I'm trying to figure out the best way to implement a UPS for an embedded system that runs from a 5V supply.

Essentially the system has a 48v to 5v, 1.2A output DCDC converter that powers my system. The 5v output supplies a bank of relays, as well as a 3.3v regulator for the embedded side of things.

When the input power fails I need to be able to keep the system alive (both 5V and 3.3V rails) for about an hour or so, and I figured the best way to do this would be with a lithium cell charger, and and boost converter. The average current of the system is around about 200mA so I know a single 18650 Lithium cell should more than suffice.

To reduce design time and complexity I hoped to find a highly integrated one-chip solution for this, but I'm unsure if it exists. If it doesn't, I don't know what my other options are either. I figured this is essentially most power banks do, so surely there must be a simplistic solution out there somewhere... I just don't know what to be searching for other than "lithium battery charger boost 5v"

Can anyone with some experience in this area please point me in right direction?

  • \$\begingroup\$ Look for "mini UPS 5V". There are plenty of examples and semi-pro boards. circuitmaker.com/Projects/Details/John-Kovacs-2/… \$\endgroup\$ Nov 14, 2018 at 16:57
  • \$\begingroup\$ a lot of cheap (<$1) micro-usb li battery charger circuits let you run on usb power if the battery is charged, and switch to the battery when usb is "unplugged"; sounds like what you need. \$\endgroup\$
    – dandavis
    Nov 14, 2018 at 21:25

1 Answer 1


What you want is not exactly simple. Most power banks are not "UPS", they are reserved storage, and usually operate either in charge mode (charging internal Li-Ion cell(s)), or in discharge mode, with external power disconnected. Many powerbanks have an explicit warning, "don't charge and discharge at the same time", see example.

More, since the only one cell is enough for your power backup requirements, but your in power source is 48V, you will need a two-step conversion, from 48V to something more manageable - I saw no ICs for single Li-Ion cells that can handle over 40 V input.

To get "true UPS" functionality, your charger IC must have something called "Power Path management", or something along this line, like the Texas Instruments BQ24xxx/BQ25xxx family of chargers do. But in no case the IC will provide you with output voltage of 5V automatically, this is left for system designer to do. Therefore you are looking for overall architecture like the following one:

enter image description here

The smart-power-path IC will provide the smooth switch from external power to battery power, but you will need to manage the variable battery output using up- and down-converters by yourself. Just as this project suggests and implements.


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