I'm designing a device with 3.6V Vcc and RGB LED. Blue part of an LED has the voltage drop in the range of 3.2..3.6V and i have to drive it with current limited to 30..50mA. This gives a restriction to the current limiters choice. I fear that a simple resistor or BJT current source will have a significant voltage drop that will disable my blue LED. I'm looking at the low Vce(sat) transistors for BJT current source but haven't got them yet (and it seems difficult to me), so i maybe need a MOSFET+inductor current source schematic. Is this possible?
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You could probably make a constant current source with a fet and cheap opamp, but there are cheap ICs (note the example is not, just one I picked out from here) that will do what you need, like the LT1932 (datasheet) which will work from 1V to 10V input (I selected a low min voltage version due to your battery - but of course you can run from the 3.6V stepped up rail), and supply up to 40mA constant current.