4 Setting Output Voltage

Figure 4-1. Constant Output Buck Converter Network

Figure 4-1 shows the complete feedback network needed. As the capacitors block DC voltages, the feedback resistors R_FBT and R_FBB will set the feedback voltage, V_FB, by attenuating the buck converter output voltage, V_out. Knowing this, the V_out of the buck can be set. The resulting feedback voltage, V_FB, of this voltage division will equal the internal reference voltage V_ref when V_out is at the desired level. The resulting relation between the resistors and the reference voltage is given by:


$V_{o u t} = V_{r e f} \cdot \frac{R_{F B T} + R_{F B B}}{R_{F B B}}$ $\Rightarrow R_{F B T} = R_{F B B} (\frac{V_{o u t}}{V_{r e f}} - 1)$

This feedback controller uses the internal DAC as the reference voltage, meaning that the output voltage can be adjusted in software. It does, however, not allow for very large adjustments, as the amplifier might have undesired characteristics closer to its supply voltage.

Figure 4-2. Variable Output Buck Converter Network With Variable Resistor

For applications using an adjustable output regulator, the bottom resistor R_FBB is replaced by a variable resistor, such as a potentiometer, as seen in Figure 4-2. This is important as the top resistor R_FBT is a part of the phase compensation network in the error amplifier, while R_FBB does not affect the poles of the system.

Note: Typical values for these resistors fall in the range of 20-200 kΩ. Higher values than this can affect the stability of the op amp, while lower values will draw unnecessary amounts of current, resulting in lower efficiency.

AN3725

4 Setting Output Voltage