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Current Mirror Feedback Challenge

Patrick Hendriks
Prodigy 40 points

I am designing a non isolated converter with a LM5027. As this unit is meant for isolated feedback by means of an optocoupler the comp input is designed as a current mirror with the opto transistor as high side configuration. I want to use a normal opamp error amplifier, but driving this comp pin directly (via a resistor) from the error amplifier would give me inverted error signal. The obvious solution to me would be to attach a PNP current mirror to invert the error signal. Is there an easier way? Eg an non inverting error amplifier? And how would the equations for a type 3 compensation network look like for such a configuration? (K method) I havent been able to figure this out.

  • 0
    TI__Expert 3545 points

    Patrick,

    Using two OPAMPs you can configure a non-inverting error amplifier,8311.evalsch.pdf, see the attached schematic.  The first stage is the feedback amplifier, you would calculated the K factor as you normal would, the second stage inverts the signal.  You still need to create a V-I for the COMP input, I'm not sure this is easier, just giving you another option.

    Terry