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My device needs 1.5A at 5V from a USBC connector. I already have 5.1k resistors in place on both CC1 and CC2 and a fuse rated for 1.5A. However, I have read on here in another thread that apparently some USBC supplies do limit the current to 500mA when the UFP does not actually negotiate. Is this true? Is there a way I could perhaps tie D+ and D- to always get the maximum the supply can output?

The reason I am asking this is that I also measure the voltage on CC1 and CC2 and will adapt the consumption, but if the DFP's pull-ups tell me it's capable of 1.5A but it still limits the supply to 500mA, my device won't know it and will run out of juice.

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2 Answers 2

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With USB C, you have to follow the negotiation rules.

  • After attaching, you are allowed to pull 100mA

  • If you detect the USB 3.0 pins are active, you have 150ma.

If you need more current, you have the following things you can do:

  • Talk over USB 2.0, requesting more power units. You can get up to 500ma@5V using this way
  • Talk over USB 3.0, requesting more power units, you can get up to 750ma@5V using this way
  • Talk over the USB data wires with proprietary charger standard, like Quick-charge or Apple
  • Read the voltage on CC1, if it is above 0.7V, you can draw 1.5A @5V (see USB Type-C Spec R2.0 - August 2019 page 241)
  • Read the voltage on CC2, if it is above 0.7V, you can draw 1.5A @5V (see USB Type-C Spec R2.0 - August 2019 page 241)
  • Read the voltage on CC1, if it is above 1.31V, you can draw 3A @5V (see USB Type-C Spec R2.0 - August 2019 page 241)
  • Read the voltage on CC2, if it is above 1.31V, you can draw 3A @5V (see USB Type-C Spec R2.0 - August 2019 page 241)
  • Communicate with the charger with active USB PD. (over either CC1 or CC2) You can get up to the chargers limit of 5A@48V

Note that the standard says any USB C source on a multiport charger should at least support 1.5A @ 5V (page 228)

If the user connects you device with an A to C legacy cable, only the first few options are available to see if you can get more current

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  • \$\begingroup\$ Thanks for your answer, I like the summary approach. Two questions: 1) Are you sure it's sufficient to check only CC1 or only CC2? Your wording seems to imply this whereas I thought that the current levels allowed came from the combination of the voltages on these pins. 2) If a A to C cable is used, but only plugged in a wall charger and not a computer USB port, does that give at least 500mA? \$\endgroup\$
    – user325962
    Nov 21, 2022 at 16:08
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    \$\begingroup\$ 1: you have to check both CC1 and CC2, both pins have their own resistor, and one can be 0V (connected to the <1k resistor in the active cable), while the other one is connected to the other side. 2: with an USB A to C cable, you are supposed to follow these rules, but allmost all sources allow devices to draw 500mA without the proper negotiation steps in the real world \$\endgroup\$
    – Ferrybig
    Nov 21, 2022 at 19:50
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This is how I interpret it: Both the source and the device must be capable of 1.5A with usb-c. This means both ends must be USB-C. So the source must have Rp resistors of 22k and the device can have 5.1k resistors and both ends must be C type connectors (A to C won't work)

enter image description here Source: https://www.st.com/resource/en/technical_article/dm00496853-overview-of-usb-type-c-and-power-delivery-technologies-stmicroelectronics.pdf

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  • \$\begingroup\$ Thanks for the answer. So do you mean that as long as I have those 5.1k pull downs, and that the charger is a proper USBC device, I should get 1.5A ? Does that mean that the chargers I've heard about were not implementing the standard correctly? \$\endgroup\$
    – user325962
    Nov 18, 2022 at 18:08
  • \$\begingroup\$ Yes, summer if the chargers don't implement the standard properly. The charger has to have a controller on it, sometimes they cheap out and just do a voltage rail \$\endgroup\$
    – Voltage Spike ♦
    Nov 19, 2022 at 2:01

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