I'm making an android accessory that uses a PIC microcontroller and a FTDI FT312D chip to poll a bunch of sensors and then report the findings back to a nexus 7 tablet.

Although I am using a 4A power supply, the tablet is not getting enough current to charge and is instead discharging.

Ive been reading up on Android Accessorys and USB specs but havnt found a way to enable a higher current draw.

  • \$\begingroup\$ I got a much better current draw, and shorter charge time, on my Nexus 7 by swapping out the 28 gauge charging cable it came with, with a 24 gauge cable. \$\endgroup\$ Mar 6, 2014 at 11:26
  • \$\begingroup\$ But will this allow me to draw more current through the USB connector? I dont think the 500mA supplied by USB 2.0 standard is enough to overcome the draw from the screen. \$\endgroup\$
    – user38278
    Mar 7, 2014 at 18:11

1 Answer 1


So you are making an accessory, and through this accessory you also want to charge your Nexus 7, if this is the case, You are feeding 5V through your USB VBUS line. Now Charging the battery over a USB connectors isn't plain simple like just applying 5V and the Tablet will charge. There is some handshake going on on the D+/D- line, just some simple voltage levels. Since your are using an FTDI on the USB line this will probably block the charging negotiation. What you could do is place a IC between your USB connector and your FTDI that will do the negotiation (I know there are some that will detect which kind of charger is applied but I don't know for sure if there are charger IC's) and will get the tablet to draw more current.

Here is an App note from Maxim that will go into more detail: http://www.maximintegrated.com/app-notes/index.mvp/id/4803

Hope this helps a bit ;-)


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