MAX507

The variable-current source generates 0 to 5 A with a compliance range of 4 to 30 V. The 12-bit digital-to-analog converter (DAC), IC2, makes it digitally programmable. The switch-mode step-down regulator, IC1, is more efficient than the alternative current source with linear pass transistor.IC3 is a high-side current-sense amplifier. In this circuit, it senses output current as a voltage drop across R5, and produces a proportional signal current at pin 8. The regulator's feedback voltage (IC1 at pin 1) is set by the DAC and modified by IC3's current feedback, which flows across a parallel combination of R2 and R3. This current feedback opposes any change in load current because of a change in load resistance. The DAC generates 0 to 10 V, producing a source current that varies inversely with code: FFF_HEX (10 V from IC2) produces 0mA, and 000_HEX (0V from IC2) produces 5 A. The circuit can be reconfigured for other ranges of output current (I_source) by resizing R₂ and R₃:

            I_source=2217[V_FB(R₂+R₃)-(R₃V_DAC)]/R₂R₃

where V_FB 2.21 V and V_DAC can range from 0 to 10 V.

The desired range for I_source defines values for R₂ and R₃:V_DAC=10V for the low value of I_source and V_DAC=0V for the high value of I_source. Substituting these two sets of values in the equation yields two equations to be solved simultaneously for the values of R₂ and R₃.