We designed a power board that can deliver 5V and 3V3. Those two voltages are provided by two boost/buck converters that can deliver 3A each. The board accepts power from a USB-C socket which is connected to a cellphone charger that can output 3A with a USB-C cable.

Currently, the only pins I'm using on USB-C is GND and VBUS. What I'm realizing now is the USB-C chargers that are for cellphones are able to negotiate the AMPs provided to whatever is being charged but I didn't setup this negotiation on our power board. I'm wondering if I'm just getting the default 500mA, or would I get the full 3A from the wall charger?

I haven't tested this yet, and I will, but I'm more interested to know about the process of negotiating the current draw from the USB-C wall charger.

An example would be this charger which can output 5V 3A or 9V 2.3A: https://www.amazon.ca/Charger-2-Pack-Super-Charging-Samsung/dp/B096PHS41G/ref=sr_1_89?crid=JIHPL6OPSI97&keywords=usb-c+charger+3A&qid=1646953774&refinements=p_85%3A5690392011%2Cp_72%3A11192170011&rnid=11192166011&rps=1&sprefix=usb-c+charger+3a%2Caps%2C75&sr=8-89

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    \$\begingroup\$ without negotiation the output should be 5 V at maximum 500 mA ... it is unknown if the power supply from amazon conforms to that standard \$\endgroup\$
    – jsotola
    Commented Mar 11, 2022 at 0:27
  • \$\begingroup\$ USB ammeter/voltmeters are valuable for this sort of thing. Great for ease of use/transparency, I wouldn't rely on their accuracy though amazon.com/dp/B07PJHDQQC \$\endgroup\$
    – raaymaan
    Commented Mar 11, 2022 at 14:36
  • \$\begingroup\$ That is a feature of "USB power delivery" (USB-PD). USB-C is nothing but the type of socket. There is no "USB-C power negotiation", you better search for "USB-PD power negotiation". \$\endgroup\$
    – Sim Son
    Commented Mar 12, 2022 at 14:23
  • \$\begingroup\$ Oh i see, so it really depends on the cell phone charger I choose. If it has USB-PD then I really would need a chip to negotiate the power on my power board to get the max current I can get. If it doesn't have USB-PD and just simply outputs 5V 3AMPs then I'm fine and I will just get the max that is advertised. \$\endgroup\$ Commented Mar 13, 2022 at 15:09
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    \$\begingroup\$ I think you really need to talk voltage primarily, rather than amps. It's the voltage you are primarily negotiating; amps follow as a function of the needs of the device. The MAX amps may be limited, but not the 'actual' amps. \$\endgroup\$
    – Steerpike
    Commented Jul 3, 2022 at 0:53

1 Answer 1


For negotiating the power, USB C has an passive and an active standard

Passive method

With the passive standard, you use your microcontrollers ADC to read the voltage on both CC pins, and take the highest voltage (make sure to properly deal when VBUS is not yet active, so use a 22k resistor to prevent the microcontrollers ESD diodes from affecting the connection test from the charger)

This can be seen in Table 4-36 Voltage on Sink CC pins (Multiple Source Current Advertisements) in the USB C spec: https://www.usb.org/sites/default/files/USB%20Type-C%20Spec%20R2.0%20-%20August%202019.pdf

Detection Min voltage Max voltage Threshold
vRa -0.25V 0.15V 0.2V
vRd-Connect 0.25V 2.04V
vRd-USB 0.25V 0.61V 0.66V
vRd-1.5 0.70V 1.16V 1.23V
vRd-3 1.31V 2.04V

If the measured voltage falls in the vRd-USB range, use USB current detection (if the host is suspended, 100mA, else 500mA or higher

Note that an USB C changer is allowed to change the maximum current while it is active, you need to keep monitoring the pins and respond to it.

Active method

With this method, it starts out with the passive method at first, but the upstream facing device sends out an list of power profiles without a second after the connection is started over USB C PD (which is digital data over the CC wire). You can then pick one of the power profiles and the upstream device will adapt the output voltage to the specified profile. You are able able to say that you do not receive enough power, in which case the upstream device will ask you how much you need before seeing what is possible to get. In most cases, it is easier to use a dedicated USB C controller chip to handle this communucations

  • \$\begingroup\$ So, in theory, if you knew for certain that the upstream device supported 20v, you could supply that voltage without negotiation and it would not harm the upstream device? (I understand this is bad practice, and extremely a hack, but I have a closed example where this hack would simplify a lot of complexity for me) \$\endgroup\$ Commented Jan 14, 2023 at 17:20
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    \$\begingroup\$ Some upstream facing devices may accept it, others do not. Also note that the inrush current at 20V is larger, which may trip protection circuity \$\endgroup\$
    – Ferrybig
    Commented Jan 14, 2023 at 21:08

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