Wide-range Voltage regulator automatically selects operating mode

the circuit in figure 1 delivers programming voltages to a 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 deliv ER a variable output voltage of 0 to 32V. As figure 1 shows, a Linear technology LT1072HV variable-boost switching regulator, IC_{1, 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, VC, 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, VCB. As long as Vcb 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 Dr OPS 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 a 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 VCE ratings of Q1 and Q3 to withstand the highest desired output voltage, values of other Compone NTS is not critical. If you substitute appropriate-D5, Q1, and Q3, the circuit can deliver output voltages as high as IC1 ' s maxi Mum Output-switch rating-75v for the LT1072HV variant-minus 3V.

Wide-range Voltage regulator automatically selects operating mode