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I see a lot of USB controllers in battery charging circuits. They have USB host controllers, embedded controller and USB port controller at host side. (Refer tps2547 )

  1. Can somebody tell me what are the minimum control and protection circuits needed for charging using USB?
  2. Is it the sole responsibility of the host to ensure reliable power to whichever device it is powering ? Is there any protection or control circuitry at device side, say load switches or similar kind?

Iam lost in the vast pool of resources with so many standards and specifications. Please note that Iam primarily looking for power transfer through USB.

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In practice, the absolute bare minimum is a stable 5V/500ma rail with the data lines shorted together. That will cause most devices to charge at "normal" speed.

That doesn't work for everything, and there is a proliferation of hacks for progressively more complicated schemes.

Is it the sole responsibility of the host to ensure reliable power to whichever device it is powering ?

Yes. You can't guarantee anything in particular at the client side, and you certainly shouldn't rely on it having any particular protection. There are all sorts of "USB" gimmick devices like fans and lights that just use it as a power rail.

You also need some kind of overcurrent protection in case a cable is damaged, resulting in a short circuit between the rails.

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