USB bus Power and Ground design

Wed Nov 16 11:21:04 CST 2022

USB-bus-Power-and-Ground-design

USB bus Power and Ground design

The USB interface has five endpoints: USB power (VBUS), D-, D+, signal ground (GND), and protection ground (SHIELD). In addition to the D+, D- differential signal design, the design of the USB bus power, signal ground and protection ground are equally important for the proper operation of the USB system.

USB power supply voltage of 5V, providing a maximum current of 500mA, the power lines should be arranged on the signal layer near the power supply layer, rather than on the same layer as the USB differential traces are located. The traces width should be more than 30mil to reduce its interference with the differential signal lines.

VCC-power-GND-with-pullup-and-pulldown-resistors

Many manufacturers of USB from the control chip operating voltage of 3.3V, when it works in bus-powered mode, requires a 3.3-5V power conversion chip, the output of the power conversion chip should be as close as possible to the voltage input of the USB chip, and the input and output of the power conversion chip should be increased capacitance capacitance and connected to a small capacity capacitor for filtering. When the USB from the control chip works in self-powered mode, the USB power line can be connected in series with a large resistor to ground.

USB interface signal GND should be in good contact with the signal GND on the PCB board, the protection ground can be placed on any layer of the PCB board, it and the signal GND split, between the two ground can be a large resistor in parallel with a high voltage capacitance, as shown in the figure below.

connection-between-Signal-GND-and-Protect-GND

The spacing between the protection GND and the signal GND should not be less than 25mil to reduce the edge coupling effect between the two GNDs. Protection GND should not be covered by large area copper cladding, a 100mli width of copper foil has been able to meet the functional needs of the protective ground.

Design Guidelines 

When drawing USB power lines, signal GND and protection GND, the following guidelines should be noted.

  • 1, USB socket pins 1, 2, 3, 4 should be within the envelope of the signal GND, not within the envelope of the protection GND.
  • 2, USB differential signal lines and other signal lines in the alignment should not overlap with the protective GND layer.
  • 3, The power supply layer and signal ground layer in the copper overlay should be careful not to overlap with the protective GND layer.
  • 4, The power supply layer should be shortened 20D than the signal GND layer, D for the distance between the power supply layer and the signal ground layer.
  • 5, If the differential line is located in the layer of the signal GND needs a large area of copper cladding, ensure that the distance between signal ground and differential line more than 35 mil, so as not to reduce the impedance of the differential line.
  • 6, Placed some vias with signal GND properties, increase the connectivity of the signal GND, shorten the signal current backflow path, as shown in the figure below.
  • 7, In the USB bus power lines and PCB board power lines, you can add magnetic beads to increase the anti-interference ability of the power supply.

add-GND-vias-pair-HornmicLink



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By HornmicLink_Henry @221116 11:29