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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?

circuit sim

sim.zip - ltspice sim with 1N4001 and LM317 models

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  • \$\begingroup\$ (13.5 V − 5 V) × 800 mA = 6.8 W. Is it possible that the model correctly simulates the thermal protection? \$\endgroup\$ – CL. Feb 24 at 9:21
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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.

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  • \$\begingroup\$ I changed the current source to a fix resistor and step it through 1 to 10ohm. The output voltage again begin to falls off when the load resistor is 6ohm and lower (which equivalent to 833mA at 6ohm). I suspect the simulation is ok, but somehow LM317 model can't output more than 800mA for whatever reason. \$\endgroup\$ – user97662 Feb 24 at 6:00
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    \$\begingroup\$ You can try another manufacturer’s model. There’s foldback current limiting on the 317 IIRC, but that should not be causing your discrepancy. \$\endgroup\$ – Spehro Pefhany Feb 24 at 6:06
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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.

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