Electronics > Beginners
Linear regulator control circuit feedback
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obuone:
Hello everybody.
I am designing my first power supply for a while now. In the design I am trying to adopt the control circuit form Dave's uSupply but I am using different Linear regulator with 1.22V reference voltage. So to be able to go down to 0V it have to driven negative which complicates things.
What are the drawbacks when having a feedback from the output (divided by 10) and directly comparing it with the set from DAC voltage. Is this configuration less stable than just injecting voltage in the set pin from the DAC with times 10 gain instead.

Thank you.
Kleinstein:
Using the extra OP in the feedback path makes stability of the control much more difficult. The extra OP is now part or even the main part of the control loop, the regulator is more or less reduced to a kind of protected transistor with no so well known AC properties and maybe relatively poor speed. So in this case using the regulator is making the control loop more difficult than using a bare transistor instead. The main advantage is that the regulator internal over-temperature protection may be still in effect - if it still works with the reference pin higher than the output.

Just adding a voltage to the reference pin is not effecting the regulator control loop very much (only the slight effect of the impedance added). So the control would be still inside the regulator chip.
obuone:
I ran few simulations and at certain loads the circuit with the output feedback was oscillating, adding something like 40-50mV ripple at the output. Thought it is just an erroneous effect from the OPamp model, but your explanation makes much more sense, thanks  :)
David Hess:
Taking feedback from the output eliminates all errors contributed by the regulator.  If you are designing for low noise or low drift, then this is the way to do it.

However like Kleinstein says, taking feedback from the output places the regulator within the control loop complicating frequency compensation.  Personally I have never had problems doing this but the frequency compensation should be verified empirically which only requires a pulse or function generator and oscilloscope.  If you do the frequency compensation this way, then it is possible to work back and derive the AC characteristics of the integrated regulator.
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