I'm working on a hobby project based on the ATmega32U4. I'd like to be able to power the device by USB or LiPo battery. I was looking at the schematic for the LilyPad ProtoSnap Plus and saw a diode that looks like it's intended to prevent the battery from powering the load during charging.

As I understand, charge managers without power-path support may never "finish charging" a battery if the load is high enough to draw power from the charge manager/battery. Some more expensive and more complicated ICs have this built in, but not the LilyPad.

Instead, they have a Schottky diode between the battery output and USB power. It seems like as long as the power consumed by the load is adequately supplied by the USB power rail the charge manager won't be tapped as a power source, but I don't really know that to be true.

TL/DR Is the Schottky diode (D1) enough to support a power-path that doesn't prevent the battery from being "fully charged" when the load is present during charging?

LilyPad Battery Charger and Regulator

  • 1
    \$\begingroup\$ TL/DR... Yes :) \$\endgroup\$
    – Finbarr
    Jan 19, 2022 at 17:31
  • \$\begingroup\$ @Finbarr woohoo!! Thanks! \$\endgroup\$
    – D. Patrick
    Jan 19, 2022 at 17:32
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    \$\begingroup\$ The voltage on the battery will always be less than the 5V on VCC_1 when the USB connector is active. D1 actually prevents charging of the battery directly from the 5V line and allows charging to be correctly controlled by the charger. \$\endgroup\$
    – Finbarr
    Jan 19, 2022 at 17:33
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    \$\begingroup\$ Yes, because VCC_1 will be at a higher voltage (5V) than the battery, so D1 will be reverse biased. It only conducts when the USB voltage goes away. However, the caveat here is that if the USB host is just powered off rather than unplugged, this circuit might end up trying to supply battery power to the host via Vbus. \$\endgroup\$
    – Finbarr
    Jan 19, 2022 at 18:03
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    \$\begingroup\$ Worth considering, some USB host ports - on older PCs especially - just connect Vbus direct to the 5V rail. \$\endgroup\$
    – Finbarr
    Jan 20, 2022 at 10:34


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