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Source: LiPo Battery (13V - 16.8V)
Consumer #1: 5V (RaspPi) ~1A
Consumer #2: 12V (variouse) 0-3A (plus ~1A if series)
DC/DC step down: two LM2596

Which would be the better solution:

LM2596 #1 on LiPo (16.8V to 12V out)
LM2596 #2 on LiPo (16.8V to 5V out)

LM2596 #1 on LiPo (16.8V to 12V out)
LM2596 #2 on LM2596 #1 (12V to 5V out)

What are the pros and cons in term of efficiency, and voltage stability?

Datasheet of the dc/dc converter,

Max output current is 3A . The battery can provide a discharge rate of 35c at 10Ah
Efficiency diagram is on page 6.

Note: The 12V output might be used to supply 12V/3A to a battery charger (Yeah, charging a liPo with a liPo,.. thinks that happen in RC-hobby). So the drawn current on the 12V out could raise by 3A instantly. So yeah, right now I see the issue with the serial concept, I'd limit my charging current to 2A instead of 3A. But are there other pros and cons?

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    \$\begingroup\$ Your question is legitimate but we need to know the current consumptions on the 5V and 12V rails and the current capabilities of your dcdc. Efficiency infos would be nice too, add a link to the datasheet at the very least \$\endgroup\$ – Vladimir Cravero Apr 22 '16 at 7:47
  • \$\begingroup\$ I've added the requested data. \$\endgroup\$ – Sempie Apr 22 '16 at 7:54
  • \$\begingroup\$ It appears that you don't have enough current for your 12 volts (0 to 7 amps from a 3 amp supply). That would dictate parallel, and suffer the shortage on 12 volts. Or a different converter for 12 volts. \$\endgroup\$ – Mark Apr 22 '16 at 8:03
  • \$\begingroup\$ I've already noticed the 7A problem, but due I only need that "high" current to charge another battery, lower amperage is not THAT much of an issue (but still, higher would be better). Could it cause problems with voltage stability, when the setup is in series and out of a sudden the 12V converter has to provide 3A instead of 1A? If yes, series is probably a bad idea, due the RaspPi could receive too low voltage for a moment which is a no go in my application. \$\endgroup\$ – Sempie Apr 22 '16 at 8:04
  • \$\begingroup\$ why do you even need a dcdc for the charger? Most lipo chargers I used work happily up to nearly 20V \$\endgroup\$ – Vladimir Cravero Apr 22 '16 at 13:12
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Both methods are certainly used. Serially connected DC/DC supplies are usually used with "high" input voltage such as mains. You first get 12V (or something) from the rectified 230VAC input and then you convert that to 1.2V or something.

Certainly the flyback can have multiple outputs but unless they're multiples of each other wound in a particular way you're going to hurt on cross-regulation.

So in this case just put them on parallel. Depending what you have on 5V you could actually use a linear regulator on that, just be aware that you have 1.4x power dissipation on regulator.

Also 3A output is nothing to sneeze at, it's entering territory where ripple currents and losses start to matter. You could use the TI webench or just copy one of the many similar examples on-line.

Bottom line, connecting DC/DC supplies in series kills your efficiency as instead of 85% you now have 85% of 85% (72%) and so on.

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