Electrical – Make ‘smart’ USB Dedicated Charging Port

chargingusb

I would like to create a circuit that is able to charge any USB connected device from my 5V rail with 1A current limit. There are many different USB devices, asking different kinds of currents, so:

1) I planned doing it with a dedicated chip, that handles all the enumarations and sets current limits that I can provide.

  • BUT I feel it's an overkill, as I want to set my current limit to 1A (So any device could get only 1A max). And this probably asks a lot of firmware.
    This is the schematic I initially planned to use:
    USB charging communication

2) Then there are wall-chargers which seems to power anything quite nicely wihout any 'smartness' – also acting as DCP.

  • Question was – Couldn't I just short the D+ D- pins, basically act as a dedicated charging port ? Without the chip?
  • Because as I've read, devices set their modes very differently on D+/- pins so there's no One way of defining it. Which was assured to be true. Probably any device I plug in with USB cable, might ask more than I can supply (since it's acting like a wall adapter) and could destroy my board.

SOLUTION_1:
So Shorting D+/- is an option (not 100% universal). This is also visible on some of the teardowned AC/DC USB wall adapters seen here. Some have resistor dividers, some seem to have other magic going on (depening on manuf. I guess).
All cheap 'travel' adapters seem to have D+/- shorted.

  • The drawback is that I have to physically limit the current if any device wants to get >1A and that can be troublesome on the design side (heating of protection components, added current consumption, possible physical faults which renders the port useless).

SOLUTION_2
Suggested also here in the answers, was to use a dedicated DCP chip (such as TPS2514x) which can communicate my 1A current limit and act as a DCP (I would still have a resettable fuse on the VUSB line though). See also here

  • That way I think, I get more control over the current I allow the device to take. (a bit more expensive, but much safer).

Thanks for the info, I believe I've reached my verdict for using solution 2 in my case (wouldn't want to destroy any phones or my own board).

Best Answer

I can refer you to this question for some answers, How do I design a 2A or more power supply for my consumer USB devices?

Probably any device I plug in with USB cable, might ask more than I can supply (since it's like a wall adapter) and could destroy my board?

On that particular subject, you are right. Assuming the device you plug in will follow the usb charging standards is not enough. I would strongly encourage you to, as good practice, implement at least overcurrent protection. With a ptc fuse for example.

I can additionally recommend that you look into the quite compact TPS2514A IC from TI, which can take care of advertising the 1A current limit you had in mind in a way that is compliant with the usb battery charging 1.2 specification (look in the datasheet, under Figure 16. 5-W USB Charger Application).

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