# 3V Current source with low voltage drop

I'm trying to make a IR laser diode driver which needs 1.9V and ~80-100 mA constant current. Most of laser driver circuits in web has LM317 as a current source but that design has always voltage drop around 2.5V. Since I have 3V supply, i'm trying to find an alternative way with low drop-out voltage. I have found LT3085 (Datasheet) which says 275mV dropout voltage.

In datasheet's typical application section there is a circuit for Current Source but Vin=10V. Since LT3085 has "Wide Input Voltage Range 1.2V to 36V". How can I use with 3V supply?

• By reading page 7 of the datasheet – PlasmaHH Nov 26 '15 at 9:23
• Should work with Vin > 1.9V [vout] + 0.275V [dropout voltage] + 0.1 * 2 [volts across 2R] – Icy Nov 26 '15 at 9:25
• @PlamsaHH or page 19 – Jasen Nov 26 '15 at 11:27

• connect the load between +3V and in
• connect Vcontrol to +3V
• connect set to 31K resistor to ground
• connect out to 3.3 ohms resistor to ground