# Design dc-dc converter for adjustable voltage

I would like to use the LM61460 (datasheet) to build a buck converter with a Vin of 24v-33v and an adjustable output from 5v at 3a to 20v at 5a for USB power delivery. I used TI Webench to generate a design for both 5v 3a and 20v 5a.

These are the biggest differences:

• Value of the inductor
• Rff and Cff aren’t present in the higher power design
• BIAS is connected to Vout at lower voltage and to gnd at higher voltage

For the inductor, I would pick the biggest value. And I would connect the BIAS pin to gnd, since the datasheets states to ‘connect to output voltage point to improve efficiency’, so it should still work when connected to gnd.

Is this correct and what should I do with Rff and Cff?

• Cff / Rff provide phase margin when using low esr ceramics (specifically they introduce an output zero), but you have this interesting conundrum to solve: If output voltage is less than 2.5 V, Cff has little effect so can be omitted. If output voltage is greater than 14 V, Cff must not be used since it will introduce too much gain at higher frequencies. So you really should be using them up to about 14V output and not above that. Jan 7, 2020 at 10:51
• How about using a varactor diode in place of Cff so that capacitance drops with bigger output voltages? Jan 7, 2020 at 10:55
• @Andyaka Wow. That's just inspired - I would never have thought of that. Sep 30, 2021 at 12:32