neg ldo problems
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From: Hammy on 25 Feb 2010 08:49 I'm guessing I'm missing the obvious here. I'm trying to build a discrete negative LDO using a NFET for the pass element. I'm basing it on the internal structure of the UCC284-5 seen here. http://i50.tinypic.com/orkroj.png I've tried inverting the output of the opamp with a BJT but its not regulating. The positive input to the opamp is tied to V4 which is a - 2.5V reference, feedback is negative. The opamp is supplied with -18Vdc and +5Vdc. My pass element needs to regulate down to 500mV in/out differential. Feeble attempt at a negative LDO schematic: http://i47.tinypic.com/29zes7.png
From: Hammy on 25 Feb 2010 09:13 On Thu, 25 Feb 2010 08:49:53 -0500, Hammy <spam(a)spam.com> wrote: > > >I'm guessing I'm missing the obvious here. I'm trying to build a >discrete negative LDO using a NFET for the pass element. I'm basing it >on the internal structure of the UCC284-5 seen here. > >http://i50.tinypic.com/orkroj.png > >I've tried inverting the output of the opamp with a BJT but its not >regulating. The positive input to the opamp is tied to V4 which is a - >2.5V reference, feedback is negative. The opamp is supplied with >-18Vdc and +5Vdc. My pass element needs to regulate down to 500mV >in/out differential. > >Feeble attempt at a negative LDO schematic: > >http://i47.tinypic.com/29zes7.png Nevermind I got it to work. http://i47.tinypic.com/20qe15l.png output load step http://i45.tinypic.com/219tdw2.png Still needs some work but I at least have something to work with now.
From: John Larkin on 25 Feb 2010 10:45
On Thu, 25 Feb 2010 09:13:36 -0500, Hammy <spam(a)spam.com> wrote: >On Thu, 25 Feb 2010 08:49:53 -0500, Hammy <spam(a)spam.com> wrote: > >> >> >>I'm guessing I'm missing the obvious here. I'm trying to build a >>discrete negative LDO using a NFET for the pass element. I'm basing it >>on the internal structure of the UCC284-5 seen here. >> >>http://i50.tinypic.com/orkroj.png >> >>I've tried inverting the output of the opamp with a BJT but its not >>regulating. The positive input to the opamp is tied to V4 which is a - >>2.5V reference, feedback is negative. The opamp is supplied with >>-18Vdc and +5Vdc. My pass element needs to regulate down to 500mV >>in/out differential. >> >>Feeble attempt at a negative LDO schematic: >> >>http://i47.tinypic.com/29zes7.png > >Nevermind I got it to work. > >http://i47.tinypic.com/20qe15l.png > >output load step > >http://i45.tinypic.com/219tdw2.png > >Still needs some work but I at least have something to work with now. Much cleaner: grounded reference, grounded ground! John |