Groups > sci.electronics.design > #431045

Re: LDO

From	John Larkin <jjlarkin@highlandtechnology.com>
Newsgroups	sci.electronics.design
Subject	Re: LDO
Date	2016-10-12 15:58 -0700
Organization	highland technology
Message-ID	<6oftvb50cdao4docroiksvrln1n35d1k0f@4ax.com> (permalink)
References	<54hrvblc24s5d7rj62noa4o2va6dl429c2@4ax.com> <hh61dd-sgh.ln1@baer-gmbh.com> <b3c954a3-2929-4faa-bf26-9444ce9f2a4a@googlegroups.com>

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On Wed, 12 Oct 2016 14:35:09 -0700 (PDT), klaus.kragelund@gmail.com
wrote:

>On Wednesday, 12 October 2016 09:54:42 UTC+2, Robert Loos  wrote:
>> Hello John,
>> >
>> > I need to make 1.5 volts (for DDR3 ram) from a 1.8 rail, and it was
>> > easier to just do it from available parts than find a new regulator to
>> > buy. Loop comp is the first wild guess, but it looks fine. At 0.3
>> > volts drain-source, the mosfet is acting like a variable resistor.
>> >
>> 
>> I love simulations. What would make me a little bit nervous is the small 
>> oscillation at ~235kHz.
>I get the same oscillation. It gets a lot worse when the load kicks in at 300ms

My sim doesn't oscillate, but there's not much safety margin.

>
>If one add a feedback capacitor from the opamp output to the inverting input of 100pF, the oscillation disappears completely. That's the normal way to compensate an LDO, at least how I normally do it :-)
>
>Cheers
>
>Klaus

It looks good with a tweak to the feedback RC: 50K + 1 uF. As I said,
the initial loop comp was a wild guess.

Giant ceramic caps have become affordable, things like 47u or 100u,
6.3 volts, 1206, so lots of output cap removes responsibility from the
control loop.



-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

Thread

LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-11 22:09 -0700
  Re: LDO Robert Loos <1209@baer-gmbh.com> - 2016-10-12 09:49 +0200
    Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-12 08:31 -0700
      Re: LDO bitrex <bitrex@de.lete.earthlink.net> - 2016-10-12 22:47 -0400
      Re: LDO John S <Sophi.2@invalid.org> - 2016-10-13 06:16 -0500
        Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-13 08:08 -0700
          Re: LDO Klaus Kragelund <klauskvik@hotmail.com> - 2016-10-13 08:57 -0700
            Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-13 09:05 -0700
            Re: LDO Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2016-10-13 09:26 -0700
              Re: LDO Klaus Kragelund <klauskvik@hotmail.com> - 2016-10-14 04:38 -0700
                Re: LDO Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2016-10-14 07:42 -0700
    Re: LDO klaus.kragelund@gmail.com - 2016-10-12 14:35 -0700
      Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-12 15:58 -0700
      Re: LDO Robert Loos <1209@baer-gmbh.com> - 2016-10-13 08:46 +0200
    Re: LDO Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2016-10-12 15:08 -0700
      Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-12 15:53 -0700
        Re: LDO Winfield Hill <hill@rowland.harvard.edu> - 2016-10-13 03:28 -0700
          Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-13 08:21 -0700
    Re: LDO piglet <erichpwagner@hotmail.com> - 2016-10-13 14:03 +0100
      Re: LDO Winfield Hill <hill@rowland.harvard.edu> - 2016-10-13 06:16 -0700
        Re: LDO piglet <erichpwagner@hotmail.com> - 2016-10-13 14:30 +0100
          Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-13 08:26 -0700
  Re: LDO doggerel <doggerel@nil.com> - 2016-10-12 16:07 +0800
    Re: LDO Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2016-10-12 15:53 +0300
      Re: LDO doggerel <doggerel@nil.com> - 2016-10-13 10:16 +0800
    Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-12 07:35 -0700
      Re: LDO doggerel <doggerel@nil.com> - 2016-10-13 10:15 +0800
  Re: LDO bloggs.fredbloggs.fred@gmail.com - 2016-10-12 08:41 -0700
    Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-12 09:20 -0700
      Re: LDO bloggs.fredbloggs.fred@gmail.com - 2016-10-13 15:52 -0700
        Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-13 16:11 -0700
          Re: LDO bloggs.fredbloggs.fred@gmail.com - 2016-10-13 21:45 -0700
            Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-14 09:05 -0700
  Re: LDO Tim Wescott <tim@seemywebsite.really> - 2016-10-12 11:21 -0500
    Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-12 09:57 -0700
      Re: LDO Tim Wescott <tim@seemywebsite.really> - 2016-10-12 15:11 -0500
        Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-12 16:11 -0700
          Re: LDO Winfield Hill <hill@rowland.harvard.edu> - 2016-10-13 04:52 -0700
            Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-13 08:33 -0700
          Re: LDO Tim Wescott <seemywebsite@myfooter.really> - 2016-10-13 18:21 -0500
            Re: LDO John Larkin <jjlarkin@highlandtechnology.com> - 2016-10-13 20:15 -0700
              Re: LDO Tim Wescott <seemywebsite@myfooter.really> - 2016-10-14 12:35 -0500
          Re: LDO Tim Wescott <seemywebsite@myfooter.really> - 2016-10-13 18:24 -0500

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