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.
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?
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.
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