USB Type-C: Consumer Convenience Comes with Design Challenges


Robert Heinzelmann at Toshiba Electronics Europe GmbH

It has been 20 years since Universal Serial Bus (USB) made its debut on an Apple iMac G3 computer, beginning a fundamental change in the way that computers and peripherals connect. During that time the standard has evolved through various iterations to the latest version — USB Type-C — that, again, promises to make fundamental changes to the way we connect computers and peripheral devices.

 

While USB-C is relatively new, indications are promising that there will be significant uptake and sales for the new standard. Market research firm IHS is forecasting that 500 million devices with USB-C will ship this year, increasing rapidly to 2 billion devices in 2019.

The consumer benefits of USB-C include high-speed data transfer, more power available for charging and powering devices and flippable connectors, making plugging simpler than ever before. In this technical article, Toshiba Electronics Europe will discuss how delivering consumer convenience brings challenges for designers and look at some of the solutions reaching the market.

USB Type-C provides a small, versatile connection scheme based around a bidirectional cable and an unkeyed flippable connector that connects to hosts and peripherals, replacing old-style Type-A and Type-B connectors and cables. Data rates of 10Gbps match the SuperSpeed USB 3.1 and the USB Power Delivery (USB PD) specification is incorporated, increasing power capability to 100W. However, these advances require careful selection of key components for power and data switching as well as interface protection.

 

Increased Power to Charge Devices Simply

USB PD allows devices to act as either a source or a consumer of power via the USB connection. It specifies profiles at 10W, 18W, 36W, 60W and 100W that operate at voltages between 5V and 20V with currents up to 5A. Devices that require power negotiate for power after the connection has started up in the default 5V/2A mode, thereby ensuring the safety of older USB2.0 devices.

 

USB PD spec contains power profiles up to 100W

Figure 1. USB PD spec contains power profiles up to 100W

 

USB PD allows for faster charging of peripherals and allowing devices such as smartphones, tablets, and laptops to be powered from their USB connection, reducing the need for mains power adapters. A mains monitor can power a laptop via USB-C, act as a hub for peripherals such as external HDDs and also receive and display video information from the laptop. In this scenario, only the monitor requires an AC-DC power adapter, removing cost and bulk from the set-up.

 

High-Speed Data and Flexibility

In USB-C, a single style of plug and socket containing 24 pins on a 0.5mm pitch in an 8.4mm x 2.6mm form factor is defined meaning that nondirectional cables can be used, allowing power and data to flow in either direction.

The USB-C plug is un-polarized and can be inserted either way. In addition, the physical layer interface for USB-C contains two data pairs (D+/D-), maintaining backward compatibility with USB 2.0, while supporting the 10Gbps rates for USB 3.0 and USB 3.1 and providing a 20Gbps capability for the future.

 

USB Type-C connector pin designations

Figure 2. USB Type-C connector pin designations

 

A USB-C cable can accommodate legacy USB type-A/B, Mini-USB, and Micro-USB connectors via an adapter as the new standard is electrically compatible with USB 2.0 while catering for USB 3.1 and PD specifications. Additional Configuration Channel (CC) pins deter- mine cable orientation and allow for PD negotiation.

Via alternate modes, standards such as DisplayPort, HDMI, MHL, Audio, and Thunderbolt can all be delivered through USB-C allowing a single cable type to be used for multiple purposes.

 

Challenges within USB-C Design

There are three principal areas of improvement within USB-C, each of which brings design challenges:

  1. The inclusion of USB PD requires power switches capable of withstanding at least 20V while having low on-resistance to minimize power loss and thermal load.
  2. High-speed transceivers are susceptible to electrostatic discharge (ESD), requiring external protection on data lines often accomplished with Transient Voltage Suppression (TVS) diodes. However, the extreme 10Gbps data speed means a trade-off between TVS terminal capacitance that avoids data distortion and the available ESD protection level is needed.
  3. The new reversible connector interface requires switching devices to ensure that the data flow is mapped according to the connector insertion while maintaining high-speed data transmission

 

Solving the Power Delivery Challenge

A simple dual-MOSFET power switch can be implemented with P-channel MOSFETs such as Toshiba’s SSM6J50xNU series with +20/-25V VGSS eliminating the need for a driver IC. The devices can be used in any of the USB PD profiles and offer RDS(on) as low as 19mΩ, ensuring efficient operation with minimal heat generation and voltage drop.

 

Simple dual-MOSFET power switch solution

Figure 3. Simple dual-MOSFET power switch solution

 

Alternatively, an integrated load switch such as Toshiba’s TCK30xG 28V device can be used. This is a very compact solution with built-in protection features including integrated thermal shutdown, under- voltage lockout, adjustable overcurrent protection, and overvoltage protection.
In addition to the low on-resistance of the switch IC valuable features in USB-C applications are integrated. This includes adjustable overcurrent protection up to 3.0A (set by a single external resistor), and adjustable slew rate (determined by an external capacitor). This combination of capabilities makes the TCK30xG family the most compact solution for USB charging up to Profile 3.

The most efficient power solution is to use discrete N-channel MOS- FETs with an external driver IC. Toshiba’s TCK40xG family of driver ICs can withstand 40V and has a built-in charge pump to provide the gate drive voltage for an external N-channel MOSFET. This external MOSFET can offer the lowest RDS(on) in a small package, like the SSM6K513NU, which is rated at only 8mΩ in a 2x2mm package. In tests, this setup ran 16°C cooler than that of a competitor’s approach when simulating Profile 5 (100W @ 5A).

In terms of space, the dual N-channel MOSFET plus TCK40xG solution requires 15mm2 while the dual P-channel MOSFET driven from the open drain output of the charger IC requires 10mm2. The integrated TCK35xG solution is significantly smaller at 2.3mm2 but does offer reduced performance.

 

Protection and Switching Solutions

In addition to devices for efficient power switching solutions, power lines also require protection, which can be realised with Toshiba’s new DF2SxxP TVS diode series, The diodes in this line-up are capable of protecting the VBUS lines from surge as they provide contact discharge protection up to ±30kV in a compact packages and allow for VBUS voltages between 5-20V, making them an ideal choice for all profiles.

 

Highly efficient USB Type-C power solution using a controller and discrete MOSFETs

Figure 4. Highly efficient USB Type-C power solution using a controller and discrete MOSFETs


Furthermore, when protecting data lines it is important to meet IEC-61000-4-2 while not unduly distorting data signals. This can be achieved with TVS diodes such as the single DF2B5M4SL (0.2pF) or the DF5G5M4N array that can provide protection for two signal pairs (RX1+/RX1-, TX1+/TX1-).

A bus switch such as the TC7PC13212MT is ideal for switching the data path to match connector orientation and will maintain full data integrity up to 10Gbps.

 

Summary

USB-C is clearly a significant step forward in convenience for users with additional power available and increased cable simplicity. However, this brings challenges for designers who have to cope with increased currents, higher voltages, and switching data paths.

Fortunately, Toshiba offers a variety of products to ease this task including several options for power management, including fully integrated designs that maximize simplicity and space saving as well as discrete designs for the ultimate efficiency.

Alongside this, bus switches allow easy management of data paths while advanced TVS diodes protect data lines from ESD without introducing signal distortion.

More information: Toshiba Electronics Europe GmbH    Source: Bodo's Power Systems, July 2018

 

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