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Setting output voltage in UC3834 with a D/A converter

Pawel Wierzba
Prodigy 30 points
Other Parts Discussed in Thread: UC2834

I would like to know UC2834 would react to setting its output voltage using a digital-to-analog converter.

The modification to the "5-10 Amp Positive Regulator" circuit from page 6 of the datasheet would involve (1) disconnecting pins 3 and 8 (1.5V ref and non-inverting input, respectively), (2) connecting the output of a digital-to-analog converter to pin 8 (non-inverting input), and (3) selecting the resistors in the voltage divider connected to pin 9 (inverting input) in such a way that the desired range of output voltage corresponds to the voltage range of the digital-to-analog converter.

As an "illustrative example": "5-10 Amp Positive Regulator" circuit from page 6 powered from a 24 V source, voltage divider connected to pin 9 providing 1/4 U_out, and a 0-5V DAC connected to pin 8.  Can anyone tell me if such circuit will give output voltage in 0-20 V range?

Moreover, I would like to know whether UC2834 controlled that way can deliver voltages below 1.5 V and what is the input voltage range  of inverting and non-inverting input pins (pin 8 and 9).

  • 0
    TI__Guru 55940 points
    Hi Pawel,

    We have not characterized the device with your application in mind but it is an interesting application. Keep in mind that by fixing the voltage divider you are fixing the ratio that the output will be with regards to the reference and your output will be a gained version of your reference (which you will now be supplying externally). Any inaccuracy in your D/A will be reflected on your output.

    LDOs will not regulate bellow their reference; however, as you are not hooking up our supplied 1.5V reference, you could theoretically regulate to a lower voltage by providing a reference bellow 1.5V.

    The Abs Max range for the error amplifier inputs is -0.5 to 35V.

    Good luck in your design, it would be interesting to hear if you have success!

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Mastermind 22610 points

    The following three documents explain how to tweak the feedback circuitry of an adjustable LDO using additional external components to achieve an output voltage lower than its reference voltage:

    SLVA216 : Regulating VOUT Below 1.2 V Using an External Reference

    www.ti.com/.../slva216.pdf

    SLUA256 : High-Current LDO Linear Regulators (UCCx81-ADJ, UCx82-ADJ, UCCx83-ADJ, UCx85-ADJ)

    www.ti.com/.../slua256.pdf

    SLVA071 : Low Power 150- mA LDO Linear Regulators. Extended Output Voltage Adjustment Range

    www.ti.com/.../slva071.pdf

    Note that LDO's are not characterized, or ensured, in this operating region, so use at your own risk.