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I am trying to design the power hierarchy if you will for a PCB that I'm designing. My choice in battery is flexible but at the moment I have a 3.7V source. I have two main power rails, 3.3V and 5V and I am having trouble picking the correct solution for my project. At first I had:

3.7V battery -> MCP1826 (3.3V LDO) -> TLV61220 (5V Boost)

I heard that chaining these regulators together reduces the efficiency, so I changed my hierarchy to

3.7V battery -> MCP1826 (3.3V LDO)
3.7V battery -> TLV61220 (5V Boost)

I am relatively new to PCB design, I've only done 3 boards so far and I am trying to learn best practice for conditioning the power inputs to my board. Efficiency is key, as my battery powered device needs to last as long as possible.

Is an LDO the right choice here? Its dropout voltage is 400mV so I believe it should be fine to output 3.3V. Should I consider changing my battery voltage? Is there a standard when it comes to powering a 3.3V board?

Also, if anyone has any resources whether they be links, books, PDFs etc about choosing the correct solution to condition the power inputs to a PCB that would be much appreciated.

Edit: After posting, and thinking about possible ambiguities, my system should never draw more than 500mA at a time, it will likely remain around 200mA when active, and a few hundred uA when sleeping, depending on the quiescent draw of my regulators.

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    \$\begingroup\$ I don't have time to write a full answer, but think about this: Assuming that your suggested 3.7V battery is a typical Li-Ion chemistry, consider its voltage range over its full state-of-charge range; 3.7V is just a nominal figure. The result of that research will change some of the assumptions in your question. \$\endgroup\$ – SamGibson May 29 '17 at 1:13
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    \$\begingroup\$ There's no reason to drop voltage through the LDO to 3.3 but then boost that to 5V. Every extra mA through the LDO is waste energy transformed into heat. \$\endgroup\$ – DoxyLover May 29 '17 at 1:13
  • \$\begingroup\$ How much of that current draw is from the 3.3V vs the 5V rail? \$\endgroup\$ – Abe Karplus May 29 '17 at 6:08
  • \$\begingroup\$ Thanks for the responses so far guys, and @AbeKarplus the 3.3V rail should pull 150mA-400mA when active, and the 5V rail could pull up to 100mA \$\endgroup\$ – Josh May 29 '17 at 13:12
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If battery life is the most important factor, you should move away from LDO regulators and go switched instead; at the cost of increased noise and complexity.

By design, linear regulators waste power to drop voltage down to an intended level. This is rarely a good idea in a battery-powered environment. You can get integrated PMICs that will condition your rails from a battery source and offer other features such as protection, brown-out detection, etc.

A quite simple solution is a pair of parallel switching regulators.

I see that you need at least 500mA maximum output and an enable line.

For down conversion, something like the

PAM2305 series ($0.91)

And for the up conversion -

FAN4860UC5X ($1.21)

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  • \$\begingroup\$ Thanks, I will certainly look towards switching regulators then, I like the PAM2305 suggestion, however for my 5V rail I need an enable or shutdown pin. What do you think about the TLV61220 \$\endgroup\$ – Josh May 29 '17 at 13:18
  • \$\begingroup\$ @Josh please see my edited answer. I think the TLV61220 is overkill (the TLV61220DBVR shows as $1.44 compared to the ones I found); and also for a standard voltage like 5V I recommend going fixed instead of configurable. \$\endgroup\$ – Reinderien May 29 '17 at 14:11
  • \$\begingroup\$ I will mark as answer, and consider looking elsewhere than the TLV61220, I am going to be assembling this myself so I would like to stay away from BGA packages. Thanks for your help. Any chance you know of any documentation regarding this subject that I could learn from? \$\endgroup\$ – Josh May 29 '17 at 15:44
  • \$\begingroup\$ You can select the FAN4860 in a UDFN package instead to make it more hand-solder-friendly (see the FAN4860UMP5X). I'm looking through this search - digikey.ca/short/3dzfc4 . You would do well to read some of these: maximintegrated.com/en/design/techdocs/app-notes/index.mvp/id/5/… \$\endgroup\$ – Reinderien May 29 '17 at 16:00
  • \$\begingroup\$ And the TI app notes - there's a lot of noise but also some good stuff - goo.gl/sLRlfz \$\endgroup\$ – Reinderien May 29 '17 at 16:00

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