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I'm working on a hobby project which combines two modules that require 3.3V @ 130mA and 5V @ 100mA power sources. I want to make my device work at wide range of power sources - from single 3.7V battery up to 12V. So far I think to use step down regulator to get 3.3V and feed that to step up regulator to get 5V.

Is this a valid solution to my problem? Is it effective solution?

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    \$\begingroup\$ Valid? Sure. Effective? probably not. Stick to just step down 6~12V input and you'll have a better efficiency than stepping down and then stepping up from that. \$\endgroup\$
    – Passerby
    Jun 9, 2016 at 19:44
  • \$\begingroup\$ Also consider a wide input range battery controller, if your device will permanently have a battery. \$\endgroup\$ Jun 9, 2016 at 19:48
  • \$\begingroup\$ Much easier to drop 12 volts to 5V and then use a linear regulator to drop 5V to 3.3 volts. \$\endgroup\$
    – PkP
    Jun 9, 2016 at 19:54

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It depends on the compromise you want.

If you want to maximize efficiency, you shouldn't chain DC-DC regulators. So you would use a SEPIC to get the 5V and a buck for the 3.3V, both supplied directly with the power input. But this requires 3 inductors and complex ICs. So it's expensive and takes a large PCB area.

If you want to favor price/PCB area instead of efficiency, do as you suggest, this is a good solution. You could even consider using a charge pump for the 3.3V -> 5V part instead of a boost, given the low currents required. It may save some more PCB estate and reduce potential EMI.

All in all, there is no bad solution, you just need to decide what you want to favor, find a few chips from manufacturers, estimate the component count/PCB area, do a few math to estimate overall efficiency, and choose.

There are a few tools from manufacturers that can help with this (I'm thinking about TI Webench and another one from Linear Technologies, but I forgot the name).

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You need to be sure that the chips don't have a problem when powering-up. If either chip needs both 5v and 3.3v, then there should be a power sequencing spec in the datasheet. Many dual voltage ICs need the 5v to come up first.

One supply will come up before the other. If the situation is that one IC runs on 3.3v and the other on 5v, and they are hooked together, then the first-powered chip may feed voltage to the not-yet-powered chip through those connections. You need to insure that those I/Os are protected.

I have always started with 5 volts, and then used a regulator without a lot of filter capacitor to regulate 5v down to 3.3v. That way, the two supplies tend to come up together. If I need core-voltage, I then regulate that down from the 3.3v.

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