The Key to USB PCB Layout

Thu Nov 17 11:39:17 CST 2022

The Key to USB PCB Layout - HornmicLink

The Key to USB PCB Layout

Due to the advantages of fast data transfer, reversable interface, support for hot-plugging and other advantages of USB devices are widely used. At present, the majority of products on the market with USB2.0 as the interface. Drawing a PCB that meets the requirements of USB 2.0 data transfer is extremely important to the performance and reliability of the product.

PCB Layout Rules

USB protocol is defined by two differential signal lines (D+, D-) to transmit digital signals, if you want USB devices to work stable differential signal lines must be strictly in accordance with the following rules to layout traces.

1. When laying out components, try to make the differential line the shortest to shorten the distance of the differential line.

Make-the-differential-line-the-shortest

2. Prioritize the drawing of differential lines, try not to exceed two pairs of vias on a pair of differential lines (vias will increase the parasitic inductance of the line, thereby affecting the signal integrity of the line), and place them symmetrically.

Two-vias-at-most-on-differential-pair

3. Symmetrical parallel tracing, which can ensure the tight coupling of the two wires, avoid 90° routing, arc or 45° are better ting methods.

Symmetrical-parallel-tracing

4. The placement of differential series resistors, test points, and pull-up and pull-down resistors.

Resistor-capacitor-series-connection-in-differential-pair

5. Due to factors such as pin distribution, vias, and routing space, the differential pairs lengths are easy to mismatch. Once the line lengths do not match, the timing will shift, and common-mode interference will be introduced, which will reduce the signal quality. Therefore, it is necessary to compensate for the mismatch of the differential pair so that the line lengths match. The length difference is usually controlled within 5mil. The principle of compensation is where the length difference occurs.

Match-differential-pairs-lengths

6. In order to reduce crosstalk, if the space permits, the distance between other signal networks and grounds should be at least 20mil from the differential line. If the distance between the ground and the differential line is too close, it will affect the impedance of the differential line;

Control-the-distance-to-differential-prair

7. The output current of the USB is 500mA. Pay attention to the line width of VBUS and GND. If 1Oz copper foil is used, the line width greater than 20mil can meet the current carrying requirements. Of course, the wider the line width, the better the integrity of the power supply.

Control the impedance

The signal line width and line spacing of the differential line of ordinary USB devices should be consistent with the signal line width and line spacing of the entire board. However, when the working speed of the USB device is 480 Mbps, it is not enough to only achieve the above rules. 

We also need to control the impedance of the differential signal. Controlling the impedance of the differential signal line is very important for the integrity of the high-speed digital signal. Because the differential impedance affects the eye diagram, signal bandwidth, signal jitter and interference voltage on the signal line of the differential signal. The differential line impedance is generally controlled at 90 (±10%) ohms (refer to the chip manual for specific values). The differential line impedance is inversely proportional to the line width W1, W2, and T1, inversely proportional to the dielectric constant Er1, and proportional to the line spacing S1, which is proportional to the distance H1 of the reference layer. The following figure is a cross-sectional view of the differential pairs.

Sectional-view-of-differential-pairs

The following figure shows the reference stack of a four-layer board, in which the middle two layers are reference layers, the reference layer is usually GND or Power, and the reference layer corresponding to the differential line must be complete and cannot be divided, otherwise it will cause discontinuous impedance of the differential line . If a four-layer board is designed as shown in Figure 2, the differential line width and spacing of 5.5 mils can meet the differential impedance of 90Ω. However, the 4.5mil line width and 5.5mil line spacing are only our theoretical design values. In the end, the circuit board factory will make appropriate adjustments to the line width, line spacing and distance to the reference layer based on the required impedance value, combined with the actual production situation and the board.

Reference-stack-of-four-layer-board

Summary

The above layout rules are based on USB 2.0 devices, in the USB layout process to grasp the shortest differential lines, tight coupling, equal length, consistent impedance and notice the USB power line current-carrying capacity, follow the above principles, then USB devices will run well.




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By HornmicLink_Henry @221117 11:50