I built the following circuit in LTSpice using LM317. With a current load at the output, stepping from 0.1A to 1A, I noticed the output voltage collapsed at around 800mA.
This is a surprise, since I read the datasheet somewhere that LM317 has a maximum output current of 1.5A.
So what did I do wrong in the simulation?
Turns out it was the model I used had a cut off at 840mA.
After I tried a LM317.SUB from someone else, it started to cut off at around 1.4A.
The "working" .sub file has the following changes than the one I previously used:
R26 32 3 .06
.MODEL QNL NPN(EG=1.22 BF=80 RB=100 CCS=1.5PF TF=.3NS TR=6NS CJE=2PF IS=6E-16 CJC=1PF VAF=100)
.MODEL QPL PNP(BF=40 RB=20 TF=.6NS TR=10NS CJE=1.5PF CJC=1PF VAF=50 IS=6E-16)
Note, the smaller R26, the higher the cut off current it will get.
The output voltage is apparently below zero when it does this (no blue line).
Your current source can pull the output below 0V, which could certainly lead to weird results both in simulation and in real life (imagine the current source powering up before or simultaneously with the input power).
Try putting a 1N5817 Schottky diode from the output to ground.
