In a standard dc-dc converter, a resistor divider typically defines a fixed output voltage. However, applications like programmable output voltage power supplies and motor control circuits require dynamic control of the dc-dc converter's output voltage. The circuit described here allows control of the converter's output voltage, VOUT, with a control voltage, VC.
In a conventional dc-dc buck converter, VOUT is:
so VOUT is fixed by the values of R1 and R2 (Fig. 1).
|Figure 1.||The output voltage in a conventional dc-dc buck converter is fixed and depends on the resistor
The added circuitry in Figure 2 enables users to control the same dc-dc converter's output voltage using VC.
|Figure 2.||The added circuitry in this version of the DC-DC converter permits control of VOUT by varying
a control voltage, VC.
In this case, R2 is not connected to the ground but, rather, to VR. Equation 1 then becomes:
Since R1 = 20 kΩ and R2 = 10 kΩ, Equation 2 can be simplified to:
R3 and R4 have the same value, 10 kΩ, so amplifier U2's output voltage is:
where VREF is the reference voltage generated by U3 after resistor divider R7/R8.
Combining Equation 4 and Equation 5:
To simplify Equation 6, choose components that make:
Then Equation 6 becomes: