after a voltage regulator with push pull op.

[colin]

Hi,
I was looking to simplify a voltage supply to generate +5/-5 v (+-10%)
<100ma from an isolated 15v supply, all it needs is a 10v regulator and
something to provide the midway/gnd, or a pos and neg regulator and some way
of balancing the gnd curremt, such as zeners acros the regulator inputs, but
what would be realy neat is if there was a 10v regulator with a built in 5v
ref op with decent src/sink capability. some voltage refs src and sink 10ma
or so wich isnt realy enuogh. some regulators are multi op but dont src and
sink.

even if i use single supply op amps i still need a midway reference for bias
wich must be able to src/sink a reasonable amount.

Colin =^.^=

 

[Joerg]

Hello Colin

even if i use single supply op amps i still need a midway reference for bias
wich must be able to src/sink a reasonable amount.

This is commonly done via a 50/50 resistor divider and an opamp as a
follower. That can source and sink as much as the data sheet says and is
cheap.

Just be careful with capacitive loading at that node. Some (most) opamps
don't like that but there are work arounds in National's app notes and
others. And in AoE I guess.

Regards, Joerg

http://www.analogconsultants.com

 

[John Larkin]

On Mon, 18 Jul 2005 20:05:27 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Hello Colin

even if i use single supply op amps i still need a midway reference for bias
wich must be able to src/sink a reasonable amount.

This is commonly done via a 50/50 resistor divider and an opamp as a
follower. That can source and sink as much as the data sheet says and is
cheap.

Just be careful with capacitive loading at that node. Some (most) opamps
don't like that but there are work arounds in National's app notes and
others. And in AoE I guess.

Regards, Joerg

http://www.analogconsultants.com

LM8261 is nice. Lots of current, r-r, unlimited cap load.

John

 

[Joerg]

Hello John,

LM8261 is nice. Lots of current, r-r, unlimited cap load.

Nice chip even if a bit on the pricey side. Colin should just keep in
mind that it's a SOT23-5 and after so many hundred mW blisters and smoke
will show.

Another option if he needs lots of power is to use an audio amplifier
IC. These are really cheap.

Regards, Joerg

http://www.analogconsultants.com

 

[colin]

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:nbUCe.712$Fk4.625@newssvr21.news.prodigy.com...

Hello John,

LM8261 is nice. Lots of current, r-r, unlimited cap load.

Nice chip even if a bit on the pricey side. Colin should just keep in
mind that it's a SOT23-5 and after so many hundred mW blisters and smoke
will show.

Another option if he needs lots of power is to use an audio amplifier
IC. These are really cheap.

ooh thanks thats quite a nice little beastie

might just about handle the current diference, at 5v the thing shldnt
vapourize too quickly even at short circuit. interesting that it handles
such capacitance.

I forgot to mention I considered an op amp, I usualy use a spare op amp but
they dont like it when you just try and hang a big capacitor on them, I need
a low impedance al the way down to dc.

Im just surprised/disapointed there isnt a regulator that is designed for
it, as im sure its a comon thing to want to do, would be nice to have the
added protection of over temp, over current, over voltage with wildly
unbalanced loads, over anything else etc. especialy when prototyping.

my idea of 7.5 zeners acros the +/- regulator inputs still seems quite
atractive from this point of view. (especialy as the regulators are already
there atm) or I cld use the op amp to balance the input voltage to the 2
regulators.

Colin =^.^=

 

[Joerg]

Hello Colin,

I forgot to mention I considered an op amp, I usualy use a spare op amp but
they dont like it when you just try and hang a big capacitor on them, I need
a low impedance al the way down to dc.

That can be handled. Look through National's appnotes. Capacitive load +
opamps.

my idea of 7.5 zeners acros the +/- regulator inputs still seems quite
atractive from this point of view. (especialy as the regulators are already
there atm) or I cld use the op amp to balance the input voltage to the 2
regulators.

