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Introduction

  • Who

  • What

    • A USB-C PD sink where the output voltage can be selected with resistors

      • DNP and fitted resistors

    • 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

USB-C Sink | Altium 365

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://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1339647/tps25730-excess-load-capacitance

https://en.wikipedia.org/wiki/Field-effect_transistor

https://www.robot-electronics.co.uk/i2c-tutorial

https://microchip.my.site.com/s/article/USB-Type-C-Layout-Recommendations#:~:text=USB%20Type%2DC%20CC%20Signals,traces%20with%2050%CE%A9%20impedance.

Engineering

PD Controller Selection

Main Options

There were a few possible options that were selected from research:

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, or similar. The TPS25730x includes this feature - however, its open drain output pins do not respond to regular USB power events (only those matching the USB-PD protocol).

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.

https://www.digikey.ca/en/products/detail/texas-instruments/TPS25730DREFR/22147394?s=N4IgTCBcDaIC4AcDOYCsB2AzABgCYgF0BfIA

Surge Protection

Inrush Current Protection

image-20241021-202116.png

Seeing as this board is primarily intended for debugging RPi Interface Rev C, which includes a buck converter with large bulk capacitance, inrush current protection is required, as recommended by USB-IF. This is due to the capacitors on the load device requesting a large amount of current, characterized by Icap = Ccap x dV/dt.

As shown above, the TPS25730 datasheet states that inrush current limiting is “implemented as described” in the USB3.2 specification. The exact section of the USB 3.2 spec regarding inrush currents is shown below.

image-20241024-193338.png

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. This is due to the possibility of a voltage surge when a cable with current actively running through it is unplugged, causing inductive ringing (as mentioned here), as well as any electrostatic discharge events. For example, the TVS2200 product is recommended for a 20V spec. The Altium schematic is configured with variants, which will change the TVS diode model to match the negotiated voltage.

image-20241024-204825.png

Power Configuration

Resistor Dividers

image-20241023-192833.png

Decoded ADC Values

image-20241023-193226.pngimage-20241023-193351.pngimage-20241023-201135.png

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 run the CAP_MIS pin high.

Schematic

image-20241029-160937.png

Rapid fire notes on the USB PD IC schematic:

  • TVS2200 for 20V model, clamping begins at 22V. TVS1400 for 12V model, clamping begins at 14V.

  • the LDO_3V3 output pin from the IC is linked to the GPIO pins (e.g. PLUG_FLIP), and can only supply max 5mA, with 1mA max per GPIO pin

  • I2C line is likely to be unused and will be DNP’ed during assembly. The PD IC is not V/I configurable through I2C as well.

image-20241029-181542.png

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