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I want to use a USB type C cable only to power my board.

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My board requires 5V input and max of 2A load.

I read that by default, the USB Type C cable can give an output of 5V & 3A.

So, I am planning to have a USB type-C input connector in my board which will give the power to all my ICs from the cable.

I only want to use it for power.

My question is, in case of only using the USB TypeC for power, what should I do with the remaining pins of the USB type-c on my board?

Should I terminate certain pins or should I leave them floating? Please help.

I saw a similar question over here. One of the answer mentions that on the device side of the things (device side of the cable), on my board, I only need to pull down the CC pins with a 5.1k. Apart from that, there is no other mention. So, all the tx, rx and other pins can be floating or should I need to terminate?

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  • \$\begingroup\$ You can't really draw 5V 3A safely because there are USB-A-to-C cables. Those will struggle above 1A when plugged into desktop/notebook PC. There are hardware solutions that can negotiate the current - use them. \$\endgroup\$
    – Turbo J
    Nov 1, 2023 at 9:10
  • \$\begingroup\$ @TurboJ, if I use a Type-C to Type-C cable, then is it possible to have 3A? \$\endgroup\$
    – Newbie
    Nov 1, 2023 at 10:07
  • \$\begingroup\$ get a "pd trigger" and buck converters to turn say, 15v1a into 5v3a. \$\endgroup\$
    – dandavis
    Nov 1, 2023 at 19:47

1 Answer 1

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By default Type-C definitely does not allow 3A.

There are many problems with this.

Assuming there is 3A available is wrong, there might or might not be, and how much is available must be known before trying to draw that amount.

There might be 3A, 1.5A, 0.9A, or 0.5A available, depending on which port will your device be connected. And it can't draw that amount unless the device has negotiated if it can draw the requested amount of current.

So a device is not allowed to draw more current than there is available.

For example, if you make a device that requires 0.5A, and you connect it to a USB hub which already has many devices so there is not 0.5A available, you need to enumerate with the PC with a request of 500mA and the PC then responds if it is OK to consume 500mA or not. So your device must perform these negotiations at 100mA max.

So sure, you can make a device which draws 2A without asking, but it may not work or damage the device which supplies power, and will not pass any USB compliance testing so you can't really say it is an USB device because it is not within USB specs. There will also be angry customers too if you intend to sell a product that may damage other devices.

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  • \$\begingroup\$ Sure, so, you mean to say, 2A from USB Type-C is OK, but not 3A? Also, what if I use a Type-C to Type-C cable, then is it possible to have 3A? \$\endgroup\$
    – Newbie
    Nov 1, 2023 at 10:08
  • \$\begingroup\$ Can you tell me how to do that then? Because, I have noticed some application that take only the power from the USB Type-C input for their device. Please let me know how its possible and what are the other requirements to be done with the other type-c pins? \$\endgroup\$
    – Newbie
    Nov 1, 2023 at 10:09
  • \$\begingroup\$ No, I am not saying 2A is OK, I am saying it isn't because you might have 0.1A available when you connect to a hub and don't want to damage it. If you use a Type-C to Type-C cable and connect to a say 3A charger with two ports, you have 3A until you plug in two devices to charger, so your device must detect this and must turn off if 1.5A is not enough. And how you should do it depends on what you are already doing and how. I don't know what ICs or MCUs or tools you already have. But yes, most non-compliant devices like rechargeable bike lights don't negotiate anything, they just draw current. \$\endgroup\$
    – Justme
    Nov 1, 2023 at 10:41
  • \$\begingroup\$ Oh Ok, But how to make my device detect it and turn it off? My device just has only power ICs. So how to do it? And why should I turn off if 1.5A is not enough? \$\endgroup\$
    – Newbie
    Nov 1, 2023 at 15:54
  • \$\begingroup\$ The details are in USB-C specification, I can't spend an hour copy-pasting copyrighted information here, but your device gets the info as voltages on CC lines. And if your device needs more that 1.5A and you are allowed only to get up to 1.5A, that is why you must turn off loads until you are allowed to draw up to 3A again so you can start up loads. And then there's USB Power Delivery which you have not taken into consideration, which is even more complex. \$\endgroup\$
    – Justme
    Nov 1, 2023 at 16:02

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