. I think
the math gets messy.

An unregulated p-p forward converter would be OK, but the transformer
would have to be 1ct to 1.1ct or something like that, custom
magnetics, always a huge hassle.

There are a bazillion buck regulators on the market, and very few
inverters.


John

LT1372 will do the job in a Cuk configuration. LT1372 is a bit pricy
at $4.30 in unit quantities, but you do get a built in switch. You can
get dual inductors from Coilcraft and Coiltronics (Digikey). You do
get some sort of overload protection with the LT1372.

---
Mark

What's the output current in your cukfun sim? National has a similar
part, LM2611, with a 1.2 amp current limit switch. In the +12 to -12
config, it's only good for 170 mA out.

All the LTC stuff is "a bit pricey."

John

I have a 24 Ohm resistor on the output = 0.5A. The LT1372 current
limit is somewhere around 1.5A. The LM6211 in a SOT23 package has a
256 deg/W thermal resistance which is probably why the output current
is limited to such a low level. The LT part is around 125 deg/W for
their SO8 package. Guess these little controllers with internal
switches are pretty limited.

---
Mark

"John Larkin" wrote in message
...
I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.

another thought,
how about a self oscillating one with a nch mosfet,
if its a steady 12vin and 12v out it should be doable with ~50% duty cycle
so just an extra winding on the inductor.
ofc with a bjt you need less turns for the base drive.
and some bias to start it up.
not sure if you could add current limit easily,
maybe when the inductor saturates it would lose base drive
(havnt given this much thought)
a 3pin device (tl434?) could shunt gate drive to regulate voltage further.

Ive also used one of those uc3843 to drive a transformer directly with no
mosfet.

Colin =^.^=

On Mon, 09 Jul 2007 14:30:47 GMT, "colin"
wrote:


"John Larkin" wrote in message
.. .
I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.

another thought,
how about a self oscillating one with a nch mosfet,
if its a steady 12vin and 12v out it should be doable with ~50% duty cycle
so just an extra winding on the inductor.
ofc with a bjt you need less turns for the base drive.
and some bias to start it up.
not sure if you could add current limit easily,
maybe when the inductor saturates it would lose base drive
(havnt given this much thought)

Interesting idea. That would be a classic blocking oscillator, with
the output taken on the flyback. The cool thing about a blocking
oscillator is that it's simple and drives the transformer to
saturation every shot, which automatically makes optimum use of the
core. It would regulate in variable-frequency mode.

I'll see if that leads to anything useful. I don't think I've seen
that topology done anywhere.

John

John Larkin wrote:
I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.


50 cents is a lot of money in electronics ;-)

How about the old CD40106? The BCX70K seems overkill as well but only by
a cent or so. I think you need to throw in another transistor or two
because you may need a current limit, unless a short isn't going to have
nasty consequences.

BTW, I'd probably be a bit squeamish about letting Vbe climb above -3V
on the BCX. But it's under abs max.

--
Regards, Joerg

http://www.analogconsultants.com

On Wed, 11 Jul 2007 18:59:17 GMT, Joerg
wrote:

John Larkin wrote:
I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.


50 cents is a lot of money in electronics ;-)

Not in my electronics!


How about the old CD40106? The BCX70K seems overkill as well but only by
a cent or so. I think you need to throw in another transistor or two
because you may need a current limit, unless a short isn't going to have
nasty consequences.

After due consideration, not to mention being yelled at by junior
staff members, it looks like we'll be going to a flyback with custom
magnetics. It was an interesting diversion, anyhow.


BTW, I'd probably be a bit squeamish about letting Vbe climb above -3V
on the BCX. But it's under abs max.

I tested some recently, and got around 5.1 typ Vbe breakdown. Beta
degradation wouldn't matter much in a common-base circuit like this!

I'm going to have to play with the blocking-oscillator converter one
of these days, just for fun.


John

John Larkin wrote:

On Wed, 11 Jul 2007 18:59:17 GMT, Joerg
wrote:


John Larkin wrote:

I need to make -12 from +12, at maybe 500 mA, and all the obvious
inverting switching regulators suck for one reason or another.

The LM5112 is a 50-cent fet gate driver.


50 cents is a lot of money in electronics ;-)


Not in my electronics!


Yeah, I know. Wish it was so in mine. But at least I got to design one
"deluxe class" board earlier this year.


How about the old CD40106? The BCX70K seems overkill as well but only by
a cent or so. I think you need to throw in another transistor or two
because you may need a current limit, unless a short isn't going to have
nasty consequences.


After due consideration, not to mention being yelled at by junior
staff members, it looks like we'll be going to a flyback with custom
magnetics. It was an interesting diversion, anyhow.


Aw, that almost sounds like taking the chicken exit ;-)

But I guess a few bucks for custom magnetics won't matter in your case.
It is hard to find off-the-shelf transformers with air gaps that are
under 50c. That is why I usually don't do flybacks.


BTW, I'd probably be a bit squeamish about letting Vbe climb above -3V
on the BCX. But it's under abs max.


I tested some recently, and got around 5.1 typ Vbe breakdown. Beta
degradation wouldn't matter much in a common-base circuit like this!


Still I wouldn't like to hit it. Somehow that feels like driving with a
check engine light that flickers from time to time.


I'm going to have to play with the blocking-oscillator converter one
of these days, just for fun.


Absolutely. Once you have a well-tested design you can add it to your
building block collection. Blocking oscillators are a pretty good path
to make sure you won't be hit with part obsolescence issues.

--
Regards, Joerg

http://www.analogconsultants.com

"Joerg" wrote in message
et...
Still I wouldn't like to hit it. Somehow that feels like driving with a
check engine light that flickers from time to time.

The tape fell off, apply a new strip ;-)

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms