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LM334: Set current at high load resistance

Jocelyn Thomson
Prodigy 20 points
Part Number: LM334

Hello,

I need a stable constant current source to supply ~100 uA to a variable load resistance (20k - 10M). I purchased the LM334 for testing and programmed it with an Rset of 680 ohm. At 0 to 1k ohm loads at ~5V it performs well and maintains a current of 92 uA, however at a greater load resistance of 577k ohm I could only draw about 30 uA @ 18V. Is it possible I'm hitting the max power limit of this IC? Do you have any suggestions for how I could achieve my goal by adjusting the schematic or using a different part? Any help would be much appreciated. 

Kind regards,

Jocelyn

  • 0
    TI__Mastermind 19580 points

    Hi Jocelyn,

    I feel you don't have enough headroom for this.

    Largest load = Vhigh = 100uA * 10M = 100V

    Smallest load = Vlow = 100uA * 2V

    In your situation with 577k Ohms @ 18V there is not enough head room. Currently the LM334 requires 1V of headroom. So that will give 17V to the load.

    17V / 577K = ~ 29uA which is in line with that you see.

    You have 2 issues in this design.

    1. You don't have enough head room, you need at least 100V + headroom to support 20M at 100uA.

    2. The LM334 only supports up to 40V.

    I would suggest to either limit the upper resistance so it does not cause the LM334 to violate 40V or to make a discrete solution that can support 100V load.

  • 0 in reply to Marcoo Z
    Prodigy 20 points

    Thank you for your help, this raises some questions for the authors of a paper this research is based off of. They've indicated to have succesfully supplied a 7M ohm load with 100 uA. But your answer is very clear, so thank you!