Adjustable Current Limiter using LM1084: Internal Vref not maintained?

[blackshadow]

i'm trying to develop a simple current regulator circuit using LM1084.

i intend to have a current limit of 3A.

R1 should be equal to 1.25V/3A = 0.413 ohms

ideally, per attached schematic, the voltage across R1 should be maintained with the internal Vref of 1.25V as we change the load current below 3A.

i simulated this on LT spice where it turned out that the Voltage across R1 is not maintained at 1.25V. instead Vref (Vout-Vload) increases from 0.2volt to 1.19V as we change load from 0.5A to 2.9A.

should'nt the Vref is kept at 1.25V? What's wrong?

 

[Roff]

You need to change the load to a variable resistor. You can't vary the load current on a current regulator. Run the attached .asc file.
If you are trying to build a voltage source with a current limit, that is not the way to do it.

 

[blackshadow]

You need to change the load to a variable resistor. You can't vary the load current on a current regulator. Run the attached .asc file.
If you are trying to build a voltage source with a current limit, that is not the way to do it.

thank you roff!

can you share the SPICE directive syntax? i'm just new to LT spice...

regards..

 

[blackshadow]

thank you roff!

can you share the SPICE directive syntax? i'm just new to LT spice...

regards..

oh i see. i'm reviewing the Help file from LT spice. thanks

 

[crutschow]

i simulated this on LT spice where it turned out that the Voltage across R1 is not maintained at 1.25V. instead Vref (Vout-Vload) increases from 0.2volt to 1.19V as we change load from 0.5A to 2.9A.

shouldn't the Vref is kept at 1.25V? What's wrong?

As you perhaps understand by now, the regulator does not work (maintain 1.25 across R1) until the load reaches the current regulated value. Below that point the regulator is trying to increase the current but can't since it is seeing a controlled current load.