LT1072 -- Wide-range voltage regulator automatically selects operating mode

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, IC,

drives a Class A amplifier comprising operational amplifier IC,

voltage-boost-stage Q, and emitter-follower Darlington transistor Q.

Resistors R and R set the amplifier's noninverting loop gain to a value of 1+(R/R).

For output voltages below 8V, switching regulator IC remains in shutdown mode,

and the output stage draws current through L and D.

Q's collector voltage, V, measures approximately 11.4V—that is, 12V minus D's forward-voltage drop.

Transistor Qmonitors the voltage drop across R, which measures a fraction of Q's collector-base voltage, V.

As long as V exceeds 1V, Q's collector current remains high enough to drive IC's feedback input higher than 1.25V, which in turn keeps IC shut down.

As the output voltage increases, the voltage differential across R decreases,

and, when it drops below 0.9V, Q's collector current decreases,

lowering the feedback voltage applied to IC and switching it on.

The boost regulator's output voltage increases, and the Q- IC feedback loop regulates

the collector-emitter voltage differential across Q to a constant 3V for all outputs exceeding 8V.

If IC's output goes to ground, cutting off Q and forcing Q into saturation,

the feedback loop around Q opens and allows the circuit's output voltage to increase.

Diode D and associated components form an overvoltage-protection clamp that limits IC's output to 37V.

Resistive divider R and R and IC determine the output voltage's range.

A part from selecting the V ratings of Q and Q to withstand the highest desired output voltage,

values of other components are not critical.

If you substitute appropriate components for D, Q, and Q, the circuit can deliver output voltages

as high as IC's maximum output-switch rating—75V for the LT1072HV variant—minus 3V.

附图所示电路可在外部DAC(未示出)控制下为EEPROM提供编程电压。还可以用一个电位器来代替该DAc．以建立从12v电源上工作．并能提供0v～32v可变输出电压的通用电源。凌特科技公司(Linear Technology)的LT1072HV型可变升压开关稳压器IC1，驱动一个由运放IC2、升压级Q3及发射极跟随器达林顿晶体管Q2组成的A类放大器。电阻器R9和R10将放大器的正相环路增益设定为1+(R9／R10)。

    当输出电压低于8V时．开关稳压器IC1保持在关机模式．输出级通过L1及D1拽取电流。Q1的集电极电压Vc测得近似为11.4v(即12v减去D1的正向压降)。晶体管Q1监视R7两端的压降，它只测量Q2的一部分集电极-基极电压Vce.一旦Vcb超过1v，Q1的集电极电流保持高得足以使Ic1的反馈输入高于1.25v．该电压反过来又使IC1关机。随着输出电压的增加．R7两端的电压差减小．并且当电压差降至低于0.9v时，Q1的集电极电流减小，从而使馈给IC1的反馈电压降低．并使其导通。若升压稳压器的输出电压增大．对于超过8V的所有输出．Q1至IC1反馈环路将Q2两端的集电极一发射极电压差调整为恒定的3V。如果将IC2输出接地，则Q3关断并迫使Q2进入饱和，Q1周围的反馈环路开路．并允许电路的输出电压增大。二极管D5及相关元件构成一个将IC1输出限制为37v的过压保护箝位电路。

    电阻分压器R9和R10以及IC2确定输出电压的范围。除选择Q1及Q3的Vce额定值以承受所需的最高输出电压外，其他元件值并不重要。如果将D5、Q1及Q3替换成其他适当的元件．则电路可提供与IC1最大输出开关额定电压(对于LT1072HV变体产品为75V)一样高的输出电压。