MicroController control for switching regulator

On 1/2/2013 3:51 AM, RogerN wrote:
> "Tim Wescott"  wrote in message
> news:ZO-dnalKjYHz2kbNnZ2dnUVZ_vOdnZ2d@web-ster.com...
>
>> On Tue, 25 Dec 2012 06:05:02 -0600, RogerN wrote:
>>
>>> I'm wanting to use a microcontroller to control the output of a
>>> switching regulator, maybe something like a LM2576 simple switcher.  I
>>> see the adjustability is on the feedback lead but what's the best way
>>> make it adjustable from a microcontroller.  I'm guessing the feedback
>>> may be used in switching timing, ie, I guess I can't just use a DAC and
>>> get an output of 1.23V more than the DAC output?  I want to monitor
>>> voltage and current and use a control algorithm in the microcontroller
>>> (Arduino at first for experimentation) for custom battery charging
>>> experimentation.
>>
>> I don't know that anyone actually answered the question that you asked.
>> You got a lot of good responses -- personally, if I were doing this I'd
>> see if the microprocessor had a suitable PWM output and think about using
>> that.
>>
> <snip>
>> --
>> Tim Wescott
>> Control system and signal processing consulting
>> www.wescottdesign.com
>
>
> I'm wondering about another idea for the control.  What if I used a resistor
> from the output to the feedback pin, and then a NPN transistor from the
> feedback pin to ground.  A DAC would feed the base through the base
> resistor, it would use the transistors current gain to control the current
> through the resistor, more current would require move voltage drop through
> the resistor and increase the output voltage.  Does that sound like it would
> work?
>
> RogerN
>
>
Sure, that works just fine as long as you want an output voltage greater 
than the reference voltage.

You'd get what you asked for...but probably not what you wanted.

The current gain of the transistor varies greatly.  You'd have to embed it
in a circuit that sought to eliminate the dependency on the actual gain
of the transistor...and the temperature characteristics of the junction 
voltages...and straightening out the nonlinearities...and...and

"mike"  wrote in message news:kc1kgl$8n4$1@dont-email.me...

>On 1/2/2013 3:51 AM, RogerN wrote:
<snip>> I'm wondering about another idea for the control.  What if I used a 
resistor
>> from the output to the feedback pin, and then a NPN transistor from the
>> feedback pin to ground.  A DAC would feed the base through the base
>> resistor, it would use the transistors current gain to control the 
>> current
>> through the resistor, more current would require move voltage drop 
>> through
>> the resistor and increase the output voltage.  Does that sound like it 
>> would
>> work?
>>
>> RogerN
>>
>>
>Sure, that works just fine as long as you want an output voltage greater 
>than the reference voltage.
>
>You'd get what you asked for...but probably not what you wanted.
>
>The current gain of the transistor varies greatly.  You'd have to embed it
>in a circuit that sought to eliminate the dependency on the actual gain
>of the transistor...and the temperature characteristics of the junction 
>voltages...and straightening out the nonlinearities...and...and

That would probably be OK for my battery charging application, the whole 
process is measure voltage & current then adjust the output until charge 
termination.  But it may be better to use some sort of rail-to-rail Op-Amp 
so the feedback controls the voltage instead of the transistors drifting 
current gain.

RogerN

Algún esquema de control con pic el lm2576?