I was thinking of avoiding an external compensation and let the internal frequency compensation of LM2576 do the job - since the adder was with the unity gain, the frequency response will not get altered within the bandwidth of interest. Will such a thing affect the stability or the response time of the converter? Of course, since the Vref pin is not available directly, there could be variations arising out of a matching Vref under varying operating conditions.

 

I am not sure I see how you can add a unity gain amp and get the thing to do what you want, which is have the output regulate to 1/2 input voltage.

If there's a way, please post it. I'd like to learn how.

 

I have attached the schematic here. Sorry about the circuit being not neat. Hope I have got this right as crutschow pointed out!
The idea is to input the feedback pin with Vref when the output is what we wanted, and higher if the output is higher and vice versa. Since this stage is only an adder with a gain of 0 db and 0 phase shift, well almost, atleast till well beyond the crossover frequency, I thought it should not affect the stability. Would that be alright?
View attachment Buck Feedback.asc

 

file won't open on this computer

 

file won't open on this computer

You need to have LTspice installed on the computer.

 

You need to have LTspice installed on the computer.

I'd be afraid to. I'm pretty sure Bob Pease would come back and haunt me......

 

I have attached the schematic here. Sorry about the circuit being not neat. Hope I have got this right as crutschow pointed out!
The idea is to input the feedback pin with Vref when the output is what we wanted, and higher if the output is higher and vice versa. Since this stage is only an adder with a gain of 0 db and 0 phase shift, well almost, atleast till well beyond the crossover frequency, I thought it should not affect the stability. Would that be alright?
View attachment 47075

Looks OK to me.

 

I'd be afraid to. I'm pretty sure Bob Pease would come back and haunt me......

and beat you up with his slide rule. After all, real engineers don't use computers....

 

file won't open on this computer

I am attaching the schematic in gif format here

 

and beat you up with his slide rule. After all, real engineers don't use computers....

We use them. We just have to hold our nose when we do.

 

I am attaching the schematic in gif format here
View attachment 47084

What is Vref?

 

What is Vref?

Vref is the reference voltage of LM2576. Since this voltage is not available at an external Pin, we need to give it from an external supply to match it.