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USB Type-C For Electronic Waste Reduction

Image Source: stokkete/stock.adobe.com

By Mohamed SAADNA, Technical Marketing Engineer at STMicroelectronics

Published January 19, 2023

"With half a billion chargers for portable devices shipped in Europe each year, generating 11,000 to 13,000 tonnes of e-waste, a single charger for mobile phones and other small and medium electronic devices would benefit everyone. It will help the environment, further help the re-use of old electronics, save money, and reduce unnecessary costs and inconvenience for both businesses and consumers."

These words are from Alex Agius Saliba, from the European Parliament, proposing the USB Type-C connector as a new harmonized standard to charge electronic devices and unbundling the power adapter for these devices.

There basically are three ways to implement USB-C to a design, depending on the power requirements of the power sinking device and following the latest specification of the USB Type-C Power Delivery:

1. Using a 5V constant voltage up to 15W at 3A maximum: in this case, the Power delivery protocol is not used, but some protections may still be required to protect downstream electronic circuits. This is the simplest and quickest way to "feel" the first benefits of this powerful standard.
2. Using any voltage between 5V and 20V at a maximum of 5A: here is where the Power Delivery protocol brings fast charging benefits as well as less thermal dissipation in the charging device if Programmable Power Supply is used [3]. This power range is called SPR: Standard Power Range.
3. The Extended Power Range "EPR" is the latest evolution of the USB-C Power Delivery standard, offering up to 240W (48V at 5A max).

After selecting the power profiles for the Power Sinking Device, the designer chooses the compliant interface circuit.

Figure 1: The USB-C connector pinout (Image source: STMicroelectronics)

This interface circuitry will focus on two main roles: ESD protection and USB-C PD compliancy.

ESD protection

First, it will have to protect against electrostatic discharges (ESD) to comply with the IEC61000-4-2 Level 4 testing. ESD protection is required for all pins where an external electrostatic discharge can be applied from the connector.

This is typically the case for CC1 and CC2 pins as well as D+ and D- pins. This ESD protection device will be ideally placed close to the USB-C connector. Choosing a 2-line device will help to minimize PCB space consumption.

For the VBUS pin, the surge protection device has to be sized much stronger as the surge waveform can be longer than an ESD discharge. In this case, a TVS specified according to 8/20µs or 10/1000µs surge waveform is preferred to avoid Electrical Over Stress to the downstream circuitry. Also, the VBUS path capacitive value should be included between 1µF and 10µF for a USB Type-C power delivery sink design.

Suppose the power sinking device embeds RF connectivity. In that case, it makes sense to improve the sensitivity of the RF receiver by adding a Common Mode Filter to the ESD protection of the D+, D- lines or SuperSpeed lines that are more likely to generate Common mode noise within the RF receiver frequencies (typically 2.4GHz or 5GHz used by Bluetooth or WiFi antennas).

On top of these basic EMC components, specific protection components for USB-C are required for VBUS and CC lines, whatever the power profile of the circuit.

Let's start with VBUS: there have been many videos around showing how the first products using improperly designed USB-C products were dramatically damaged due to faulty power adapters or USB-C cables. Unfortunately, this is likely to happen again as USB-C will gain popularity after the EU mandate. Therefore, a power sinking device must protect itself against a defective source or cable that can apply a VBUS voltage higher than negotiated, whatever the power profile of the application: 5V or any power profile from the Power delivery protocol. Against this hazard, the most robust solution is to add an Over Voltage Protection (OVP) that will trigger at a threshold ideally being set using a voltage divider resistor bridge: whatever the faulty voltage on VBUS, this ensures the integrity of the electronic device.

CC lines are usually rated up to 6V DC maximum voltage. A short-to-VBUS event happening when removing the USB-C plug from its receptacle has been frequently observed with the USB-C connector. This is due to the connector's small pitch (0.5 mm), which could apply the VBUS voltage to the CC line when twisting the plug in the receptacle. Here again, a common best practice is to place an Over Voltage Protection on the CC lines. Also, if the USB-C Power Delivery protocol in SPR or EPR modes is necessary, the standard asks to add EMI capacitors on CC lines with values specified between 200pF and 600pF.

USB-C PD compliancy

Second, the interface circuitry must ensure functional compliance with the USB-C Power Delivery specification. Let's focus on functional compliance rather than voltage and timing levels defined in the specification that can be found in any integrated circuit datasheet.

What's new for the designer here is the so-called "dead-battery" mode that allows using the fast-charging protocol when the power sinking device is fully depleted.

How does it work? Dead battery behavior is basically a pull-down (Rd) or a voltage clamp when a USB Type-C source voltage is applied to both CC lines. It is interpreted as a request by the sink to receive a VBUS voltage at 5V. The source then powers up the sink, and it can run the power delivery protocol to advertise its sink power profile that will allow fast charging.

Now that these essential functions are clarified, there are two kinds of circuitry to consider to build a USB Type-C Power Sinking device. The choice of implementation directly impacts the cost of the device. The cost of the USB-C connector and its complex circuitry has often been the critical limiting factor in expanding this solution in battery-operated devices.

