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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.

5v 3a 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?

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  • \$\begingroup\$ 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. \$\endgroup\$ Jan 7, 2020 at 10:51
  • \$\begingroup\$ How about using a varactor diode in place of Cff so that capacitance drops with bigger output voltages? \$\endgroup\$
    – Andy aka
    Jan 7, 2020 at 10:55
  • \$\begingroup\$ @Andyaka Wow. That's just inspired - I would never have thought of that. \$\endgroup\$ Sep 30, 2021 at 12:32

1 Answer 1

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The datasheet recommends the design number 1 with the bias connected to Vout. However, this is not a typical USB PD, as a single design should be able to deliver 5V/9V/15V/20V up to 3A with a communication protocol to communicate with your load to define which voltage is needed. It looks like to me as 2 different design as you will need physical modifications for the 5V and the 20V.

For the Inductor value, it depends on the footprint, if it's the same, no matter, you can decide the values later, but bigger means less ripple however higher Rdc and price obviously => trade off

Concerning Rff and Cff, this might impact your compensation, you might want to place the footprint in your layout, if you don't need it then keep it NC.

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  • \$\begingroup\$ I wanted to use the STUSB4700 ([datasheet] [1]). The schematic on page 21, figure 6 looks like what I need, but the ST1S14 used has a max current of 3a, while I’m looking for 5a. Maybe someone could point me to a different ic/solution that can generate 5v, 9v, 12v, 15v and 20v at 5a without having to change any components? [1]: st.com/resource/en/datasheet/stusb4700.pdf \$\endgroup\$
    – Cecemel
    Jan 7, 2020 at 17:35
  • \$\begingroup\$ I can’t get the link to work.. could someone please fix that for me? \$\endgroup\$
    – Cecemel
    Jan 7, 2020 at 17:37
  • \$\begingroup\$ The USB PD can be used at 5A, if you look on the data sheet you will confirm it, however there few things to take into account: if you use 5A in the output you will need a special USB cable as the standard ones are design for 3A max. \$\endgroup\$
    – Delphesk
    Jan 7, 2020 at 19:16
  • \$\begingroup\$ I know it can be used at 5a, but if I use the schematic I linked to, the max output power will be 60w (20v, 3a). I want to be able to output 100w (20v, 5a), which is why I'm looking for a higher power buck converter. As for the cable, I think that the PD controllers at source and sink won't negotiate any higher than 3a, unless there's an eMark chip in the cable that tells them it can handle more than 3a. \$\endgroup\$
    – Cecemel
    Jan 7, 2020 at 19:50
  • \$\begingroup\$ If you want to use the 5A configuration, you will have to configure the PDO current configuration via I2C, it might require some design change from the schematic in the data sheet. But as specify in the data sheet it’s possible but it might require more design effort unfortunately... \$\endgroup\$
    – Delphesk
    Jan 7, 2020 at 20:03

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