Can be risky. Zeners have a lot of tolerance. If Vz is too high the
central node meanders around. If too low they'll short the supply. With
zeners there ain't much wiggle room in between these two situations.

Regards, Joerg

http://www.analogconsultants.com

 

[colin]

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:EbVCe.5757$NU2.3862@newssvr13.news.prodigy.com...

Hello Colin,
my idea of 7.5 zeners acros the +/- regulator inputs still seems quite
atractive from this point of view. (especialy as the regulators are
already
there atm) or I cld use the op amp to balance the input voltage to the 2
regulators.

Can be risky. Zeners have a lot of tolerance. If Vz is too high the
central node meanders around. If too low they'll short the supply. With
zeners there ain't much wiggle room in between these two situations.

I would need vz to be in the range 7.5 to 9v with ldo regs thats assuming
the 15v is steady wich it isnt realy, the supply has some decoupling
resistance so it wouldnt hurt if they both conducted slightly anyway.

I also considered 2 transistors, normaly one or the other would conduct
keeping the input centered within a volt or so.

-+--+-----reg----
| | |
5kr | |
| c |
+-b |
| e |
300r +------+-----
| e |
+-b |
| c |
5kr | |
| | |
-+--+-----reg----

gona use it in several places tho so would be realy neat to have a little
soic 8 chip that did it all realy, maybe some semi conmpany is listening

Colin =^.^=

 

[Joerg]

Hello Colin,

gona use it in several places tho so would be realy neat to have a little
soic 8 chip that did it all realy, maybe some semi conmpany is listening

Nah, why should they if it can be done with amps? Ok, so now I looked it
up for you. The magic trick is shown in figure 15 and the text around it
at the end of this application note:
http://www.national.com/an/AN/AN-4.pdf#page=1

Regards, Joerg

http://www.analogconsultants.com

 

[Pooh Bear]

colin wrote:

Hi,
I was looking to simplify a voltage supply to generate +5/-5 v (+-10%)
100ma from an isolated 15v supply, all it needs is a 10v regulator and
something to provide the midway/gnd, or a pos and neg regulator and some way
of balancing the gnd curremt, such as zeners acros the regulator inputs, but
what would be realy neat is if there was a 10v regulator with a built in 5v
ref op with decent src/sink capability. some voltage refs src and sink 10ma
or so wich isnt realy enuogh. some regulators are multi op but dont src and
sink.

even if i use single supply op amps i still need a midway reference for bias
wich must be able to src/sink a reasonable amount.

TI I'm sure and no doubt others make dedicated ICs to provide that midway
reference.

You don't need 'single supply' type op-amps if you go that route.

Graham

 

[colin]

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news7WCe.6040$NU2.4094@newssvr13.news.prodigy.com...

Hello Colin,

gona use it in several places tho so would be realy neat to have a
little
soic 8 chip that did it all realy, maybe some semi conmpany is listening


Nah, why should they if it can be done with amps? Ok, so now I looked it
up for you. The magic trick is shown in figure 15 and the text around it
at the end of this application note:
http://www.national.com/an/AN/AN-4.pdf#page=1

thanks thats a usefull appnote, although r4 doesnt afect the output
resistance so much it would afect the output curent although 50ohm would
just about do, it does make that other op amp look so cool.

Colin =^.^=

 

[colin]

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:42DC2FC7.9B458AD0@hotmail.com...

colin wrote:

Hi,
I was looking to simplify a voltage supply to generate +5/-5 v
(+-10%)
100ma from an isolated 15v supply, all it needs is a 10v regulator and
something to provide the midway/gnd, or a pos and neg regulator and some
way
of balancing the gnd curremt, such as zeners acros the regulator inputs,
but
what would be realy neat is if there was a 10v regulator with a built in
5v
ref op with decent src/sink capability. some voltage refs src and sink
10ma
or so wich isnt realy enuogh. some regulators are multi op but dont src
and
sink.

even if i use single supply op amps i still need a midway reference for
bias
wich must be able to src/sink a reasonable amount.