The entire hardware solution consists of integrated circuits that will implement a USB-C power delivery controller and all the high-voltage controls (OVP for a sink, OCP for a source) for all the USB-C pins… and there are many. These ICs were the first answer (legacy) from different suppliers when the USB-C connector was mainly populated in laptops or desktops. They are usually a one-fit-all solution that cannot be recommended for cost-sensitive devices. Lower-cost ICs exist, but they lack full compliance with the latest evolution of the USB-C Power Delivery specification (like PPS) or protection features (like OVP on VBUS). Also, the cost is not yet optimized due to the high voltage silicon technology used in these ICs, which is not ideal for logic integration of the power delivery protocol.

Figure 2: Moving from full-hardware architecture to a cost-effective, MCU-based solution (Image source: STMicroelectronics)

Another solution consists of moving these pure logic functions into an IC already present in any embedded device: the MCU. Indeed, the USB Type-C Power Delivery (UCPD) protocol will have a more optimized cost inside an MCU rather than inside a high-voltage IC that will not feature the cost advantage of an MCU using thinner lithography. Hence, the high-voltage controls of the VBUS path and CC lines protections can be integrated in another much smaller device, ideally tailored for power sinking devices that can be used as a Type-C Port Protection (TCPP). For this device, there is no need to feature the exhaustive functions of the USB type-C Power Delivery if the standard's basic requirements are met.

Thanks to the latter solution, new applications can benefit from the fast-charging capability without sacrificing cost.

Let's review in detail this second solution in two steps, first focusing on the MCU that features the UCPD and then on the TCPP.

Figure 3: STM32+TCPP functions for a turnkey USB-C Power Delivery solution (Image source: STMicroelectronics)

Deploying the full features of the USB Type-C Power Delivery may require expertise in different areas, such as wired connectivity, power management, data communication and authentication. STMicroelectronics STM32 microcontrollers are compliant with the latest USB PD r3.1 specifications. They simplify the deployment of USB PD in embedded systems for state-of-the-art application functionality. The latest STM32 MCU series, such as the STM32G0, STM32G4, STM32L5, and STM32U5 series, have a built-in certified USB PD controller (UCPD). STM32G0 is the only family with built-in 2 UCPD controllers, optimizing the cost of applications featuring a dual USB-C connector. The UCPD peripheral can now be considered a standard peripheral, just like I2C or ADC, and will be featured in the new STM32 series in the future.

When the USB-C PD is not used, any MCU can be used as long as CC lines are pulled-down with a correct value (typically 5.1k for CC1 and CC2 lines; both lines must not be connected).

But to enjoy the fast-charging benefit of the USB-C PD, an MCU with UCPD with its companion chip TCPP to handle the high-voltage controls and protections is the best choice. For battery-powered devices, which are typically power-sinking devices, the STMicroelectronics TCPP01-M12 integrates the gate driver for an external N-Channel MOSFET for VBUS overvoltage protection with an externally adjustable OVP threshold using a voltage divider bridge. Also, it features system-level IEC61000-4-2 Level 4 ESD protection on CC lines to guarantee up to +8kV contact discharge applied on the connectors pins. Also, two integrated FETs on these CC lines will protect them from the short to VBUS event. Both devices have been designed to work together flawlessly during a dead battery event: TCPP01-M12 is advertising its own Rd (dead battery) resistors on CC lines when the STM32 is off and removes them when the STM32 is powered-up again after the source reads the voltage clamp on the CC lines and apply 5V on VBUS. In normal conditions, the TCPP01-M12 will be powered up by the STM32 only when a USB-C cable attachment is detected to maximize the battery lifetime. These are the kind of tricks difficult to achieve when using alternative solutions.

The main benefit of using the combo chips STM32+TCPP01-M12 is the availability of a Nucleo expansion board X-NUCLEO-SNK1M1 to experiment with all these features and feel the real power of the Power Delivery protocol. It is good to know that this board and its free software example code X-CUBE-TCPP have been certified by the USB-IF under the TID: 5205.

Outlook

Are you not designing a power-sinking device? STMicroelectronics has released an entire series of TCPP products according to the use case (Source with TCPP02-M18, Dual Role Power with TCPP03-M20), and for each, an affordable Nucleo expansion board: X-NUCLEO-SRC1M1 for TCPP02-M18 and X-NUCLEO-DRP1M1 for TCPP03-M20. The software example code is available for free download for each of them: X-CUBE-TCPP.

Figure 4: Hardware and Software tools facilitating USB-C Power Delivery solution development (Image source: STMicroelectronics)

For each expansion hardware board cited above, there is plenty of documentation to quickly start a USB-C PD project, such as a Quick Start Guide, a Databrief, a User Manual, and a complete list of Application Notes. Time being precious, you may consider the AN5418: How to build a simple USB-PD sink application with STM32CubeMX and the AN5225: USB Type-C Power Delivery using STM32 MCUs and MPUs to quickly become an expert on USB-C Power Delivery using STM32 MCUs.