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I am working on a project that needs about 2A current for the system to work. I don't need data communication just need power. How can I achieve this?

I saw several articles say that USB 2.0 supports 1.5A and USB 3.0 supports even more. But I am not sure this can be configured at the slave side. If possible how can I do this?

This question discusses about sensing the pull up resistor. Can someone explain this?

I done research how to implement this. I couldn't find any material yet someone have actually done this.

I have read in this article, connecting 200ohm resistor between D+ and D- can draw 1.5 A. But my understanding this is something to do from the host side.

To activate the DCP, the D- and D+ pins are internally connected by a resistor of 200 ohms or less. This distinguishes the DCP from the original USB ports that carry data. Some Apple products limit the charge current by connecting different resistor values to the D+ and D- pins.

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  • \$\begingroup\$ Does this answer your question? USB Type-C configuration necessary for battery charging only \$\endgroup\$ Dec 7 '20 at 1:09
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    \$\begingroup\$ You cannot within specification do this with a traditional USB port, but many USB phone "chargers" will support it. Their signalling is to inform the phone of their capabilities; they don't expect a reply or negotiation. Of course what a given "charger" allows is an off topic usage question you'd have to address from its ratings or manufacturer. \$\endgroup\$ Dec 7 '20 at 3:28
  • \$\begingroup\$ 2 amps from a mac or pc laptop is likely to cause issues. 2 amps from many PCs may be fine since they tend to tie directly to the 5V psu lines. But many others may have problems with that much current. From your average usb wall charger rated for 2 amps or above you'd be hard pressed to find any that enforce a limit. \$\endgroup\$
    – Passerby
    Dec 7 '20 at 11:43
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Are you trying to get power from a USB-C port, USB-A port, or potentially both? Will the host supplying power be a computer, a "dumb" wall wart charger, or potentially both? You say "about 2 amps", is this above 2.4 amps? Depending on how you answer can determine your best options.

USB 2.0, and the USB-BC (battery charger) spec that came with it, allows up to 2.4 amps current draw (or 12 watts) from a host USB-A port. USB 1.1 allowed for 1 amp after "enumeration", or whatever it's called, but only 100 mA before. Most USB-A chargers are quite "dumb" and just supply 5 volts and hope the device behaves itself by not drawing too much current, most chargers have some kind of current limiting to avoid becoming a fire hazard.

Before USB 2.0 and USB-BC caught on things got pretty wild, and there were a number of crazy ways for a charger to tell the device how much current could be safely drawn. Some devices would try to draw as much as 3 amps from a charger without checking first if it was safe. Some devices and chargers could negotiate a voltage higher than 5 volts and/or current above the USB 2.0 limit of 2.4 amps, these are very dangerous and definitely violate the USB spec which lead to some strongly worded letters from the USB spec group to remove these products from the market.

After the USB 2.0 and USB-BC spec came out, and was enforced by the USB group with their strongly worded letters, chargers were expected to put resistors across the D+/D- lines to tell the "smart" device how much current could be safely drawn. The device was expected to draw no more than 100 mA until it detected the resistance or negotiated power from a computer (or "smart" charger which is really just a small computer). There was an incentive to keep chargers "dumb" to maintain backward compatibility with old (pre-USB-BC) hardware and keep costs low.

If what you need is 5 volts and less than 2.4 amps you could likely get away with just connecting your load to +5 and ground on a USB-A cable. That might be "rude" to a PC or smart charger as it will see more current going out than is explained by the enumerated devices. Depending on the host providing power to the port it might limit power output somehow, or it might not.

USB-C ports are designed to act as host or device ports, especially those on a laptop or smartphone, and so will not output power until it sees some indication of a connected device. How this works was defined under the USB-PD (power delivery) and USB-C spec. This is done with the right pull-up and/or pull-down resistors on the configuration pins. Get that right and you might be able to get away with being "rude" and drawing up to 2.4 amps from the +5 line like a stupid legacy device. Again, this resistor only tells the host that a device is connected and therefore safe to put +5 volts on the legacy power pins, it would be wise (or "nice") to negotiate the power draw if taking more than 100 mA. By not being nice then the host might cut power since to be backward compatible with USB 1.1 it only had to allow for 100 mA until the device told the host how much current it wanted.

There are chips, little micro-controllers, that should be fairly easy to find that can perform this USB-PD power negotiation to be "nice" to a smart charger or computer. If you are getting power from a dumb power brick then just keep current draw under 2.4 amps and be prepared for some variability in the voltage. The USB spec allows for the voltage to be as high as 5.25 volts and sag to about 4.5 volts. The allowed voltage sag depends on which version of the spec is followed.

Any USB-PD controller chip will have a spec sheet specifying how to wire it up. The USB-PD spec allows for up to 5 amps from a USB-C port. With a USB-PD chip there is the option to get 5 volts at up to 5 amps safely from a USB-C host, though few power bricks and computers will provide more than 2.4 amps at that voltage. Depending on if the host supports the old USB-BC standard it's possible to get up to 2.4 amps from USB-C or USB-A by using only configuration resistors. If the host is really "dumb" then it can be generally safe to draw up to 2.4 amps by connecting only the +5V and ground, just take care not to use any 5 watt chargers that are still quite common.

I doubt I answered all of your questions, and that wasn't my intent since there's only so much room here to type. My intent was to give you enough of a start so you know what questions to ask next, as well as some history to explain why USB acts like it does.

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  • \$\begingroup\$ Great answer... \$\endgroup\$
    – Passerby
    Dec 7 '20 at 11:44

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