Adjustable current sources often exhibit a non-linear control characteristic, but the inclusion of bootstrap feedback provides an inherently linear control that works well at all levels of output current. Fitting an op amp into the control loop allows the output current to be adjusted while keeping the control potentiometer outside the main current path, thereby enabling use of a high-impedance, low-drift potentiometer. The circuit shown in Fig. 1 provides an output-current range of 0 A to 1.25 A, linearly controlled by potentiometer RPOT.
Note that the regulator's ADJ terminal connects to the op-amp output, and that RPOT bridges that terminal to the regulator's OUT terminal. Thus, VPOT = VOUT - VADJ. Also, because the op amp is configured for negative feedback (VIN+ = VIN-), therefore:
VIN+ = (IOUT) × (RLOAD)
VIN- = (VIN+)+(IOUT) × (RSENSE) - (VPOT) × (X),
where X = (RX/RPOT). In this design, VPOT = 1.25 V and RSENSE = 10 Ω. RMIN ensures the minimum load current needed for regulation, which is 5 mA over the operating temperature range. To obtain this value, the parallel combination of RPOT and RMIN must be 1.25 V/5 mA, or 220 Ω. For a 1-kΩ RPOT, RMIN should be 330 Ω. The op amp is guaranteed to sink at least 5 mA.
The operating range is bounded at the maximum load resistance by the regulator's dropout voltage, and at the minimum load resistance by its power-dissipation limit. The regulator shown, for example, reaches its thermal limit for the conditions of VIN = 30 V, a 1.25-A output current, and a load resistance below 7 Ω. Lowering the supply voltage below 30 V allows the circuit to operate with loads down to 0 Ω. The linearity and load-regulation performance of this circuit are shown in Fig. 2.