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Re: LDO

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On 10/12/2016 10:31 AM, John Larkin wrote:
> On Wed, 12 Oct 2016 09:49:36 +0200, Robert Loos <1209@baer-gmbh.com>
> 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 don't see an oscillation in my sim. Maybe you have different Spice
> settings; LT spice is a speed demon and it can be quirky.
>
> It will oscillate in my sim if R5 is doubled, which is cutting things
> close. But then a tiny bit of added ESR in C2 fixes it again.
>
> The transient load regulation is super good, so the loop is needlessly
> hot. 1 uF and 20K is more prudent, and those values are already on the
> BOM.
>
> The DDR3 ram will probably pull current in short bursts, as CPU cache
> is topped off. The 47uF ceramic cap helps; I could put two of them on
> the board for luck.
>
>
>
>> Doubling the output C silences it but I tried to add small C's here and
>> there and it only got worse. Not even the frequency changes notably.
>> So I wonder what determines the frequency? The rise time of the OpAmp?
>> A better idea how to silence it anyone?
>>
>> Cheers
>>
>> Robert
>

Hey, John, try this setting to see the oscillations:
.tran 0 500m 10m 10u uic

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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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