Charging your phone with USB 2.0 from PC

[tree]

Hello folks,

I've been pondering this lately and I'm sure there's someone here with experience in this field. So we know that the charging circuitry in modern smartphones charges the Li batteries inside with constant current up until some voltage and then switches to constant voltage charging. The problem I'm having is with the current limit on the USB 2.0 ports on your PC which are limited to 0.5A, 5V (2.5W). So suppose we're capable of charging our Lithium cells at 2A and our current battery voltage is 3.6V and we're charging in constant current mode. This means that we can supply ~ 7.2W. Assume it's 100% efficient.

As you can see this presents problems. The input is limited in power. So how does the charging circuitry inside the phone KNOW how much power it can draw before the input supply droops?

I hope this is clear...

Thanks.

[Howardlong]

The phone uses signalling of numerous different types to figure out the capabilities. USB-C introduces a more complex signalling protocol to allow for multiple voltages and currents.

http://www.eetimes.com/document.asp?doc_id=1279469

You can get chips to figure it out for you, for example the Microchip USB3751A series you can probe the capabilities at the device using I2C.

[stmdude]

The phone uses signalling of numerous different types to figure out the capabilities.

Yep, have worked on that particular part of cellphones, and you're quite right.  We also did "foldback" detection, which means that we monitor VUSB, and if we see it dropping, we start pulling less current, as we're probably overloading the supply. This is needed to deal with crappy DCP implementations (aka, chinese chargers).

We also backed off for a multitude of other reasons as well, mostly temperature-related.