You don’t want to use the usual microUSB as the power connector in your design and want to use the new standard USB-C, but USB power delivery is complicated, the C connector has extra pins and you don’t know what to do with them. The quick answer: 5.1kΩ ±20% pull-down resistors to the ground on both CC1 and CC2 pins. If you want to know WHY, continue! The Micro USB was pushed over a decade ago to replace the charger hell caused by the phone manufacturers, and in 2022. the European Union decided to standardize the charging ports again, everything must have a USB-C charging port.
This is possible because the USB standard became a monster and continues to evolve, adding more and more features and complexity. Connect peripherals, get network connectivity, stream pixel data to monitors, and charge your equipment with 240W power, over a single connector? It’s today’s reality, and an engineer’s nightmare: if one device can do so many things, it must be a mess, and it is. But learn, how did we get here.
USB power history
The very first USB ports were simple: 5V with 100mA or 500mA limit, providing up to 2.5W. Did I say simple? This already included handshaking and negotiating power limit! This is where all the troubles began, it’s normal to discuss the possibilities with a port.
While 2.5W is enough for most mice and keyboards, it’s not enough for many other devices. This was increased to 900mA/4.5W and USB Battery Charging standard added 1.5A/7.5W mode.
The famous “short the data lines for extra power” comes from this USB BC standard, dumb chargers and power banks don’t want to implement power negotiation. So the standard defines: if the charger (not the charged device!) shorts D+ and D-, that’s the confirmation of providing 5V 1.5A without any further questions.
Something changed at this point: tablets and powerful phones wanted way faster charging modes, because 7.5W is not really much. The USB standard had to introduce a fast charging method.
USB Power Delivery (USB-PD)
While increasing the current limit could work, no one wants to carry 4 AWG welding cable for phone charging. All the other quick charge methods include a charging voltage greater than 5V. Because increasing the voltage increases the power too, so the wires don’t have to be thicker. USB PD introduced 9V, 15V and 20V in 2012. The preferred max current is around 3A to save cable thickness. The 20V mode increased this to 5A, providing up to 100W charging! To further increase this, 28V, 36V and even 48V was announced in 2021. to reach up to 240W.
Practically all smartphones (except iPhones and iPads with Lightning), tablets, and better laptops in 2022. already have USB-C charging port. Only high-end gaming and workstation laptops had to use custom, high-power power supplies, but 240W USB PD will surely replace them too. So, forget the microUSB connector for your new designs, all hail USB-C!
Compliance for your simple design
I admit, microUSB was easy to use in a circuit: connect the 5V and GND to your design, and that’s all! At least for devices plugged only in wall warts, because the maximum allowed capacitance is 10μF! Slapping a 1000μF capacitor on the Vbus already violates the standard, so I recommend using a 4.7μF as an input filter to leave headroom for your other decoupling capacitors scattered everywhere.
If you still insist to charge a 10F supercap, go ahead, use a power management IC and other fancy stuff. But we are focusing on the simplest possible design, the microUSB replacement. You just want a stable 5V power supply, up to 3A.
The CC (configuration channel) pins are used to tell the role of a port/device. Pull-up resistors sets a port as source and the supply current, pulldown goes in our gadgets. Because USB-C can be plugged in 2 different ways, both CC pins must have a resistor. If you are okay with the bare minimum, just use the 5.1k 20% pulldown resistors.
Simple power “negotiation”
If you want to know the source’s power capability, just ask the port, it’s eager to tell you! Luckily, no fancy messages are needed, just measure the voltage on your side. (well, it’s mandatory to monitor the voltage on the CC pins, at least for a source) In this case, 5.1k 10% is recommended for precision. The downstream (source) port can advertise the maximum current with the following:
|CC pins on the source side||Pulled to 5V||Pulled to 3.3V|
|500mA 2.0, 900mA 3.0 5V||56k||56k|
If you do stuff like this, you already figured out: this is a voltage divider. So, here is a chart with the voltages (without tolerance!):
|Voltage on CC pin||5V||3.3V|
|500mA 2.0, 900mA 3.0 5V||0.417V||0.275V|
Don’t go over 10μF, 5.1k on both CC pins, and check if the port can provide the high current, if needed!
So, now you can make a compliant gadget with USB-C for getting power!