I came across this application note at OnSemi for a current regulated LED driver circuit. That's all well and good- but I'm not exactly sure how to apply the second formula listed in the document. Any ideas? The first one is pretty straightforward - however I want to know how to calculate Rsense and R4 from the second diagram. It is not clear from the formula which R value is being referred to - and I wasn't sure which Vsat value should be used from the MPS2222 data sheet. There are Vsat listed for Collector-Emitter, and Base-Emitter. Thanks!

These are some pics pulled from the link above - for a quick idea of what I'm talking about. I'd suggest you read the PDF also- there is explanatory text with the diagrams.





Here are links to the datahsheet PDFs of both the LM317 and the MPS2222
LM317
MPS2222

This is my interpretation:

Vsat is the saturation voltage drop across the collector and emitter of the transistor. For a MPS2222 at 150ma it will be 0.4v (taken from a data sheet)

So the equation becomes (1.25+0.4)/Rsense=IOUT per leg

Rsense will have 1.25v+Vsat=1.65V (different to what they say in the text) across it and the base-emitter junction will generally drop 0.7v. So that leaves 0.95v to be dissipated by R4. For 150ma to flow in the collector (saturated), 15ma needs to flow though the base (HFE = 100). To find R4 it will be 0.95/0.015=63ohms

I’m a little rusty on transistors so there is most probably some errors in there some ware. Easiest way would be to set up one leg and take some measurements

DJ

Daz - Thanks for the clarification. Next round of questions...

The saturation voltage drop - does that assume that 150ma is flowing to the LED string? As I understand it, the transistor is acting as a switch - which means it needs to be saturated so that current can flow from collector to emitter. The current flowing through each LED is the same - and the standard for most LEDs is 20ma. Does this mean that the transistor will require 150ma, and the load of LEDs only 20ma?
If each Rsense resistor is different, will differing amounts of current flow through each leg? Intuitively it doesn't seem that one regulator would supply different currents from 1 output. I do realize that intuition probably is a poor tool for understanding electronics...

The saturation voltage drop will vary with differing loads. The data sheet only list two values, 150ma and 500ma, so I just chose the lower value. The main thing is that it is saturated so it doesn’t waste too much power. I’m sure there is a way to calculate the actual current flow though the transistor but I’ve forgotten how to do that long ago .

The voltage drop across Rsense is what regulates the current flow so if one leg is removed the regulator backs off the total current to the rest of the branches. The regulator chip doesn’t acually monitor the current or resistance, it monitors the volage across Rsense. So each leg could have different resitsors and current flow. As long as Rsense is satified, all is well.

DJ