Question: We’re planning to add ESD diodes to the USB lines of our FX2LP based design. Is this recommended?
Answer: Adding of ESD diodes on the USB lines will add capacitance and resistance on the line and we've seen customers face issues meeting USB compliance when they add components on the USB line (choke, ESD diode, switch etc). So we do not recommend adding components on the USB lines.
Here are the parameters that the device is expected to meet as per spec, D+ and D- should have 45 ohm +/- 10% to ground and a differential impedance of 90 ohm +/- 10% between them. For capacitance, in high speed
Capacitance to Ground on each line: CHSLOAD≤ 10 pF
Matching of Capacitances to Ground: ≤ 1.0 pF
The differential capacitance across the transceiver inputs should be no more than 5.0 pF.
For full-speed due to change in termination it will be 100pF +/- 10% (measured at B-receptacle).
It would be a good idea to check with your PCB manufacturer if the board can be fabricated to meet these values even with the addition of the ESD diode before incorporating them in the design.
