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, V_{OUT}, with a control voltage, V_{C}.

In a conventional dc-dc buck converter, V_{OUT} is:

(1) |

so V_{OUT} 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 resistordivider, R1/R2. |

The added circuitry in Figure 2 enables users to control the same dc-dc converter's output voltage using V_{C}.

Figure 2. |
The added circuitry in this version of the DC-DC converter permits control of V_{OUT} by varyinga control voltage, V _{C}. |

In this case, R2 is not connected to the ground but, rather, to V_{R}. Equation 1 then becomes:

(2) |

Since R1 = 20 kΩ and R2 = 10 kΩ, Equation 2 can be simplified to:

(3) |

or:

(4) |

R3 and R4 have the same value, 10 kΩ, so amplifier U2's output voltage is:

(5) |

where V_{REF} is the reference voltage generated by U3 after resistor divider R7/R8.

Combining Equation 4 and Equation 5:

(6) |

To simplify Equation 6, choose components that make:

(7) |

Then Equation 6 becomes:

(8) |