On the Current Source for Battery Charger section in the November '08 Q&A column, I got feedback that caused me to think about how to make a precision current source. My solution was to use a rail-to-rail (input and output) op-amp and precision voltage reference to directly regulate the current through a resistor; see Figure A. The LM6492 inputs can go 0.3 volts above and below the power rails; the LM4040 is available in 0.1% to 1% tolerance and 2.048 volts to 10 volts. The minimum current through the LM4040 is 80 microamps which sets the minimum output voltage at 80 millivolts when R3 is 1K. R3 could be larger if a higher minimum output voltage could be tolerated.
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