neg ldo problems

[Hammy]

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

 

[Hammy]

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.