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I'm currently studying different approaches to supplying a PCB which needs both 3.3V and 5V from a wide range DC input source (i.e., 6-36V.)

The most common layout I've come across is V_in(6-36) -> buck (3.3) -> boost (5.) Is there a reason for this being the most common? What are the potential drawbacks of V_in (6-36) -> buck(5) -> buck(3.3,) or not putting them in series at all and simply using two buck converters side by side?

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  • \$\begingroup\$ To some extent it depends how much current you need on each rail. Which you haven't mentioned. You can make a case for any of those scenarios (and a couple of others) for different requirements. \$\endgroup\$ Sep 7, 2021 at 16:27
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    \$\begingroup\$ I would not say most common (far from it actually) but one reason to do that is dropout with a 6V input being unable to support 5V.. \$\endgroup\$
    – DKNguyen
    Sep 7, 2021 at 16:28
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    \$\begingroup\$ Another reason could be supply sequencing, eg. when the 3V rail must always be on before the 5V rail. \$\endgroup\$
    – DamienD
    Sep 7, 2021 at 16:33
  • \$\begingroup\$ Thanks a lot, could you elaborate on how different currents affect the choice? In my specific case its going to be in the 100-400mA range for both. But I'm interested in the general case and the reasons for that. \$\endgroup\$
    – paulst
    Sep 7, 2021 at 16:35
  • \$\begingroup\$ Downstream converters load the upstream converters they are connected to. So if you have a 10A supply, you probably want it running through minimal upstream converters, \$\endgroup\$
    – DKNguyen
    Sep 7, 2021 at 16:36

2 Answers 2

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If the 5V requires only a small current, using a boost (or even a flying-cap converter) from 3.3V would give wide flexibility for the input voltage. For higher 5V current it could instead be a buck-boost directly from Vin, which would be more efficient than boosting from the 3.3V rail.

One challenge you face is the large input voltage range required. This has a profound effect on the DC-DC: the stepping ratio down to 3.3V varies from about 1.8:1 (55% duty cycle) for 6V to about 11:1 (9% duty cycle) for 36V. Why does this matter? It affects the inductor choice and switching frequency: the higher the stepping ratio, the shorter the minimum on-time becomes.

If you can step down to 5V instead for your bulk voltage, then make 3.3V from that, then these extents become 83% and 14% for 6V and 36V, respectively. This will allow a higher switching frequency. As it is, many DCDC's will run at close to 100% duty, so the lower input is doable even though the Vin-Vout overhead voltage is just 1V (some regulators will even switch to linear mode as Vin gets close to Vout.) Meanwhile your 3.3V gets to be a low-voltage input type, so possibly less expensive and able to run at a higher switching frequency.

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Because the 3.3 V current is so small, I would use a buck regulator to take the input down to 5 V, and a linear regulator to pick off some of the 5 V and turn it into 3.3 V. 0.7 W is not a lot of heat to move.

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