# Voltage Controlled Voltage Source

This circuit uses a single voltage regulator, but allows selection between two different output voltage supplies. A microcontroller can control the Vc input to the circuit, and pull it low for a lower output voltage, or a high for a second higher voltage.

**Selectable Voltage Source Schematic**

**Circuit Analysis**

When the control voltage VC is set low to ground, the circuit looks like a traditional voltage regulator circuit with R4 in parallel with R3 and in series with R2:

R_{B}= (R2+R3//R4)= R2+R2*R4/(R2+R4),

and the output voltage is governed by the typical voltage regulator equations:

V1= R1*Vp_{L} /(R1+RB)

R_{B}=R1(Vp_{L}-V1)/V1

However, the analysis becomes more complex with VC set to a non-zero voltage:

Vp_{H}=
V1+V1*R2/R1+Vx

We have a multitude of free parameters so we can just set R3=R2.

Through superposition:

Vx= V1*(R2*R4)/(R2+R4)/R1 +R2*Vc/(R2+R4)

Thus:

Vp_{H}= V1+V1*R2/R1+Vx+V1*(R2*R4)/(R2+R4)/R1 +R2*Vc/(R2+R4)

Vp_{L}=V1+V1*R2/R1+Vx+V1*(R2*R4)/(R2+R4)/R1

Combining the two equations:

Vp_{H}= Vp_{L}+R2*Vc/(R2+R4)

R4= R2*(Vc-(Vp_{H}-Vp_{L}))/(Vp_{H}-Vp_{L})

Let A= (Vc-(Vp_{H}-Vp_{L}))/(Vp_{H}-Vp_{L}),

so R4= R2*A.

R_{B}= R2+A*R2^2/(R2+R2A)

R2= R_{B}(1+A)/(1+2*A)

So the design
methodology is to first design a standard 2 resistor voltage regulator,
with 2 resistors. Then split the grounded resistor R_{B} into three
resistors, two in parallel with one in series. Then use the
equations above to find R2, and then R4.

Note that V1 is usually around 1.25V.