USB 3.0 Circuit Protection Considerations
The three most common protection issues for USB 3.0 circuit designs are related to: over-current protection, over-voltage protection and electrostatic discharge (ESD) protection. Hot plugging / insertion and short circuit events should also be taken into account when developing USB products.
This article is intended as an introduction to USB protection methods – it explores each of these and discusses methods to help protect against circuit damage. More in-depth information, application notes, design examples and product specifications can be found in the links at the end of this article.
It’s important to note that the methods, and protection devices, used for USB 2.0 designs are not sufficient for USB 3.0 applications. However, protection solutions for USB 3.0 are backward compatible to USB 2.0 designs. The USB 3.0 protocol provides an additional four data channels to support 5 Gbps data rates, and the maximum current per port has been increased to 900 mA in order to work with more complex devices. The table below calls out the key parameters for USB 2.0 and 3.0.
|Parameter||USB 3.0||USB 2.0|
|Operating Voltage (V)||4.45 – 5.25||4.4 – 5.25|
|Max. Operating Current – Low Power Port (mA)||150||100|
|Max. Operating Current – High Power Port (mA)||900||500|
|Total Data Line Capacitance (pF)||< 1||< 10|
|Max. Data Rate||5 Gbps||480 Mbps|
Additionally, as IC fabrication technology improves, the physical geometries of the transistors continue to shrink. There is a direct correlation between transistor size, gate oxide thickness and substrate thickness to the magnitude of voltage transients a device can withstand before incurring a short term fault or long term damage. All of these factors combine to make circuit protection for USB 3.0 devices both more critical and more complicated.
The USB 3.0 protection considerations can be divided into two categories – one for power ports with charging capabilities and a second for data ports. It’s vital that any protection implementations must not interfere with the actual data transmissions, limit the data throughput for the port or restrict the power supply.
Over Current Protection:
The USB specification requires that over current limiting mechanisms must be resettable without user mechanical intervention. This restricts the types of protection devices to resettable solid state fuses, silicon switches and Polymer Positive Temperature Coefficient (PPTC) devices.
On the VBUS port, it’s necessary to limit the amount of current sourced to an attached device in the event of a short circuit condition. A PPTC device can be used to prevent in-rush current which may damage the device.
It’s generally recommended that the power carrying traces on the PCB be constructed to handled a minimum of 150% to 200% of the maximum specified current draw. This is so that the physical trace does not become a fusing element, which would be a permanent system fault. It is also important to select an over current protection device which has a low on resistance to ensure a low voltage drop through the protection circuit.
Over Voltage Protection:
This relates to both transient and sustained voltages on either or both the power line(VBUS) and the ground line (GND), rather than the data lines. Overvoltage events can occur for a number of reasons including hot removal / insertion of a device, poorly regulated power supplies and even user error.
Silicon diodes, polymer fuses and ceramic devices can be used to protect these lines from over voltage conditions. In some instances, a simple clamping diode is sometimes sufficient.
The most prevalent failure mechanism for USB devices results from electro static discharge (ESD) in which a high voltage spike is introduced into the circuitry. This spike can come from a direct contact or through a small air gap, and can exceed many kV. Given the portable nature of most USB devices, these were developed for hot plugging / insertion as well as removal from a powered connection.
There is also a significant amount of human body contact while handling, transporting and attaching USB peripheral devices to systems. It’s very likely that a user will, on one or more occasions in the lifetime of a device, touch the exposed pins of the connector or a cable attached to a USB port.
As with the previous methods to mitigate unwanted effects on the data lines, the ESD protection solution should not affect the underlying circuit operation. Data throughput and signal levels need to be maintained after the addition of the protection circuitry.
One of the most common, and inexpensive, solutions is to use ESD suppressors connecting each individual data line to the ground plane. These components can be silicon diodes, TVS diodes, multi-layer varistors (MLV) and polymer resettable fuses.
Hot Insertion and Removal:
Hot insertion and removal of a USB device can also create a start-up in-rush current condition as well as an over voltage condition. The techniques described above should be used to mitigate unwanted effects from ordinary “plug-and-play” usage.
In general, a number of these methods will be used in combination for a complete system protection at the interface port. Many suppliers offer devices which can protect lines individually or in groups (e.g., diode arrays). This comprehensive protection will ensure robustness of the device as well as provide for an extended product life. In many cases, these supplementary protection circuits will be necessary for a product to meet all the necessary USB standards and safety requirements.
Where To Find Protection Devices And Additional Information:
A number of companies supply USB protection devices, including:
(Image Credit – TE Connectivity)
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