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I discovered USB power banks for a mere $1 (excluding battery) that seem to have an IC that handles everything from charging the battery to boosting output to 5v. It is apparently rated at being able to supply 500mA. The PCB and IC can be seen here:

enter image description here

What's most curious is it has so few components: three capacitors, an inductor and the IC; And two LEDs which indicate power and charging status.

I have a couple of questions:

I have not been able to find any information on the IC. Are there ICs that perform both charging and boosting, from/to 5v? I have not been able to find any - so is this power bank is potentially dangerous? Or perhaps the IC is an ASIC developed by / for the manufacturer of the actual power banks? What are the chances that I could develop such a power bank circuit by using some widely available IC?

The simplest circuit I can conceive of would have a minimum of 2 ICs, probably 3 or 4, plus quite a few more components, and that's ignoring USB specs that require that current draw greater than 100mA shoukd be negotiated (Which is known to be commonly ignored). I get the sense I'm missing something.

I know that many of these 18650 batteries incorporate charging and discharging protection circuits, so perhaps this powerbank relies on that, meaning it just needs to worry about boosting the output?

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Yes, such special-purpose chips exist. I don't see a protection power MOSFET etc. on that particular board so it probably doesn't have good protection.

The chances are very good you could develop such a board with well documented chips from TI, Intersil etc. that would be technically good. Cost is another matter.

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  • \$\begingroup\$ Thought -- in a monolithic synchronous reversible design (i.e. boost when discharging, buck when charging) -- the top switch can be made a NMOS as charge pumps can be snuggled in on chip now, and since the body diode on-chip goes to substrate not source, that's out of the way, so it can do double duty as a protection FET. (Can't do this with discrete MOSFETs though as the body diode won't let you.) \$\endgroup\$ Commented Nov 6, 2016 at 16:09

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