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?

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

that regulator has low drop out, but it requires a supply voltage for the internal circuitry (not for the load) thats a 1.5V more than the output voltage. configure it as a current sink.

  • 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
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  • \$\begingroup\$ Is there a equation to calculate different Iload values? \$\endgroup\$ – rodeomusty Nov 26 '15 at 14:59
  • \$\begingroup\$ Ihe formula is something like r1/r2*10uA (but I simulated it in ltspice which is a free download) \$\endgroup\$ – Jasen Nov 27 '15 at 19:36

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