LTC3780 Season 3

 Quick summary of the previous seasons :

Season 1 was about adding current limiting to LTC3780 + arduino : Sort of ok

Season 2 was about having full digital external control using stm32 + DAcs by redoing the feedback loop completely.

 It was more or less working but was very prone to noise i.e. unusable. 

(main reason is the LM358 used cannot sink large amount of current => big resistances => noise sensisitve)


Season 3 is a hybrid.

Let's see what we want :

1- Use digital potentiometers to control voltage & current

2- Make sure it does not blow up when tinkering with it. that means the "unconnected/reset" state should go to a safe output voltage and not crank it up to the max ( i 've blown a couple of transistors that way).

3- Have the control part very close to the DC/DC to limit noise

The digital pots i have are 10k & 100K ohm BUT they can only conduct 1mA **MAX**

The default LTC3780 configuration is as follows :


FB is 0.8, so the max output voltage is 0.8*(1+100/3.2)=~ 25 V
We'll change it to the following :



(R1 is a 100k pot + 10k pot, so it's 110k)

Why so complicated ?

1- If the pot is disconnected, Vout = 2x FB =~ 1.6v, so the DC/DC will not blow.
The schottky  is a placeholder for when current limiting will be implemented.

2- What about max current ? It's well below Vout/100k
So with VoutMax=30v, iMax=0.3mA, well within the 1mA margin (the actual number is 200 uA)

3- it's better to have the 2 inputs of the digital pot within it's supply voltage i.e. 5v. So they should be on the low side.

The main problem is the curve is behaving like (1/(a+b)) where b is the potentiometer value.
It looks like that :

Quick test :
* nothing connected : ~ 1.6v , ~ 2x FB, as expected since Rup=Rdown)
* 6.8k connected : 13.5 v, in theory it should be 16, some calibration to do later on



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