LT1085 constant current. will this work?

[wall]

Can an LT1085 be used as a constant current LED driver? There is no mention of it in anywhere in the datasheet, but since this is an LM317 replacement can I use it the same way?

 

[abraxalito]

The old LM1085 has a current source (as a battery charger) in its application schematics so I don't see why not. I have had stability issues when using an LM317 as CCS so I'd pay attention to oscillations nevertheless.

 

[wall]

I had seen that example, but what is the purpose of R1 and R2? Is it for voltage limiting?

 

[abraxalito]

Yes, I think so.

 

[Michael Bean]

The LT1085 requires a minimum load current of 5mA to 10mA, so it may not be the best choice unless you want the LED's to glow quite brightly. A better choice for that application, you might try a simple discrete current source/sink, or try something like the Lm4041 for example.

Mike

​

 

[wall]

View attachment 1156033

Should D2 go before or after Rsense?

 

[wall]

The LT1085 requires a minimum load current of 5mA to 10mA, so it may not be the best choice unless you want the LED's to glow quite brightly. A better choice for that application, you might try a simple discrete current source/sink, or try something like the Lm4041 for example.

Mike

​

Doesn't R1 consume about 10mA from the regulator? Check the schematic in the first post.

 

[MarcelvdG]

Can an LT1085 be used as a constant current LED driver? There is no mention of it in anywhere in the datasheet, but since this is an LM317 replacement can I use it the same way?View attachment 1156030

D2, R1 and Pot are all useless in this schematic. C4 can be reduced to whatever is needed to keep the regulator stable.

 

[MarcelvdG]

Doesn't R1 consume about 10mA from the regulator? Check the schematic in the first post.

Unlikely, as R1 is shorted.

 

[wall]

D2, R1 and Pot are all useless in this schematic. C4 can be reduced to whatever is needed to keep the regulator stable.

D2 may be unnecessary, but are you sure R1 and Pot(R2) are useless? They seem to limit the output voltage and can be useful for open-circuit/no-load voltage limiting.

 

[MarcelvdG]

R1 is shorted, Pot is in parallel with the LEDs.

 

[wall]

See post #3

 

[MarcelvdG]

My comments apply to the schematic in the opening post, as you wrote: "Check the schematic in the first post."

In the circuit of post #3, R1 and R2 limit the output voltage, as is essential for a lead-acid battery charger. For a LED driver, you might as well let the output voltage increase to the input voltage minus the drop across the regulator when the load is disconnected.