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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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  • \$\begingroup\$ Consider increasing the Vcc a little bit, and then using an LED driver IC like the TLC5940. You can configure individual currents per channel using the dot adjustment settings, which get saved to internal EEPROM. This answer has some detail. \$\endgroup\$ Dec 4, 2012 at 18:39
  • \$\begingroup\$ Thank you but increasing the Vcc is not an option for me because i'm getting those 3.6 volts from step-upping the 1.5V battery and poor battery is already loaded with 100 to 800mA which is top margin of alkaline's possibilities. Every additional millivolt of Vcc will drain the battery even faster. \$\endgroup\$ Dec 4, 2012 at 18:44
  • \$\begingroup\$ Just a stab in the dark, but have you considered rail-to-rail output op-amps with a current sense resistor in the feedback path? \$\endgroup\$
    – akohlsmith
    Dec 4, 2012 at 18:55
  • \$\begingroup\$ What is your budget and Volume , Low gate drive low Ron FETs exist that use charge pump boost for gate drive. Is this PWM regulated or you want analog regulated or fixed on off? Given cheap choice of battery and no voltage margin, overall requirements need clarification \$\endgroup\$ Dec 4, 2012 at 20:09

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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.

LT1932

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  • \$\begingroup\$ It even has the dimming input. I think this IC should work for me, thank you! \$\endgroup\$ Dec 5, 2012 at 7:13
  • \$\begingroup\$ Just a note: i have chosen the NCP5006 for my application. \$\endgroup\$ Dec 6, 2012 at 11:52

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