About DVI-I signals and ESD

[Roky]

Hi everybody
In the schematic of voipack baseboard, in the DVI-I sheet,
there is no connection to 5V for the protection diodes on DVI-I signals
going output to the connectior. Is it correct? Why are there "No ERC" simbols on the power supply of the BGX50AE6327 ICs ?
..maybe some zener diodes are missing !?

thank you very much
Rocco

[Paul van Avesaath]

i dont think it is needed for differential usage.
different device but with more details in the datasheet..

https://www.onsemi.com/pub/Collateral/NUP4202W1-D.PDF

[Roky]

yes ...right .....that is a solution .....but this is another solution:



look at your circuit ...there is a zener


the point is another: BGX50A has a reverse voltage of 50V
so when you start ESD test there are 50V on differential outputs on DVI IC
I do not think it is ... so good



anyhow thank you for your reply
Rocco

[robertferanec]

I guess, the same way it was connected also on the reference board which was used to design VOIPAC baseboard - and I guess, that is the reason why it is connected that way. There was probably a reason why they did not connect it in the original reference board, but I am not sure why.

For today's designs, if you are not sure, the best is to choose ESD chips specific for the interface you need to protect (many chips directly say - use for LVDS, HDMI, USB, etc). Otherwise you can use a generic ESD protection (which may be cheaper), but then be sure to check influence of that protection on signals and passing all the validation and tests.

PS: I personally go with reference schematic or ESD protection chips for specific interface or with something what we used in past projects and we know it works ok.

[Roky]

yes ...I got the point for the apparent "missing" connections ..it is ok

without a zener it is not good to discharge ESD impulses through the power supply lines (VCC) and ICs.

But however BGX50A is not good for ESD protection for its high reverse voltage.
When an ESD impulse is applied, its voltage actives a diode in a reverse conduction.
So a zener diode is needed in parallel to diodes in order to fix voltage at less than the max
voltage allowed at the differential signal ports of the DVI IC.

So, as you said, it would be better to use a specific (DVI, HDMI, etc) ICs for ESD protection,
to fix voltage at about 5V with a zener and maybe to reduce parasitic capacitor effects of all the diodes
for high speed LVDS signals.

Thank you for your reply
Rocco