TS3USB30

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SCDS237F – AUGUST 2007 – REVISED AUGUST 2015

10 Power Supply Recommendations

Power to the device is supplied through the VCC pin and should follow the USB 1.0, 1.1, and 2.0 standards. TI

recommends placing a bypass capacitor as close as possible to the supply pin VCC to help smooth out lower

frequency noise to provide better load regulation across the frequency spectrum.

11 Layout

11.1 Layout Guidelines

Place supply bypass capacitors as close to VCC pin as possible and avoid placing the bypass caps near the D+

and D– traces.

The high speed D+ and D– traces should always be matched lengths and must be no more than 4 inches;

otherwise, the eye diagram performance may be degraded. A high-speed USB connection is made through a

shielded, twisted pair cable with a differential characteristic impedance. In layout, the impedance of D+ and D–

traces should match the cable characteristic differential impedance for optimal performance.

Route the high-speed USB signals using a minimum of vias and corners which will reduce signal reflections and

impedance changes. When a via must be used, increase the clearance size around it to minimize its

capacitance. Each via introduces discontinuities in the signal’s transmission line and increases the chance of

picking up interference from the other layers of the board. Be careful when designing test points on twisted pair

lines; through-hole pins are not recommended.

When it becomes necessary to turn 90°, use two 45° turns or an arc instead of making a single 90° turn. This

reduces reflections on the signal traces by minimizing impedance discontinuities.

Do not route USB traces under or near crystals, oscillators, clock signal generators, switching regulators,

mounting holes, magnetic devices or ICs that use or duplicate clock signals.

Avoid stubs on the high-speed USB signals because they cause signal reflections. If a stub is unavoidable, then

the stub should be less than 200 mm.

Route all high-speed USB signal traces over continuous planes (VCC or GND), with no interruptions.

Avoid crossing over anti-etch, commonly found with plane splits.

Due to high frequencies associated with the USB, a printed circuit board with at least four layers is

recommended: two signal layers separated by a ground and power layer. The majority of signal traces should run

on a single layer, preferably Signal 1. Immediately next to this layer should be the GND plane, which is solid with

no cuts. Avoid running signal traces across a split in the ground or power plane. When running across split

planes is unavoidable, sufficient decoupling must be used. Minimize the number of signal vias reduces EMI by

reducing inductance at high frequencies. For more information on layout guidelines, see High Speed Layout

Guidelines (SCAA082) and USB 2.0 Board Design and Layout Guidelines (SPRAAR7).

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