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Original question:

LM5175: LM5175

LM5175: Variable output current

Cora Ferrer
Prodigy 80 points
Part Number: LM5175
Other Parts Discussed in Thread: LMV431A, INA213

Hi,

This question follows the thread: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1098782/lm5175-lm5175?tisearch=e2e-sitesearch&keymatch=LM5175# 

We want for safety, to be able to control the current output of LM5175. We are designing as suggested on  "Visio-LM5175 Variable VOUT and IOUT_General.pdf "

We would like to know the use of LMV431A in this case. Why is it needed? 

Thanks!

Cora

  • 0
    TI__Genius 14085 points

    Hello Cora,

    Thanks for using e2e.

    Just for clarification:
    The target that was discussed in the original thread was to find a way to control the output voltage and the limitation of the output current DYNAMICALLY during operation.
    The purpose of that circuitry is NOT an overcurrent protection.

    To be able to adjust both, the output voltage and the maximum current that the LM5175 is allowed to deliver to the load, two DACs shall be used which will be driven by a microcontroller.
    These DACs deliver an adjustable VOLTAGE.

    When this concept is used, it is important that the settings will be changed slowly, otherwise you would simulate a very steep change of the load.

    To influence the output voltage of the LM5175, you simply build a resistor-adder, which adds the signal from the DAC into the existing feedback voltage divider.

    In case of the current measurement and control loop, you do not want any distortion of the signal from the sense resistor, so it first gets amplified and de-coupled by the INA213.
    On the output of the opamp you will find the same resistor-adder as on the voltage feedback path, where the signal from the DAC gets added to the actual feedback path from the current sense resistor.
    Finally, to feed that signal back into the LM5175 (which expects to see a voltage drop across a sense resistor between ISNS+ and ISNS-), you need to convert it back into a current (which will be sent through this resistor).

    The LMV341A  will do this conversion.
    From a high level perspective you can look at it as a converter which turns a voltage on its input into a current on the output.
    In fact it compares the incoming voltage with a precise internal reference and turns the delta into the output current.

    Best regards,
    Harry