Groups > sci.electronics.design > #431032

Re: LDO

From	Tim Wescott <tim@seemywebsite.really>
Subject	Re: LDO
Newsgroups	sci.electronics.design
References	<54hrvblc24s5d7rj62noa4o2va6dl429c2@4ax.com> <K5ednXGmuP6A_mPKnZ2dnUU7-c2dnZ2d@giganews.com> <u4qsvbp936mb9vjhnfk1tcob5eu7vv5bn2@4ax.com>
Message-ID	<krWdnRBkJdBqBWPKnZ2dnUU7-LednZ2d@giganews.com> (permalink)
Date	2016-10-12 15:11 -0500

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On Wed, 12 Oct 2016 09:57:17 -0700, John Larkin wrote:

> On Wed, 12 Oct 2016 11:21:49 -0500, Tim Wescott
> <tim@seemywebsite.really> wrote:
> 
>>On Tue, 11 Oct 2016 22:09:59 -0700, John Larkin wrote:
>>
>>> 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.
>>> 
>>Don't have time to put this on LT Spice, but do an AC analysis of the
>>input and output impedances when it's running at design current and
>>maybe 2x and 1/2 as much.  Look for where the real part goes negative,
>>and consider whether that's going to find fertile ground for
>>oscillation.
> 
> The transfer curve of the fet is very nonlinear, and it's acting like a
> variable resistor, so the output time constant is continuously changing.
> I wouldn't want to try to write closed-form equations for the load-step
> response, or to evaluate stability over the load range. This is clearly
> a case where Spice wins.
> 
> The loop is unnecessarily hot, but was a first guess. It needs tweaking.
> What's important is to design a circuit that can be tweaked.
> 
> The LDO is powering a 4 gbit DDR3 ram and the FPGA bank that interfaces
> it. I have no good estimate of either the active or the idle current, or
> the time profile of the load current. The big output cap helps a lot
> here. I may need a dump resistor (R1) but probably not; gotta research
> the FPGA and the dram to be sure. The output can't go up much, given
> there's only 1.8 volts available.
> 
> 
> 
> 
>>(If you've got power gain and more than 180 degrees of phase shift,
>>you're pretty much guaranteed that the output impedance will go negative
>>at some frequency.)
> 
> Simulated load steps are a great way to estimate, well, load step
> response. Ringing suggests low phase margins.
> 
> 
>>To sweep for an impedance shove a voltage or current source in there
>>someplace (or use an existing one).  Set the DC value to zero (or use
>>what you had been for your simulation), set the AC value to 1, run the
>>sweep, and plot the current or voltage vs. frequency.  The admittance
>>(or impedance) jumps right out at you.
> 
> Load steps do that, in the time domain. But they account for the large
> nonlinearities, which AC analysis doesn't.

You've reminded me again of what I would have made my main line of 
research, had I been able to get it piled higher and deeper.

Basically, the world needs an extension of Lyapunov's 2nd method for 
determining stability that doesn't require a bunch of messing around 
solving nasty nonlinear differential equations, but that does cough up 
answers that are as intuitive as a resonant circuit's Q, or a phase 
margin on a Bode plot.

Dunno if you remember it, but Lyapunov's 2nd method is a way of 
determining if a nonlinear system is stable, and, to some extent, how 
fast it'll settle.  It involves cooking up a cost function that's 
analogous to the total system energy, then proving that the system always 
moves to a lower cost state.  I see no reason that it can't be done 
numerically, allowing one to basically say "is this system globally 
stable" and having something like LT Spice say "yes" or "no".  Having it 
cough up something analogous to a measure of Q in a resonant circuit 
would be a plus, so you know if it's on the verge of breaking into song.

-- 
www.wescottdesign.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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