Forums

another LDO

Started by John Larkin May 27, 2012

Seriously, I need another LDO for a new gadget. It needs to work down
to maybe 1 volt drop, and needs to be very accurate, especially
longterm drift and TC. 

How about this?

http://dl.dropbox.com/u/53724080/Circuits/T680_LDO.JPG

Joerg will be pleased to see a depletion fet. Jim will find something
to whine about.

It might be fun to power the opamp from the +10 rail, if I could prove
that's safe at startup. That does keep from enhancing the fet and
preserves its Idss current limit. I only need about 50 mA out, with a
nearly constant load. I could even add a source resistor.

Maybe I should add a gate resistor, just in case the fet wants to
oscillate on its own.

I guess I could also servo the ADJ pin of an LM1117.


-- 

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
On Sun, 27 May 2012 14:17:40 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

> > >Seriously, I need another LDO for a new gadget. It needs to work down >to maybe 1 volt drop, and needs to be very accurate, especially >longterm drift and TC. > >How about this? > >http://dl.dropbox.com/u/53724080/Circuits/T680_LDO.JPG > >Joerg will be pleased to see a depletion fet. Jim will find something >to whine about. > >It might be fun to power the opamp from the +10 rail, if I could prove >that's safe at startup. That does keep from enhancing the fet and >preserves its Idss current limit. I only need about 50 mA out, with a >nearly constant load. I could even add a source resistor. > >Maybe I should add a gate resistor, just in case the fet wants to >oscillate on its own. > >I guess I could also servo the ADJ pin of an LM1117.
http://dl.dropbox.com/u/53724080/Circuits/T680_LDO_1117.JPG Running the opamp off +10 is probably too cute for its own good. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On 5/27/2012 2:17 PM, John Larkin wrote:
> > > Seriously, I need another LDO for a new gadget. It needs to work down > to maybe 1 volt drop, and needs to be very accurate, especially > longterm drift and TC. > > How about this? > > http://dl.dropbox.com/u/53724080/Circuits/T680_LDO.JPG > > Joerg will be pleased to see a depletion fet. Jim will find something > to whine about. > > It might be fun to power the opamp from the +10 rail, if I could prove > that's safe at startup. That does keep from enhancing the fet and > preserves its Idss current limit. I only need about 50 mA out, with a > nearly constant load. I could even add a source resistor. > > Maybe I should add a gate resistor, just in case the fet wants to > oscillate on its own. > > I guess I could also servo the ADJ pin of an LM1117. > >
I don't see the advantage of a depletion fet. If anything, it insures the regulator will go overvoltage on start up since the fet is conducting at zero time. Certainly true for a high DV/DT on the supply rail. A mos pass device is good. Roll your own regulators is usually asking for trouble. COTS devices have well behaved start up, short circuit protection, and often reverse power protection.
On Sun, 27 May 2012 14:36:37 -0700, miso <miso@sushi.com> wrote:

