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Topic: Low capacitance ESD diodes? ...dosent seem so? (Read 516 times) previous topic - next topic

jtw11

Sep 14, 2013, 10:53 pm Last Edit: Sep 14, 2013, 10:57 pm by jtw11 Reason: 1
Hi all,

I'm looking for diodes to protect some CAN lines from ESD, and stumbled across a device produced by NXP, specifically for protecting CAN bus lines from ESD - and is advertised specifically as having ultra low capacitance.

Here's the device http://www.nxp.com/documents/data_sheet/PESD2CAN.pdf

So, max 30pF at 1MHz, VR = 0V.

Take a bog standard 1N4148

http://www.nxp.com/documents/data_sheet/1N4148_1N4448.pdf

Max 4pF at 1MHz, VR = 0V.

So, am I missing a trick? What's so special about these ESD 'low capacitance' protection diodes? They have higher capacitance than everyday use diodes?

westfw

A normal diode is not a protection diode.  A zener diode can have a capacitance of about 100pF, and that's still not a "protection diode" of the sort used against ESD.

LarryD

#2
Sep 15, 2013, 03:20 am Last Edit: Sep 15, 2013, 03:29 am by LarryD Reason: 1
My guess is: capacitance will act as a direct short to very short duration spikes.
But, too much capacitance would degrade your signal.
Hence there is a tradeoff.

Some times you add a small capacitor to deal with ESD.
The way you have it in your schematic isn't the same as how you have it wired up!

MarkT


Hi all,

I'm looking for diodes to protect some CAN lines from ESD, and stumbled across a device produced by NXP, specifically for protecting CAN bus lines from ESD - and is advertised specifically as having ultra low capacitance.

Here's the device http://www.nxp.com/documents/data_sheet/PESD2CAN.pdf

So, max 30pF at 1MHz, VR = 0V.

Take a bog standard 1N4148

http://www.nxp.com/documents/data_sheet/1N4148_1N4448.pdf

Max 4pF at 1MHz, VR = 0V.

So, am I missing a trick? What's so special about these ESD 'low capacitance' protection diodes? They have higher capacitance than everyday use diodes?


Yes, you are, these diodes are TVS, ie back-to-back zener/avalanche breakdown diodes.
They are pulse rated at 230 watts, can take 5 amps....

They are big diodes in a small package, and the large diodes (ie large junction area) tend to have more
capacitance (because of the junction area), which obviously will load down a bus like CANBUS, hence
they have striven to keep the capacitance as low as possible while still taking 5A 40V spikes without damage.
[ I won't respond to messages, use the forum please ]

jtw11

Thanks for the replies.

I thought the current capability would perhaps be the case here, as has been mentioned.

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