Thursday 22 September 2016

lm317 - High Side Constant Current Source


I am trying to create a constant current source of 100mA to charge a single cell Li-ion battery. I see the LM317 thrown around a lot as a solution, but I'm looking for something that does not require a 1.2V drop (Due to the Vref=1.2V for a LM317). The problem is when I look at more modern LDOs, they tend to have a ground as well as a ADJ pin, and I'm wondering if these can be floated the same way the traditional LM317 regulators could. Once such device I was looking at was a ADP123AU due to it having a Vref of 0.5V.


enter image description here


So looking at the block diagram they do not mention what the 0.5V is ref to but it would have to be ground as far as I can understand, so if you float ground as shown, would it work? Or does anyone know another small regulator / specialty IC that can do this?


schematic



simulate this circuit – Schematic created using CircuitLab


EDIT: Thanks for the input, it is interesting but I may not have been clear in the initial question. I am looking to do this as simple as possible, something such as the LM317 with a single resistor. Multiple op-amps and transistors is something I would like to avoid.


Does anyone have an opinion on if what I proposed in my question would work?



Answer




So, I downloaded LTSpice, and found an appropriate LDO Regulator made my LT that met my specifications of a low dropout and low Vref. I created the example circuit from above and simulated it.



After testing the circuit shown below with both a small Rload and large Rload (full and empty battery) I discovered that 'any' adjustable regulator even with a ground can be floated as the old LM317 could. I suspect that this is due to the small Quiescent current that can be sunk by just about any load.


The image shows a 20mA current source that is comprised of 2 components, a LDO and resistor.


enter image description here



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