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LM334: Zero Temperature Coefficient Configuration

Sean Cornish
Prodigy 20 points
Part Number: LM334

Hello,

I have been using an LM 334Z in its standard (single resistor) configuration to produce a 10 micro-amp current source for some 1 Kelvin temperature diodes I am using. In order to make a more robust system, I tried modifying the LM 334 circuit to the zero temperature coefficient configuration seen in Fig.15 of the data sheet. However, the math to sort out the exact R1 and R2 values has confused me. By time i fully worked it out and tested it for what I though would produce a 10 micro-amp source, I was producing mili-amps. If anyone has set up this configuration successfully, please let me know how you calculated your R1 and R2 values, or if you feel charitable, please let me know what R1 and R2 values I should use for Iset = .00001A.

Thanks in advance,

Sean

lm334.pdf

  • 0
    TI__Genius 15940 points

    Hi Sean,

    Thanks for reaching out to us. Do you have the tempco of the diodes, as mentioned in the data sheet example?

    What temperature will you be using this system at? The V_R mentioned in the data sheet is quoted at 214uV/K (so V_R = 214uV * degrees K) and, when including the bias current in calculations, is quoted to be 277uV/K.

    The calculations used in this example are split into sections: the ratio of R1/R2 and the magnitude of the resistors you use.

    Once you have the ratio of R1/R2 (see equations 6 and 7 in the data sheet), you can simply divide your temperature-based V_R by your desired current (0.00001A) to determine R1 and your ratio to determine R2.

    Does this help? Can you share the above information (tempco of diodes and operating temperature)?

    Thanks,

    Patrick

  • 0 in reply to Patrick Shalton
    Prodigy 20 points

    Hey Patrick,

    The Temp coefficient of the diode is 300 deg C/W. Operational temperature is ~22 deg C. The diode data sheet is attached for reference.

    Thanks,
    Sean

    1N457A-D.pdf

  • 0 in reply to Sean Cornish
    TI__Genius 15940 points

    Hi Sean,

    Thanks for sending the datasheet. This application requires the temperature coefficient in V/degC. From a quick browse: this document lists the tempco of the 1N457 is -2.5mV/degC. However, we cannot verify that information and we recommend to test it yourself or reach out to the manufacturer instead.

    Using this tempco, you can follow equations 6 and 7 to obtain your resistor ratio. Using this temperature coefficient, that part should be identical.

    Does that make sense?

    Thanks,

    Patrick