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my question is simple but USB standard and how it works, let me going crazy. I need to sink up to 3 A (5 V) from a USB type C connected to a 15 W (5V @3A) wall power supply. My board should have only a type C connector (receptable) with 5.1K resistors on CC1 and CC2 lines, no more hardware. Is this enough to sink up to 3A from power supply? I don't worry about compliance with USB standard, it's a custom board. Someone says that with no hardware control on CC lines in the board, the suppply can't deliver more than 500 mA, but I think is the supply that should monitor CC lines on DFP side and according to its pullup resistors delivers up to 3 A.

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    \$\begingroup\$ Depends on the USB supply. Many just do 5V at whatever their rated current is without any control/handshaking. This makes it cheaper, easier and not offically USB compliant, but it works. Others DO have control and handshaking (especially those which are higher power as they can vary the voltage up to get more power). We don't know which you have. \$\endgroup\$
    – Puffafish
    Mar 9, 2023 at 7:58
  • \$\begingroup\$ A "(5V @3A) wall power supply" will supply 3A at 5V no matter what. If you don't plan to connect your device to any TYPE-C ports other than your own specific power supply, you can do whatever you want, no resistors are necessary. \$\endgroup\$ Mar 10, 2023 at 3:53

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What you propose should work in your case, even if it does not follow USB specs.

It's the USB charger that tells to the device how much current is available based on the pull-up resistor on CC.

A 3A charger should be assumed to be able to provide 3A, but in some cases a single 3A supply might be shared between two charging ports, so when you have two devices connected, the signaling can change to allow only up to 1.5A for each port, and therefore, as per the specs, a device must monitor how much is available and must not consume more than available.

But if you are sure the charger will always signal that 3A is available, then there should also be no problems always drawing 3A without checking.

The charger just needs to see the resistors on CC pins to activate the output. No other negotiation should be needed from your receptacle side.

The other current limits you mention are true, but don't apply when you have a native Type C supply that can provide 3A.

In all other cases, when the power is supplied by e.g. a PC, you might have a limit of 500mA, 900mA, or 1500mA, depending on which kind of port on PC is used (USB 2.0, single-lane 3.2 or dual-lane 3.2). So don't try connecting a 3A load to a PC or you will burn something, at least a fuse if nothing else.

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