The circuit in Figure 1 delivers programming voltages to an EEPROM under the control of an external DAC (not shown). You can replace the DAC with a potentiometer to create a general-purpose power supply operating from 12V and able to deliver a variable output voltage of 0 to 32V. As Figure 1 shows, a Linear Technology LT1072HV variable-boost switching regulator, IC₁, drives a Class A amplifier comprising operational amplifier IC₂, voltage-boost-stage Q₃, and emitter-follower Darlington transistor Q₂. Resistors R₉ and R₁₀ set the amplifier‘s noninverting loop gain to a value of 1+(R₉/R₁₀).

For output voltages below 8V, switching regulator IC₁ remains in shutdown mode, and the output stage draws current through L₁ and D₁. Q₁‘s collector voltage, V_C, measures approximately 11.4V—that is, 12V minus D₁‘s forward-voltage drop. Transistor Q₁monitors the voltage drop across R₇, which measures a fraction of Q₂‘s collector-base voltage, V_CB. As long as V_CB exceeds 1V, Q₁‘s collector current remains high enough to drive IC₁‘s feedback input higher than 1.25V, which in turn keeps IC₁ shut down.

As the output voltage increases, the voltage differential across R₇ decreases, and, when it drops below 0.9V, Q₁‘s collector current decreases, lowering the feedback voltage applied to IC₁ and switching it on. The boost regulator‘s output voltage increases, and the Q₁- IC₁ feedback loop regulates the collector-emitter voltage differential across Q₂ to a constant 3V for all outputs exceeding 8V. If IC₂‘s output goes to ground, cutting off Q₃ and forcing Q₂ into saturation, the feedback loop around Q₁ opens and allows the circuit‘s output voltage to increase. Diode D₅ and associated components form an overvoltage-protection clamp that limits IC₁‘s output to 37V.

Resistive divider R₉ and R₁₀ and IC₂ determine the output voltage‘s range. Apart from selecting the V_CE ratings of Q₁ and Q₃ to withstand the highest desired output voltage, values of other components are not critical. If you substitute appropriate components for D₅, Q₁, and Q₃, the circuit can deliver output voltages as high as IC₁‘s maximum output-switch rating—75V for the LT1072HV variant—minus 3V.

Wide-range voltage regulator automatically selects operating mode