PowerSource

Jul 17 2008 11:31AM | Permalink | Email this | Comments (3) |
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Many MP3 players have already bailed on the concept of having a separate power connection/wall wart combination to charge their internal battery pack, and instead use only power from their USB cable, both for operating and battery-recharge power. Relying on USB power gets rid of wall wart efficiency problems as well as differences in international line power standards. The USB power standard calls for the host, usually a desktop or laptop computer, to provide 500mA of USB current to power peripherals.

But what about when you want to connect two peripherals -- say, a digital still camera to a cell phone ? Which device provides the power then?

USB on-the-go (USB OTG) relies on a mini USB connector and bi-directional power. When a non-host device, such as a cell phone, connects to a host, such as a laptop, the cell phone charges from the host, and its charger power supply acts as a buck converter to charge the cell phone battery, dropping the USB 5V down to the lithium ion battery charge voltage of around 3V. When a cell phone capable of USB OTG connects to another peripheral device via its mini-USB connection, the cell phone power supply acts as a boost converter, boosting its battery voltage up to the USB 5V required by the other peripheral.

I heard about USB OTG from TI as it prepared to introduce its new bg24150 battery charger, which not coincidentally supports bi-directional USB OTG power. Here’s a link to the OTG supplement for the USB standard.