Electronic – Powering a circuit from either USB or LIPO

dc/dc converterlipolow-powerpower supply

I have a small hobby project which includes MCU and some peripherals.
It is powered by 3.3V power source and consumes 200mA-800mA for up to a minute. However, most of the time it's in deep sleep mode where is consumes very little current (as low as 5 µA, hopefully) in order for it to last longer when powered by battery.

Now I would like to power my project by USB when USB is connected, or by one cell LIPO battery when USB is disconnected. When USB is connected I would like it to both power the circuit and charge the LIPO.

Here is the high level scheme I had in mind:

schematic

simulate this circuit – Schematic created using CircuitLab

Since USB voltage (5V) is always higher than LIPO voltage, D1 and D2 make sure that when USB is connected the LIPO is disconnected from the DC converter, and when USB is disconnected the LIPO charger is disconnected.

My questions are:

  1. Does my high level scheme make sense? Any special consideration for selecting D1 and D2?
  2. My main concern is how to select the DC converter.
    Input voltage ranges from 3V to 5V although I can probably live with the converter shutting down when its input is below 3.3V as most of the LIPO power is consumed before it gets lower.
    I'm looking for a high efficiency converter that can handle efficiently both high currents and very low currents, in order to make the battery last longer.
    Would a step down converter be a better choice than LDO? What parameters should I look for in the data sheet?

Best Answer

Your thinking goes in right direction but the choice of components could be better. The task of charging single lithium battery from USB and powering system load at the same time is so common nowadays that there are literally dozens of chips designed exactly for this purpose.

What you are looking for is called "Power Path", and it is a technology that allows input power distribution between charging and system load, powering system from the battery when external adapter disconnected and even boosting external power with battery when system draws more current than available from adapter/USB.

Note, however, that while they all have either LDO or switching buck to get input voltage down to battery charging voltage, they do not have another one for 3.3 V. So, they usually output either regulated 4.2 V or direct cell voltage. Considering your 800 mA maximum system load I'd recommend using buck converter rather than LDO.

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