LM234: How load resistance impacts set current?

Bojan Jovanovic

Part Number: LM234
Other Parts Discussed in Thread: LM334, TL431LI, LM4041-N

Hello guys,

I am using LM234Z-3 in my current design with the intention to give me either 1mA (Rset = 68Ohm) or 2mA (Rset = 34Ohm) constant current for my load that can be in the range of up to 5Kohm.

Bellow is the schematic around LM234. As you can see, my V+ node of LM234 is supplied with +12V. What I noted while testing on the board is that LM234 delivers 1mA constant current for loads of up to 3.2KOhms and 2mA constant current for loads of up to 1.7Kohm. When the load resistance is above the threshold, constant current starts dropping down.

Do you, guys, have any idea what might cause constant current to drop? By reading LM234 datasheet I could not find any info on how load resistance influences constant current.

Also, do you have any better idea on how to design constant current source for the loads whose resistance can vary in a range from 100Ohm to 5Kohm?

over 5 years ago

0 Abhinav.Sharma over 5 years ago

TI__Genius 9375 points

Hi Bojan,

I'm going to get in touch with someone who's an expert on these parts and get back to you with an answer tomorrow.

Thanks,
Abhinav.

0 Bojan Jovanovic over 5 years ago in reply to Abhinav.Sharma

Intellectual 320 points

Hey Abhinav,

Sounds good to me. Thank you very much for your help!

Cheers,

Bojan.

0 Marcoo Z over 5 years ago in reply to Bojan Jovanovic

TI__Mastermind 19550 points

Hi Bojan,

Interesting use case. How much current deviation are you currently measuring?

I am thinking it might be the "Average change in set current with input voltage" but If the error is too large, then it might be something different.

-Marcoo

0 Bojan Jovanovic over 5 years ago in reply to Marcoo Z

Intellectual 320 points

Hi Marcoo,

For your info, on the place of Rload from the above image I used 10KOhm potentiometer. When I set the current to be 1mA (Rset = 68KOhm), I have desired current for the potentiometer values from 0 to 3.2Kohm. After that, the current starts dropping down to 200mA when Rload = 10Kohm.

I am not sure what I am missing but it seems that the current delivered by LM234Z-3 is influenced by load reisistance.

Sincerely,

Bojan.

0 Marcoo Z over 5 years ago in reply to Bojan Jovanovic

TI__Mastermind 19550 points

Hi Bojan,

There is a dependency but I am curious why this is happening. This is an old device so I am not as familiar with it as I should be. Let me go to the lab this week and try it out.

0 Bojan Jovanovic over 5 years ago in reply to Marcoo Z

Intellectual 320 points

Hello Marcoo,

I just wanted to touch the base and check with you if there are some news about LM234 from your side. Did you have some time to play with it in your lab this week?

Thanks in advance!

Sincerely,

Bojan.

0 Marcoo Z over 5 years ago in reply to Bojan Jovanovic

TI__Mastermind 19550 points

Hi Bojan,

I am waiting for the devices to arrive to test them in the lab. I did not have this variant on hand. I expect to receive them by next week.

-Marcoo

0 Bojan Jovanovic over 5 years ago in reply to Marcoo Z

Intellectual 320 points

Hi Marcoo,

Thank you very much. I really appreciate your effort! :-)

Have a nice week-end,

Bojan.

0 Marcoo Z over 5 years ago in reply to Bojan Jovanovic

TI__Mastermind 19550 points

Hi Bojan,

Due to shipping delays, I have tested this on a LM334 which I had near by.

The main difference should be in the accuracy and current limit.

The LM334 performed per datasheet spec, matched figure 11, and your schematic with load requirements.

The main difference I see wit the LM334 and LM234Z is that the LM234Z is not specified with current above 1mA.This means the LM234Z-3 should not function above 1mA.

I would try a LM234 (non Z grade) or LM334 and see if it performs better.

If you need more accuracy, try a LM4041-N in current source configuration or TL431LI in current sink.

-Marcoo

0 Bojan Jovanovic over 5 years ago in reply to Marcoo Z

Intellectual 320 points

Hello Marcoo,

Thank you very much for your great support! It makes sense what you write.

Will try with the suggested modules.

Cheers,

Bojan.

Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

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LM234: How load resistance impacts set current?