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LM234: Current source LM134/234 max output voltage can't go above 17V

Part Number: LM234

Hello,

I'm using current source LM234 chip to output constant 1mA current, the load vary from 1K to 30K. The V+ of LM234 is provide by +32V from DCDC.

The output of LM234 can only drive the load below 15K, when the load above 15K, the output voltage will not increase equal proportion, the max output voltage is only about 17V.  So when the load is 30K, the output voltage is only 17V, the current is only about 0.566mA(17/30). Any one can help me to solve this issue? Thanks very much.

Below is my schematic and reference circuit from TI datasheet, VDD is 32V, load is below W4 and W3.

Thanks and Regards

Yanjun

  • Hi Yanjun,

    I will try to replicate this on my bench. What exact part number are you using?

    When you mention loads about 15K, is this an effective load resistance? I am a little confused on the circuit regarding your load.

    I will message you privately and we can take this this question offline.

    Thanks,
    Ethan
  • Hi Ethan,

    Thanks for your reply. I found the issue is caused by the unqualified varistor. The circuit works OK after I remove the varistor.

    Another question:
    Could you please help to recommend some SMD diode which can replace the 1N457 in the reference circuit? We need to get accurate 1mA output. IN457 is EOL now, i can not find any SMD diode which has same forward voltage(0.6V) and same temperature coefficient(minus 10 times of LM234 chip).

    Regards
    Yanjun
  • Hi Yanjun,

    The MMBD1201 / MMBD1202 (SOT-23-3), or FDLL914A (SOD-80), are similar in performance.

    The Fairchild dataheet for the FDLL914A has a Typical Performance curve (Page 3, Figure 6) that suggests the forward voltage tempco will be about -1.43mV/°C, with a If of 1mA, and a typical forward voltage of about 615mV at Ta=25°C.

    www.fairchildsemi.com/.../1N914.pdf

    Hopes this helps,
    Ethan
  • Ethan, thank you very much for your support.