TI I'm sure and no doubt others make dedicated ICs to provide that midway
reference.

You don't need 'single supply' type op-amps if you go that route.

oo thanks, thats got my hopes up, I spent some time looking today at voltage
regs/references but didnt see any, maybe il look again ..
for some reason not all devices seem to be easily found on some
manafacturers sites unless u know the url or partnumber, maybe they want to
keep them for themselves ? :g

Colin =^.^=

 

[Joerg]

Hello Colin,

thanks thats a usefull appnote, although r4 doesnt afect the output
resistance so much it would afect the output curent although 50ohm would
just about do, it does make that other op amp look so cool.

Remember that this is a very old app note. If you use a modern amp with
a more stiff output R4 can be much lower. Plus, the mid rail wouldn't
see that resistor because it is regulated out. The fast stuff is taken
care of by, well, the cap.

Regards, Joerg

http://www.analogconsultants.com

 

[colin]

"colin" <no.spam.for.me@ntlworld.com> wrote in message
news:GPWCe.4874$F_2.2050@newsfe6-win.ntli.net...

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:42DC2FC7.9B458AD0@hotmail.com...

colin wrote:

Hi,
I was looking to simplify a voltage supply to generate +5/-5 v
(+-10%)
100ma from an isolated 15v supply, all it needs is a 10v regulator
and
something to provide the midway/gnd, or a pos and neg regulator and
some
way
of balancing the gnd curremt, such as zeners acros the regulator
inputs,
but
what would be realy neat is if there was a 10v regulator with a built
in
5v
ref op with decent src/sink capability. some voltage refs src and sink
10ma
or so wich isnt realy enuogh. some regulators are multi op but dont
src
and
sink.

even if i use single supply op amps i still need a midway reference
for
bias
wich must be able to src/sink a reasonable amount.

TI I'm sure and no doubt others make dedicated ICs to provide that
midway
reference.

You don't need 'single supply' type op-amps if you go that route.

oo thanks, thats got my hopes up, I spent some time looking today at
voltage
regs/references but didnt see any, maybe il look again ..
for some reason not all devices seem to be easily found on some
manafacturers sites unless u know the url or partnumber, maybe they want
to
keep them for themselves ? :g

oo after much searching i came across reg1118 and lt1118 it seems both
originaly intended for scsi termination voltage source supplies, and utc560,
however they al seem to be somewhat unavailable exept for the 2.85 v
version, but they can source and sink plenty of current.

Colin =^.^=

 

[John Larkin]

On Mon, 18 Jul 2005 22:22:50 GMT, "colin"
<no.spam.for.me@ntlworld.com> wrote:

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:EbVCe.5757$NU2.3862@newssvr13.news.prodigy.com...
Hello Colin,
my idea of 7.5 zeners acros the +/- regulator inputs still seems quite
atractive from this point of view. (especialy as the regulators are
already
there atm) or I cld use the op amp to balance the input voltage to the 2
regulators.

Can be risky. Zeners have a lot of tolerance. If Vz is too high the
central node meanders around. If too low they'll short the supply. With
zeners there ain't much wiggle room in between these two situations.

I would need vz to be in the range 7.5 to 9v with ldo regs thats assuming
the 15v is steady wich it isnt realy, the supply has some decoupling
resistance so it wouldnt hurt if they both conducted slightly anyway.

I also considered 2 transistors, normaly one or the other would conduct
keeping the input centered within a volt or so.

-+--+-----reg----
| | |
5kr | |
| c |
+-b |
| e |
300r +------+-----
| e |
+-b |
| c |
5kr | |
| | |
-+--+-----reg----

gona use it in several places tho so would be realy neat to have a little
soic 8 chip that did it all realy, maybe some semi conmpany is listening

Colin =^.^=

Somebody, TI I think, used to make a rail-splitter thing; don't know
if it's still around.

John

 

[Ian]

"colin" <no.spam.for.me@ntlworld.com> wrote in message
news:XsZCe.4260$jo3.204@newsfe5-win.ntli.net...

