Signal 1

GND Plane

Power Plane

Signal 2

14

TS3USB3200

SCDS333B – JUNE 2012 – REVISED JULY 2016

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Copyright © 2012–2016, Texas Instruments Incorporated

10 Layout

10.1 Layout Guidelines

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

the D+/D– traces.

The high-speed D+/D- must match and be no more than 4 inches long; 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 must match the cable characteristic

differential impedance for optimal performance.

Route the high-speed USB signals using a minimum of vias and corners which reduces 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 must be less than 200 mm.

Route all high-speed USB signal traces over continuous GND planes, 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 as shown in Figure 20.

Figure 20. Four-Layer Board Stack-Up

The majority of signal traces must 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. Minimizing the

number of signal vias reduces EMI by reducing inductance at high frequencies.