Electronic – 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?


simulate this circuit – Schematic created using CircuitLab

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?

Best Answer

You can use this idea: -

enter image description here

The voltage across R1 sets the voltage that is forced across R2 by the op-amp's negative feedback. Let's say that voltage is 0.1 volts and let's say R2 is 1 ohm. This means that when 0.1 volts is forced across R2 there can only be 100mA flowing through it.

99%+ of that current flows through the load so it's basically a constant current generator. Less than 1% flows into the output of the op-amp to drive the base.

Sounds simple but you'll need a rail-to-rail op-amp with inputs capable of getting to within 100mV of the positive rail. Not too tricky of course. The voltage across R1 being set to 0.1V won't be stable if the V+ rail moves about so some kind of positive supply referenced shunt regulator can be used and this can be further potted down to provide 0.1 volts across R1.

Saturation of the PNP might be 100mV so overall, this design could be expected to "drop" about 200mV. It's a circuit I use a lot for the excitation of strain gauges.