LT3080 "0" volts

[gregfox]

Hi all,
I’m using a LT3080, and I have a problem. (See attached) If the LT3080 does not have a current draw on its output (pin 3) of a least .5mA it will not stay in regulation. A LM334 was added to the output to keep it in regulation, but the problem is the LM334 operates only down to 1 volt, therefore as the voltage drops down below a volt the LM334 will no longer operate, and the LT3080 will be out of regulation (you can’t get it below ~2 volts)
In the attached schematic I show the bread board, and I can only get it down to 2.4 volts even after grounding the set pin. Does anyone know how to keep it in regulation and get “0” volts as an output?

 

[Harald Kapp]

This seems to be a "feature" of the LT3080. Have a look at the datasheet, page 17, figure at top right. There an additional transistor is inserted to assert 0V output voltage in the absence of a load.
The issue is probably that there is no discharge path from the output of the regulator to GND. So any charge that builds up in the output capacitor is stored and generates an output voltage. Instead of the shutdown transistor you can add a bleeder resistor parallel to the output capacitor to asssert a minimum output current.

 

[gregfox]

Thank you Harald, but the schematic I included was working in other circuits. I tried 3 more 3080 in circuit and the third one worked. bad batch I guess.

 

[Harald Kapp]

bad batch I guess

Or two good ones and one from a bad batch with more leakage than the others - which in your case turnes out to be beneficial.