Now I hope that this is done, for now. I chose the LTC4055 from Linear, which, besides having a Li-ion charger from USB power, it can also seamlessly switchover the source power from battery and the USB line, thus allowing the robot to be used also while recharging. The LTC4055 datasheet says that the output current is only 900µA, but, according to some feedback I got from asking to the professors and to stackexchange, I’m almost sure that this figure must be wrong (probably they meant 900mA). A design note from linear proposes a circuit that can balance the load from the USB power and the battery, which means that we can not only use the robot while it is recharging, but also to draw some power from the battery if the USB power is not enough to power the robot.
Most of the last week, including the weekend I spent searching for an architecture for the USB charger + a buck-boost converter for the board components, which would require a source for 3.3V and 5V, plus another separate regulator of 3V exclusively for the motor.
The USB charger schematics was mostly done by this Sunday (only the schematics of the CI was missing in the library, thus I left it to place in a second time) until we meet with the professor Monday. We found some downsides:
- The battery would provide from 2.75V to 4.2V, thus the buck-boost CI that I chose could not provide the right regulation at 3.3V (since the CI could do buck or boost, but not both at same time).
- There was an issue for using the robot while recharging.
- The motor had to be changed to another one that needs 9V of input.
- The components that required 5V were removed.
So all the time spent making the schematics was written-off. At least I could study how to build a (more or less) proper USB charger. Now I hope that the new power source architecture is ready and can be written on stone, or I risk to increase even more the time of the development.