Introduction
Who
Daniel Puratich Architecture & Management
Kenny Na EE Designer, Documentation
What
A USB-C PD sink where the output voltage can be selected with resistors
DNP and fitted resistors, should be foolproof 0805s
Intent for requesting 20V 65W when debugging https://uwarg-docs.atlassian.net/wiki/spaces/EL/pages/2701197313/RPi+Interface+Rev+C?atl_f=content-tree setup without the full drone, but should support 12V and 5V as well for testing other boards
Use a USB-C PD chip and the associated required FETs that are required for negotiation
Ideally does not require a microcontroller, due to cost constraints
USB-C input connector / XT30 output connector
don't worry about board area too much but it can be tiny
no buck converter
Resources
https://www.youtube.com/watch?v=W13HNsoHj7A&t=615s
https://hackaday.io/project/192576-picopd-usb-c-pd-30-pps-trigger-with-rp2040
https://www.ti.com/interface/usb/type-c-and-power-delivery/products.html#1241=PD%20controller&
TI | ESD and Surge Protection for USB Interfaces
https://www.usb.org/usb-charger-pd
Wikipedia | USB-C Debug Accessory Mode
https://en.wikipedia.org/wiki/Field-effect_transistor
Engineering
PD Controller Selection
Main Options
There were a few possible options that were selected from research:
https://www.digikey.ca/en/products/detail/stmicroelectronics/STUSB4500QTR/9092189
https://www.digikey.ca/en/products/detail/texas-instruments/TPS25730SRSMR/22147461/https://www.digikey.ca/en/products/detail/texas-instruments/TPS25730DREFR/22147394?s=N4IgTCBcDaIC4AcDOYCsB2AzABgCYgF0BfIA
https://www.digikey.ca/en/products/detail/infineon-technologies/CYPD3177-24LQXQT/10238323
Cheap PMIC + RP2040 (ex. https://www.digikey.ca/en/products/detail/onsemi/FUSB302B10MPX/7356099 + https://www.digikey.ca/en/products/detail/raspberry-pi/SC0914-13/14306010?s=N4IgTCBcDaIEoAUwAYAsyQF0C%2BQ)
Decision Matrix
USB-C PD PMIC | Option 1 | Option 2 | Option 3 | Option 4 |
|---|---|---|---|---|
Name | STUSB4500 | TPS25730x | CYPD3177 | FUSB302B + RP2040 |
Description | USB-C PD Sink IC | USB-C PD Sink IC | USB-C PD Sink IC | USB PD IC + MCU |
DigiKey $/ct | $5.22 CAD | $3.76 CAD | $3.51 CAD | $2.59 + $1.08 CAD |
Configuration | NVM config editing in EEPROM via I2C TPs | Strapping resistors on ADC pins | Strapping resistors on pins | Firmware control on RP2040 via I2C |
Misc. Pros | Popular and well documented USB-C sink device within hobbyist community | Newest controller, supports USB-C PD rev 3.1, simple to implement, extensive schem and layout guidelines, D-model has integrated FET gate | Cheapest option, simple implementation, some online examples | Most configurable option, no need for strapping resistor variants, completely firmware controlled via I2C |
Misc. Cons | Expensive | PD 3.1 is irrelevant for the purpose of this project, also doesn’t support 240W | Complicated implementation |
The TPS25730x was selected for its recent release, comprehensive datasheet, relatively low price, and simple implementation.
FET Selection
A typical USB-C PD controller will require a gate to block the USB-C input voltage while arbitration occurs. Once the negotiation is successful, the controller sends voltage to the gate to enable the transport of power.
In the case that PD negotiation fails, there is sometimes a fallback “safe power” rail. These might typically supply 5V @ 900mA. The TPS25730x does not include this feature.
The TPS25730S recommends the https://www.digikey.ca/en/products/detail/texas-instruments/CSD87501L/5126233, while the TPS25730D comes with an integrated gate in the package. Given the small price delta between the S and D models, the TPS25730D is a practical choice, and very simply concludes our FET selection.
Surge Protection
Inrush Current Protection
Seeing as this board is primarily intended for debugging RPi Interface Rev C, which includes a buck converter with large bulk capacitance, we will need to implement surge protection, as recommended by TI.
This is due to the capacitors on the load device requesting a large amount of current, characterized by Icap = Ccap x dv/dt.
The surge current limiting section of the USB3.2 specification is shown below.
Input Voltage Protection
While the source device should do most of the voltage regulation, TI recommends a transient-voltage suppressing diode be implemented on the VBUS_IN rail. The TVS2200 product is recommended within the PMIC’s datasheet.
TI additionally recommends a (Schottky) diode be placed from VBUS to ground. This is due to the possibility of large ground currents during sudden disconnects due to inductive effects in a cable.
Power Configuration
Resistor Dividers
Decoded ADC Values
If the source device cannot supply voltage between the minimum and maximum voltages set by pins ADCIN1 and ADCIN2 OR the operating/maximum current is violated, then the controller will signal a capabilities mismatch and (assumedly) run the CAP_MIS pin high.