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Right now I am designing a circuit that will charge a Li-ion battery via USB, using the MCP73831 (at 100 mA).

The battery voltage (nominally 3.7 V) will be regulated to 3.3 V to power a microcontroller like an ATtiny85 or ESP8266 for example.

Below is what I have so far. My question is: Are there any flaws/mistakes I made, and how can I charge the Li-ion battery while simultaneously powering the MCU?

Because right now I can only charge OR use the MCU, depending on the SPDT switch.

enter image description here

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    \$\begingroup\$ Why do you have the switch? Why do you need it? \$\endgroup\$ Commented Dec 30, 2020 at 13:53
  • \$\begingroup\$ I was afraid that charging the lipo, while using the regulator would cause a fire or something. I have never worked with lipo batteries before. So i used a switch to only be able to do one thing at the same time. My question then is: would it be safe to remove the switch altogether? I am using a battery like this one sparkfun.com/products/13851 \$\endgroup\$
    – powan
    Commented Dec 30, 2020 at 14:04

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Most Li-ion chargers don't charge a Li-ion battery with a load attached correctly, because the load current interferes with the charging algorithm.

Charger ICs that can support a load while still charging correctly exist, but your MCP73831 isn't one of them. The MCP73871 is; it supports load sharing and can supply a load while charging the battery.

You could add a load sharing circuit to your MCP73831 if you want to get rid of the switch; see MCP73831 Li ion battery charger problem with load sharing circuit for an example of how to go about it. There's also a Microchip application note that describes how to do it.

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    \$\begingroup\$ That was very helpful. Thanks, I will definitely look into the MCP73871. \$\endgroup\$
    – powan
    Commented Dec 30, 2020 at 16:09
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    \$\begingroup\$ I get the impression that that app note is also applicable to the MCP73831 (your current part) however I suspect that this circuit suffers a brown-out fault if the AC input to the USB is turned off. \$\endgroup\$ Commented Dec 30, 2020 at 20:59
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The problem with charging and using a Li-ion battery at the same time is that end of charge is not properly detected. The MCP73831 uses the current level at the end of the constant voltage stage for charge termination.

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    \$\begingroup\$ Aha that makes sense, thanks for clearing that out. I think i will leave the schematic as it is and not remove the switch. \$\endgroup\$
    – powan
    Commented Dec 30, 2020 at 14:20

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