"colin" <no.spam.for.me@ntlworld.com> wrote in message
news:GPWCe.4874$F_2.2050@newsfe6-win.ntli.net...
"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:42DC2FC7.9B458AD0@hotmail.com...

colin wrote:

Hi,
I was looking to simplify a voltage supply to generate +5/-5 v
(+-10%)
100ma from an isolated 15v supply, all it needs is a 10v regulator
and
something to provide the midway/gnd, or a pos and neg regulator and
some
way
of balancing the gnd curremt, such as zeners acros the regulator
inputs,
but
what would be realy neat is if there was a 10v regulator with a built
in
5v
ref op with decent src/sink capability. some voltage refs src and
sink
10ma
or so wich isnt realy enuogh. some regulators are multi op but dont
src
and
sink.

even if i use single supply op amps i still need a midway reference
for
bias
wich must be able to src/sink a reasonable amount.

TI I'm sure and no doubt others make dedicated ICs to provide that
midway
reference.

You don't need 'single supply' type op-amps if you go that route.

oo thanks, thats got my hopes up, I spent some time looking today at
voltage
regs/references but didnt see any, maybe il look again ..
for some reason not all devices seem to be easily found on some
manafacturers sites unless u know the url or partnumber, maybe they want
to
keep them for themselves ? :g


oo after much searching i came across reg1118 and lt1118 it seems both
originaly intended for scsi termination voltage source supplies, and
utc560,
however they al seem to be somewhat unavailable exept for the 2.85 v
version, but they can source and sink plenty of current.

Colin =^.^=

Search on SDRAM terminators, e.g. the LP2996. These are readily available

and cheap.

Regards
Ian

 

[colin]

"Ken Smith" <kensmith@green.rahul.net> wrote in message
news:dbj180$pp8$2@blue.rahul.net...

In article <dQTCe.1376$yH4.896@newsfe2-win.ntli.net>,
colin <no.spam.for.me@ntlworld.com> wrote:
[...]
100ma from an isolated 15v supply, all it needs is a 10v regulator and
something to provide the midway/gnd, or a pos and neg regulator and some
way


This is a bad way of doing it. If the -5V supply gets overloaded, the +5V
supply ends up being 10V. It is better to use a pair of 5V regulators
like this:


--------
---------+-------------+-----! LM7805 !-------- +5V
! ! --------
! ! !
! \! !
------- ! \ !
! 5VREF !-------! >---------+------------ 0V
------- ! / Push pull amp.
! /!
! !
---------+-------------+------------------------- -5V

Hi, thnks yes good point, although for the op amps it wldnt be a problem,
id thought of doing it that way but instead if the 5v rail gets overloaded
then the -5 v rail becomes unregulated, but im also using it for a max038
wich I wouldnt like to risk overloading either rail so im thinking il use
the pos and neg regulator aproach.

However idealy a single device would shut down totaly under any fualt
condition, another aproach i might use a charge pump for the -5v
this would mean lower suply voltage and also one side of the supply is gnd
wich seems neater, maybe save me a wire too, they seem quite neat small
package and only a smal capacitor just lose about .6v on the -5 rail, might
be able to compensate, also fail safe.

still, some intersting little beasties have eventualy come out of the
woodwork

Colin =^.^=

 

[colin]

"Ian" <Ian_Buckner_not_@agilent.com> wrote in message
news:1121762800.913229@newsreg.cos.agilent.com...

oo after much searching i came across reg1118 and lt1118 it seems both
originaly intended for scsi termination voltage source supplies, and
utc560,
however they al seem to be somewhat unavailable exept for the 2.85 v
version, but they can source and sink plenty of current.

Search on SDRAM terminators, e.g. the LP2996. These are readily available
and cheap.

cool seems like these are actualy stocked by RS anyhow, thanks
adjustable too. be real neat if they added a normal output

Colin =^.^=

 

[Ken Smith]

In article <dQTCe.1376$yH4.896@newsfe2-win.ntli.net>,
colin <no.spam.for.me@ntlworld.com> wrote:
[...]

