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, 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.

A part 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.

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。