Improving current limiting with LT3780,season 2


Now that' i've been using my LT3780 power supply for some times, some limitations are showing.
In particular :
* Cannot set max current to less than ~ 100 mA
* Resets if actual current is way above limit , when limit is low
* Linear pot, so painful to set low values

As far as requirements are concerned, i aim at max ~4A

First things first, let's have a closer look at the first stage of our current sensing measure :

It is a non inverting  opAmp amplifier , that we'll simplify here that way.


It is done on a breadboard using cheap LM358 from ebay (~1$ for 20)

Let's drive it from 0 to 14 mV, representing a current sensing from 0 to 2 A
The result is as follows, output voltage vs input voltage in mv:



We can see the  input voltage offset, it should be ~  2mV according to the datasheet.
It is only 0.74 mV here.

That graph  can be approximated as

VOut = (Iin*7mOhm-0.74mv)*153  (value <0 are grounded)

That can also be expressed  as :

VOut=IIn-113mV  (value <0 are grounded)

Gotcha, that's the ~ 100 mA minimum we were looking for.

So, if we add  an output offset of ~ 113mV, we are back in business.

Let's have a look at TI nice application note

We already have the multiplier part

* Rf= 1.5 MOhm
* Rg = 100kOhm

Let's assume R1 and R2 are very small compared to those
The offset becomes

B=5v*150*R2/R1=113.10-3

So 

R2=6500*R1

And we end up with a rough computation :

* R1 = 10 Ohm
* R2=  65 kOhm  (still small compared to Rf)

R2 will be an adjustable multiturn pot of 200k, to account for rounding errors and component accuracy.





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