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Variable reference voltage for SMPS error amplifier

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treez

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Hello
The attached schematic varies the reference voltage shown from 0V to 1v5. (In the actual circuit it will be the variable reference to the error amplifier of an SMPS..thus giving a soft start by slowly incrementing the reference node voltage).

Do you have any “less component count” ways of doing this?
I need to keep the LT1634 in there. –Because in oe circui the cctry that makes the reference variable will be “Not fitted”.
Pdf schem and LTspice sim attached.
 

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  • Variable reference.pdf
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  • variable reference.txt
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Flip the opamp (-) and (+) pins and ditch the PNP stuff. I'd replace R8/C4/R10 with a single series R between the opamp and the NPN base as your single point of gain control. Also, add an integrating cap between opamp out and the inverting pin to prevent stability problems.

Or, go with a rail to rail output opamp and let it directly pull down your reference with a schottky, though that won't allow full 0V operation and doesn't really save parts.

There are other options in general, such as biasing your feedback pin up rather than biasing your reference down. Or add a large C on the reference and rely on the RC time constant to 'ramp up' (use the NPN in an ON-OFF fashion to reset the C).
 
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