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TVS diode for the CP2102N USB-to-UART bridge

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Nikos A.:
Hi I want to include ESD protection for the CP2102N USB-to-UART bridge. The datasheet (page 7) recommends the SP0503BAHTG TVS diode array. This diode has a max. positive clamping voltage of 8.5V, while the maximum rating of the bridge is 5.8V. If the diode clamps at 8.5V, won't destroy the chip?

https://eu.mouser.com/datasheet/2/368/cp2102n_datasheet-1634912.pdf
https://eu.mouser.com/datasheet/2/240/Littelfuse_TVS_Diode_Array_SP05_Datasheet_pdf-777043.pdf

T3sl4co1l:
The chip is probably already rated for 1kV or so ESD, but you don't know what clamping voltage it experiences during that pulse.  The question is, will making a current divider with an external TVS, sink enough current to make e.g. 15kV contact discharge look like 1kV to the chip?

The answer is much improved with a series resistor between TVS and IC.  Typically one can afford a couple 10 ohms here.

Which, curiously, the transmitter impedance is rated 28-44 ohms, while 45 is needed for USB; evidently you should add 9 ohms, but they omit this (out of laziness / not worthwhile?).

Probably it's still fine with the "traditional" 22 ohms in series as well, though I would expect somewhat reduced maximum range in rare cases (i.e. when a random unit happens to have max. output resistance: it's a manufacturing distribution).  You might never notice this.

Direct ESD strike is also unlikely, as USB is fully shielded and the shield is generally struck preferentially over the pins.  Especially for mini/micro/USB-C.  (I would still prefer a TVS, myself.)

Tim

Nikos A.:

--- Quote from: T3sl4co1l on February 05, 2023, 06:59:18 am ---Which, curiously, the transmitter impedance is rated 28-44 ohms, while 45 is needed for USB; evidently you should add 9 ohms, but they omit this (out of laziness / not worthwhile?).

Probably it's still fine with the "traditional" 22 ohms in series as well, though I would expect somewhat reduced maximum range in rare cases (i.e. when a random unit happens to have max. output resistance: it's a manufacturing distribution).  You might never notice this.


--- End quote ---

Thank you for the tip, I didn't notice that. I will include the 9ohm termination resistor since 36ohms is the typical value of the transceiver.

So it's safe to proceed with the TVS suggested in the datasheet right?

T3sl4co1l:
Yes.

Or, round it to 10 ohms as the next common value.

Tim

tooki:

--- Quote from: Nikos A. on February 04, 2023, 11:17:23 pm ---Hi I want to include ESD protection for the CP2102N USB-to-UART bridge. The datasheet (page 7) recommends the SP0503BAHTG TVS diode array. This diode has a max. positive clamping voltage of 8.5V, while the maximum rating of the bridge is 5.8V. If the diode clamps at 8.5V, won't destroy the chip?

https://eu.mouser.com/datasheet/2/368/cp2102n_datasheet-1634912.pdf
https://eu.mouser.com/datasheet/2/240/Littelfuse_TVS_Diode_Array_SP05_Datasheet_pdf-777043.pdf

--- End quote ---
We don’t know much about the reaction times of each. It’s entirely possible the IC can handle 8.5V for some very short amount of time. (So while a sustained 8.5V would damage it, a very brief pulse — as happens in an ESD event — of 8.5V wouldn’t.)

Just be sure to lay out your USB data lines carefully, using 90 ohm impedance-controlled differential traces. I’ve seen a CP2102N board fail completely because of poor USB data line layout. I found it easiest to put the TVS array on the opposite side of the board so it could sit directly beneath the USB traces without affecting their placement.

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