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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).

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  • \$\begingroup\$ shorting the pins will help older samsung phones take more current, that's about it. if you only want 1A anyway, most phones will draw near that from naive adapters, save iphones. \$\endgroup\$
    – dandavis
    Jun 13, 2017 at 14:55
  • \$\begingroup\$ You're right about there not being "one way". Since there's no guarantee that no device will draw more than 1 A at most, you should make your design such that it can survive a device asking for more. That protection could be an IC or a polyfuse. \$\endgroup\$
    – Bimpelrekkie
    Jun 13, 2017 at 15:00
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    \$\begingroup\$ There is no way to set the current limit to 1A via enumeration. Pin fiddling is device-specific, there is no generic setup, as explained here. \$\endgroup\$
    – Dmitry Grigoryev
    Jun 13, 2017 at 15:00
  • \$\begingroup\$ Take a look at the TPS2514A. It will pretty much convince any device to draw the maximum current it can without going over 5V. But make sure you are able to supply at least 2.4A. \$\endgroup\$
    – user57037
    Jun 14, 2017 at 3:55
  • \$\begingroup\$ Thanks for the comments. I read upon these and I think I've made my decision for using a DCP chip this time. A bit more expensive but easier on the protection circuit design. \$\endgroup\$
    – crypton
    Jun 14, 2017 at 8:41

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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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