Your explanation is a bit off: a Type-C UFP (power sink) always has 5.1kΩ pulldowns on the two CC pins (separate pulldown per CC pin, do not tie them together). The UFP does not get to advertise its current requirements using Type-C negotiation, the DFP (power source) advertises how much you’re allowed to draw from VBUS and you’re required to comply with that. The DFP may also change its current advertisement at any time and you’re required to comply with such changes within 60ms.
The DFP advertises its VBUS current capability by varying the pull-up on its CC pins:
- 5V 3A capability is advertised by sourcing 330 μA (±8%) into each CC pin (e.g. using 10 kΩ pullup to 5V or 4.7 kΩ pullup to 3.3V)
- 5V 1.5A capability is advertised by sourcing 180 μA (±8%) into each CC pin (e.g. using 22 kΩ pullup to 5V or 12 kΩ pullup to 3.3V)
- “Default USB Power” is advertised by sourcing 80 μA (±20%) into CC (e.g. using 56 kΩ pullup to 5V or 36 kΩ pullup to 3.3V)
Default USB Power means:
- If you connect as USB device then the rules of the applicable USB specification must be followed.
- If a charger is detected using the D+/D- lines as per the USB Battery Charging 1.2 standard then you may draw up to 1.5 A but the voltage may drop as a result, see that spec for details. This allows for detecting non-Type-C chargers, although Type-C chargers are also still required to support this.
- Otherwise, you may draw up 500 mA from VBUS.
To determine the current advertised by the DFP you measure the voltage on the CC pin (i.e. across the 5.1kΩ resistor):
- below 0.20V means no source is connected
- between 0.20V and 0.66V means you’re limited to Default USB power as described above.
- between 0.66V and 1.23V means you’re allowed to draw up to 1.5A
- above 1.23V means you’re allowed to draw up to 3A
Beware that only one of the two CC pins will get pulled up (this is used to detect cable orientation) so you have to monitor both CC pins.
If your device cannot reduce its current consumption below what’s advertised by the DFP then you should simply power off (e.g. using a power switch on VBUS) to avoid overloading the DFP.
Finally, USB Power Delivery (PD) negotiation overrules everything and is the also only way to be allowed to draw more than 3A and/or receive a higher VBUS voltage than 5V.