Bench power supply revisited: Digital Control


The usual way to control a DC/DC power supply is to send a fraction of the output voltage to the feedback pin. The fraction-ing is done using a potentiometer i.e. a resistance divider.



The DC/DC converter will adapt the output voltage  so that the feedback pin reaches a constant value (typically 0.8v or 1.2v)

If Vo*Divider < 1.2 v, the DC/DC will increase Vout
If Vo*Divider > 1.2 v, the DC/DCDC will decrease Vout


Nice ans simple , but two caveats :

  • You cannot have Vo < Feedback, so not possible to do 0.5v for example
  • It cannot be controlled digitally. Using a digital potentiometer is not really an option. They have strong limitations regarding voltage and resistance.

So instead, what we will do is directly drive the feedback pin in a slightly more complicated way.

To do that, we add a difference amplifier to compare 1/11th of Vout to a voltage reference (Vref).
That voltage Vref will be set digitally.




Same story as before, if Vout/11 < Vref, the Feedback pin will be < 1.2v and the DCDC will raise Vout.

So with a 0--3.3 v Vref, we can have ~0---35 v Vout.


It is important to note that the comparator must be  amplifying, i.e. we get N*(Vout/11-Vref)
If we dont it that way, it will oscillate between 0 and MaxVout

There will be a slight offset between Vref and Vout/11 , i.e. Feedback/Gain
So the gain must be high enough so that the offset is a few mV.

Additionally, we will use the same scheme to add  current limiting.
We add a shunt resistor to get a voltage proportional to the current, then compare it to Iref.
If I > Iref, the FB pin will go higher than 1.2 v and Vout will go  down.


Both Vref and Iref will be driven by cheap DACs.

Bingo, we have a generic digitally driver  DC/DC controller with current limiting.




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