>On 5/27/2012 2:17 PM, John Larkin wrote: >> >> >> Seriously, I need another LDO for a new gadget. It needs to work down >> to maybe 1 volt drop, and needs to be very accurate, especially >> longterm drift and TC. >> >> How about this? >> >> http://dl.dropbox.com/u/53724080/Circuits/T680_LDO.JPG >> >> Joerg will be pleased to see a depletion fet. Jim will find something >> to whine about. >> >> It might be fun to power the opamp from the +10 rail, if I could prove >> that's safe at startup. That does keep from enhancing the fet and >> preserves its Idss current limit. I only need about 50 mA out, with a >> nearly constant load. I could even add a source resistor. >> >> Maybe I should add a gate resistor, just in case the fet wants to >> oscillate on its own. >> >> I guess I could also servo the ADJ pin of an LM1117. >> >> > >I don't see the advantage of a depletion fet. If anything, it insures >the regulator will go overvoltage on start up since the fet is >conducting at zero time. Certainly true for a high DV/DT on the supply rail. > >A mos pass device is good. Roll your own regulators is usually asking >for trouble. COTS devices have well behaved start up, short circuit >protection, and often reverse power protection.
A regular n-channel mosfet won't have enough gate drive available. I guess a p-channel would be OK, but I'd need to add some sort of current limit. A commercial LDO won't be as accurate as I need; I'd like to hold a few hundred PPM over time and temperature, and tracking the DAC reference is good here, too. A powerup overvoltage blip wouldn't bother me... there's only 12 volts available, and that wouldn't damage anything. Probably the LM1117 thing is more prudent, with the opamp powered off +12. I've tried to damage LM1117s by doing rude things to their ADJ pins and they didn't care. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
John Larkin wrote:
> > Seriously, I need another LDO for a new gadget. It needs to work down > to maybe 1 volt drop, and needs to be very accurate, especially > longterm drift and TC. > > How about this? > > http://dl.dropbox.com/u/53724080/Circuits/T680_LDO.JPG > > Joerg will be pleased to see a depletion fet. Jim will find something > to whine about. > > It might be fun to power the opamp from the +10 rail, if I could prove > that's safe at startup. ...
At 12 ohms on resistance that's iffy. Depends on what the opamp output does when at only 1-2V supply. If it won't come up enough then you'd lose the 10V output.
> ... That does keep from enhancing the fet and > preserves its Idss current limit. I only need about 50 mA out, with a > nearly constant load. I could even add a source resistor. >
You could also use a series resistor towards the gate and then a diode from gate to drain. Then it won't enhance to more than 700mV or so.
> Maybe I should add a gate resistor, just in case the fet wants to > oscillate on its own. > > I guess I could also servo the ADJ pin of an LM1117. >
But with a depletion mode FET you get a stable LDO, why give that up? -- Regards, Joerg http://www.analogconsultants.com/
miso wrote:
> On 5/27/2012 2:17 PM, John Larkin wrote: >> >> >> Seriously, I need another LDO for a new gadget. It needs to work down >> to maybe 1 volt drop, and needs to be very accurate, especially >> longterm drift and TC. >> >> How about this? >> >> http://dl.dropbox.com/u/53724080/Circuits/T680_LDO.JPG >> >> Joerg will be pleased to see a depletion fet. Jim will find something >> to whine about. >> >> It might be fun to power the opamp from the +10 rail, if I could prove >> that's safe at startup. That does keep from enhancing the fet and >> preserves its Idss current limit. I only need about 50 mA out, with a >> nearly constant load. I could even add a source resistor. >> >> Maybe I should add a gate resistor, just in case the fet wants to >> oscillate on its own. >> >> I guess I could also servo the ADJ pin of an LM1117. >> >> > > I don't see the advantage of a depletion fet. ...
Stability. It behaves like the usual follower structure of a non-LDO. John, the depletion mode FETs from Infineon cost less, in case that's a concern.
> ... If anything, it insures > the regulator will go overvoltage on start up since the fet is > conducting at zero time. Certainly true for a high DV/DT on the supply > rail. > > A mos pass device is good. Roll your own regulators is usually asking > for trouble.
Au contraire, monsieur.
> ... COTS devices have well behaved start up, short circuit > protection, and often reverse power protection. >
Yeah, right. I've seen a lot of COTS LDOs with behavioral issues. All the way to going *PHUT* after a while. -- Regards, Joerg http://www.analogconsultants.com/
On May 27, 11:17=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> Seriously, I need another LDO for a new gadget. It needs to work down > to maybe 1 volt drop, and needs to be very accurate, especially > longterm drift and TC.
There's always the LM10. Supply voltage from 45V down to 1V and includes a 200mV reference voltage source. Farnell still stocks it. http://www.newark.com/national-semiconductor/lm10cn-nopb/ic-op-amp-90khz-0-= 2v-%C2%B5s-dip-8/dp/41K3353?ref=3Dlookahead -- Bill Sloman, Nijmegen
John Larkin wrote:
> > Seriously, I need another LDO for a new gadget. It needs to work down > to maybe 1 volt drop, and needs to be very accurate, especially > longterm drift and TC. > > How about this? > > http://dl.dropbox.com/u/53724080/Circuits/T680_LDO.JPG > > Joerg will be pleased to see a depletion fet. Jim will find something > to whine about. > > It might be fun to power the opamp from the +10 rail, if I could prove > that's safe at startup. That does keep from enhancing the fet and > preserves its Idss current limit. I only need about 50 mA out, with a > nearly constant load. I could even add a source resistor. > > Maybe I should add a gate resistor, just in case the fet wants to > oscillate on its own. > > I guess I could also servo the ADJ pin of an LM1117. >
2V drop is within LM317 range at low currents. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
On Sun, 27 May 2012 19:22:59 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>John Larkin wrote: >> >> Seriously, I need another LDO for a new gadget. It needs to work down >> to maybe 1 volt drop, and needs to be very accurate, especially >> longterm drift and TC. >> >> How about this? >> >> http://dl.dropbox.com/u/53724080/Circuits/T680_LDO.JPG >> >> Joerg will be pleased to see a depletion fet. Jim will find something >> to whine about. >> >> It might be fun to power the opamp from the +10 rail, if I could prove >> that's safe at startup. That does keep from enhancing the fet and >> preserves its Idss current limit. I only need about 50 mA out, with a >> nearly constant load. I could even add a source resistor. >> >> Maybe I should add a gate resistor, just in case the fet wants to >> oscillate on its own. >> >> I guess I could also servo the ADJ pin of an LM1117. >> > >2V drop is within LM317 range at low currents. > >Cheers > >Phil Hobbs
"2V" is now called "LDO" ?? ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Sun, 27 May 2012 19:22:59 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>John Larkin wrote: >> >> Seriously, I need another LDO for a new gadget. It needs to work down >> to maybe 1 volt drop, and needs to be very accurate, especially >> longterm drift and TC. >> >> How about this? >> >> http://dl.dropbox.com/u/53724080/Circuits/T680_LDO.JPG >> >> Joerg will be pleased to see a depletion fet. Jim will find something >> to whine about. >> >> It might be fun to power the opamp from the +10 rail, if I could prove >> that's safe at startup. That does keep from enhancing the fet and >> preserves its Idss current limit. I only need about 50 mA out, with a >> nearly constant load. I could even add a source resistor. >> >> Maybe I should add a gate resistor, just in case the fet wants to >> oscillate on its own. >> >> I guess I could also servo the ADJ pin of an LM1117. >> > >2V drop is within LM317 range at low currents. > >Cheers > >Phil Hobbs
I prefer the 1117. It's very similar to the 317 but has somewhat lower dropout. 317 is a Darlington, and 1117 is a pnp-npn pair, what we used to call a "pseudo-PNP." I think there's somebody's name attached to that. I'm doing a mess of DAC-trimmed current sources, to make fast ramps, and I can grossly simplify everything if I can make a really good +10 supply. I've used 1117s as power amps before, and they work great. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators