Optimization of USB 3.0 Current Limiting Solution

Need for speed. Need for power. As the USB 3.0 specification was finalized last November, not only did the transfer speed increase to a blazing 5.0 Gbps, the power budget was also boosted from 500mA to 900mA. The increase in speed and power brings new challenges to both system and component level, from integrating new data transfer protocol and new power management schemes, to successfully transferring data between host and device. Among those upgrades in USB 3.0, one critical thing remains unchanged is the safety requirement. System builders now need to allocate enough current supply for these ports, and at the same time carefully incorporate the current-limiting function.

Polytronics offers a complete PPTC product portfolio to help customers to meet their design requirements and USB 3.0 over-current protection at the same time. The small footprint SMD 1206 (0.12” x 0.06”) and 1210 (0.12” x 0.10”) series save precious board space and offer single or dual port protection. Although USB 3.0 lowers the supplied voltage requirement from hosts, Polytronics still recommends the following products in Table 2, which guarantee PPTC voltage drop of less than 0.1V under full load, to ensure greater compatibility with USB 2.0 peripherals and give greater voltage margins to other on-board components and trace. In addition, these current limiting devices can hold full load current for high-power device above 50°C, avoid unwanted trip conditions due to thermal derating especially on the rear USB ports of desktop PCs.

has developed new super-low-resistance PPTC material platforms (low-rho) which overcomes the oxidation issue from nickel powder systems. Breaking the traditional carbon-black volume resistivity limit, the new surface-mount SLR series can achieve high holding current with low profile of less than 0.75mm. In addition to battery PCM application, it is also ideal for over-current protection in cellular phones, mobile internet devices, or other ultra-slim electronics. The SMD SLR series can protect up to 2 USB 3.0 ports in 1206 size or 4 USB 3.0 ports in 1812 size.

With advanced research in polymeric technology, Polytronics

Polytronics continues to provide circuit protection products to meet the new technology requirement and market demand. Together with over-voltage protection and thermal management solutions, Polytronics provides a total protective solution to customers. A multichannel polymeric ESD suppressor is also targeted to launch in Q3 2009 to protect USB 3.0 differential signal pins.

Polymeric Positive Temperature Coefficient, commonly known as PPTC, can be treated as a non-linear temperature-dependant resistor. Under normal operation, PPTC is highly conductive so the circuit can operate normally. However, if an over-current event occurs, faulty current will generate enough heat on PPTC to exceed its switching temperature, causing a 104 to 106 times resistance jump. Therefore, the over-current condition can be eliminated, and the objective of protecting the circuit device can be achieved.

The USB 3.0 specification requires current limiting device for power protection, and PPTC devices or low-voltage solid-state switches are often used for over-current limiting. Most of the time design engineers prefer to use PPTC to prevent the circuit from damage for various reasons. Due to the increase of power budget in USB 3.0, current limiting devices must pass through more power and keep a voltage drop limit. Table 1 shows PPTC having more advantages than low-voltage solid-state switches in USB 3.0 application, such as price, resistance, maximum fault power, and ESD sensitivity.

Compared to USB 2.0, USB 3.0 creates new power management schemes to achieve overall platform power efficiency, by defining new link states and mechanisms. However, in terms of power distribution, 3.0 remains similar to 2.0, with increased power budget and more generous voltage drop requirement. SuperSpeed devices can now draw up to 900mA after initiation with the host. The supplied voltage requirement at the connector of hub or root ports has been decreased from 4.75V to 4.45V,

and bus-powered devices should be capable of operating with input voltage as low as 4.00V. Other requirements such as inrush current limiting and suspend current limitation stay as stringent as before, with updated power allocation of 150mA or 900mA for high-power device.

PPTC Introduction and Over-Current Protection

Power Delivery Di�erence from USB 2.0

USB 3.0 Over-Current Protection

Low Profile, Super Low Resistance SMD Series

Total Protective Solution

PPTC Solid-State Switch

PriceResettableFault Current LimitingFault Power LimitingESD SensitivityTime-to-Trip (LPS)Resistance

LowYesHigh (>100A)High (800W)N/AMillisecondLow (Typical 100±20mΩ)

MidYesLowLowHighMicrosecondMid (Typical 200±20mΩ)

Coming Soon

1max 0.12RSMD1206P150TFT

R1max 0.110Ω SMD1210P150TFT

R1max 0.100Ω SMD1206P110SLR

R1max 0.050Ω SMD1210P260TF†

R1max 0.055Ω SMD1206P200SLR

- - R1max 0.018Ω

SMD1812P370SLR

Single Port

Dual Port

Quad Port

SMD 1206 Series(0.12”x0.06”)

SMD 1210 Series(0.12”x0.10”)

SMD SLR Series(Profile < 0.75mm)

In addition to transfer speed enhancement in USB 3.0, the requirement for power supply is also increased to meet various peripheral demands. The article first introduces Polymeric Positive Temperature Coefficient (PPTC), which is an over-current protection device often used in the industry, and compares it with low-voltage solid-state switch under USB 3.0 application. Regarding to the power delivery in USB 3.0, difference from USB 2.0 is explained and detailed PPTC devices are suggested for over-current protection. Finally, benefiting from the new development in conductive filler, the new low profile, super low resistance surface-mount PPTC is launched which is ideal for over-current protection in ultra-slim electronics.

Table 1: PPTC and solid-state switch comparison in 1A application.

No. 24-1 Industry E. Rd. IV, Hsinchu Science Park, Hsinchu 300 TaiwanTEL : +886 (3) 564-3931 FAX : +886 (3) 564-4624E-mail : sales@pttc.com.tw http://www.pttc.com.tw

Table 2: USB 3.0 over-current protection solution.*R1max: Maximum resistance of device at 20°C measured one hour after tripping or reflow soldering.