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Author Topic: Low capacitance ESD diodes? ...dosent seem so?  (Read 410 times)
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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?
« Last Edit: September 14, 2013, 03:57:27 pm by jtw11 » Logged

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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.
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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.
« Last Edit: September 14, 2013, 08:29:11 pm by LarryD » Logged

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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.
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Thanks for the replies.

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