I have ATmega328P circuit that drains from 0.1 to max 150 mA. Most of the time it's asleep (0.1 mA - with other little components attached) and once a minute it wakes up and drains 80 mA && occasional user input can increase drain up to 150 mA. So drain isn't constant, but most of the time it is less than 1 mA.

I need my circuit run from 320 mAh LiPo batterry and also to be able to charge it while the device is being used. One problem is that there is no datasheet for my LiPo batterry (CELLEVIA BATTERIES L303040), so I don't know if it's possible to charge it in 1C rate...

If the charge current was 160 mA (0.5C), with AVR circuit's 150 mA drain the charging current equals 10 mA. But this is only temporary (miliseconds), not a constant drain. So I guess that battery charging process won't be affected. Or I can set charge current up to 320 mA (1C), so with drain 150 mA there should be 170 mA left for the battery.

What do you think about charging no-datasheet battery in 1C?

I have MCP73831T-2ATI/OT charge IC and BU33TD3WG 3.3 V 200 mA regulator available. Will this circuit work?


There is 220 mV dropout voltage on the regulator so it won't work with VIN < 3.52 V?

I know there is a solution suing P-channel MOSFET to switch power sources, but I don't have any access to this type. I hope the circuit above should work.

Thank you very much!

  • 1
    \$\begingroup\$ Are you expecting to keep the device plugged in normally, or only to recharge it? 0.5C should work just fine for "normal charging" the vast majority of polymer packs. If normally plugged in, charging current can be much lower, or even charge a supercap instead. Link to MCP73831T. \$\endgroup\$
    – rdtsc
    Commented Oct 6, 2015 at 13:24

1 Answer 1


A typical LiPo battery can be charged with 1C however, will your USB supply this? If the device is only registered as a USB low power device, you should draw no more than 150mA from the USB or there is the danger that the port either shuts down or (best case) limits the current to 150mA.

Furthermore you are worried about that the battery is charged while your controller is running. If that's the reason for needing 1C charge current I would add some capacity to decouple the uC current draw.

It is correct that the linear voltage regulator will subtract the drop-out voltage from the input voltage. This voltage is depending from the current. You can find the applicable dropout voltage if you check the datasheet. Normally there is a graph showing dropout-voltage over current.

  • 1
    \$\begingroup\$ If vin of the LDO is below the (regulated) output voltage it will still supply voltage but unregulated. And it will still deduct the dropout voltage depending on current draw. E.g. Vin = 2V; Vout =2V - v_drop \$\endgroup\$
    – optronik
    Commented Oct 6, 2015 at 13:33
  • \$\begingroup\$ Eh, there are a lot of USB hubs that are non-compliant and supply 500mA with no registering. I've heard they do that because of proliferation of data-less gadgets like USB-powered fans, massagers and other similar stuff. My Dell monitor has such a juicy hub for example. \$\endgroup\$ Commented Oct 6, 2015 at 14:40
  • \$\begingroup\$ Most common tablet chargers that have USB are rated for 12W of energy, that's usually about 2.3 amps at 5.2V \$\endgroup\$
    – user86234
    Commented Oct 6, 2015 at 14:52
  • \$\begingroup\$ @tuskiomi: yeah, but there's usually a complicated story behind the high-powered chargers. \$\endgroup\$ Commented Oct 6, 2015 at 14:59

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