100ma from an isolated 15v supply, all it needs is a 10v regulator and
something to provide the midway/gnd, or a pos and neg regulator and some way

This is a bad way of doing it. If the -5V supply gets overloaded, the +5V
supply ends up being 10V. It is better to use a pair of 5V regulators
like this:


--------
---------+-------------+-----! LM7805 !-------- +5V
! ! --------
! ! !
! \! !
------- ! \ !
! 5VREF !-------! >---------+------------ 0V
------- ! / Push pull amp.
! /!
! !
---------+-------------+------------------------- -5V
--
--
kensmith@rahul.net forging knowledge

 

[Rich Grise]

On Tue, 19 Jul 2005 14:26:09 +0000, colin wrote:

Hi, thnks yes good point, although for the op amps it wldnt be a problem,
id thought of doing it that way but instead if the 5v rail gets overloaded
then the -5 v rail becomes unregulated, but im also using it for a max038
wich I wouldnt like to risk overloading either rail so im thinking il use
the pos and neg regulator aproach.

However idealy a single device would shut down totaly under any fualt
condition, another aproach i might use a charge pump for the -5v
this would mean lower suply voltage and also one side of the supply is gnd
wich seems neater, maybe save me a wire too, they seem quite neat small
package and only a smal capacitor just lose about .6v on the -5 rail, might
be able to compensate, also fail safe.

This was the first thing I was going to suggest - it has all of the
benefits you've mentioned, and you're going to need caps anyway.

Have you bothered to figure out how much current your negative
rail really has to sink?

Good Luck!
Rich

 

[colin]

"Rich Grise" <richgrise@example.net> wrote in message
newsan.2005.07.19.16.51.37.504847@example.net...

On Tue, 19 Jul 2005 14:26:09 +0000, colin wrote:

Hi, thnks yes good point, although for the op amps it wldnt be a
problem,
id thought of doing it that way but instead if the 5v rail gets
overloaded
then the -5 v rail becomes unregulated, but im also using it for a
max038
wich I wouldnt like to risk overloading either rail so im thinking il
use
the pos and neg regulator aproach.

However idealy a single device would shut down totaly under any fualt
condition, another aproach i might use a charge pump for the -5v
this would mean lower suply voltage and also one side of the supply is
gnd
wich seems neater, maybe save me a wire too, they seem quite neat small
package and only a smal capacitor just lose about .6v on the -5 rail,
might
be able to compensate, also fail safe.


This was the first thing I was going to suggest - it has all of the
benefits you've mentioned, and you're going to need caps anyway.

Have you bothered to figure out how much current your negative
rail really has to sink?

Well yes ballpark figures anyway, for neg supply its virtualy just the
quiescent supply for the op amps, few ma depending on what op amps I find
perform best (ultra low noise), not sure I like the idea of the effects of
switching near <1uv signals tho, probably do it diferently here, although
mains hum is probably the only real problem, Il prob try it, I decided I
like the idea of -vin being same as gnd. (wires will be going through slip
rings).

For the max038 (seperate unit) its at least 50ma just for the quiescent
supply (wich seems a bit high) plus output current, so I wld be looking at a
100ma to 200ma pump device wich are available, you can get regulated
negative charge pumps too wich is nice, however their max input voltage is
5.5 v wich means you cant hang it on the unregulated input, 5v in wld mean
it couldnt regulate the full -5v.

If I use 5v supply op amps I might use 3v logic and +3/-2 to save having 3rd
supply, and being able to provide -v regulation.

Im not sure the max038 wld need regulation much better than ~0.4v tbh, or
maybe taking a second output of a single chip flyback converter wld actualy
end up simpler here, and maybe a bit of extra eficiency to boot.

Sometimes I feel like ive gone round in circles, gues its time to just order
some chips and plug and play ... hmm i cld go for some fish and chips
actualy, I feel like ive been rambling now, thanks for listening

Colin =^.^=