ESDS302: Problem with pat failing during hotswap

Hi Sean,

It's not recommended to have DC voltages above the working voltage at the input of ESD diodes. Our diodes will burn up if there is a DC voltage above the breakdown voltage, they were not designed to be continuously operated in the breakdown region. 

You'll need a device with a working voltage of at least 12 V to prevent the diode from burning up from the hotswap. Something like TPD2E007  however that device might not have a low enough capacitance depending on the data rate of the DM/DP lines. 

Regards,

Sebastian 

Thanks for the input, however you misunderstood my comment.  There is no DC on the DM/DP pair.  I was stating that instead of 5V on the 4th wire I have 12V.  Hope that helps.

HI Sean,

If those data lines are 5 V you should be using a diode with a working voltage of at least 5 V.  The ESDS302 has a working voltage of 3.6 V and can breakdown at voltages above 4.5V. Even if it doesn't breakdown during normal operation you would be leaking larger amounts of current than the specified leakage at Vrwm. 

Also to clarify, during the hot swap there is 12 V present at IO2 of the ESDS302? or that 4th wire is always carrying a 12 V signal? 

Regards,

Sebastian 

The schematic is posted above Sebastian.  Hopefully that will help.  Thanks.

Hi Sean,

I'm assuming the hotswap is on the J1 connector. It's likely the 12 V line is shorting to the D+/- lines and causing the diode to fail short (short to gnd I'm assuming). If that wire is carrying 12 V signals it's possible that miswiring could be causing the short. 

Regards,

Sebastian 

Hi Sabastian,

Correct J1 and while I cannot absolutely prove that the 12V shorting is not happening, there are two thoughts to consider:  That is why I put the PTCs in line so that they would open the line in the case the 12V got on DM or DP and the second thought is that the frequency is so high and is randomly happening during power cycling of the product.  No handling is taking place when the parts shorts.  Any other ideas or questions?  Thanks,Sean

Hi Sean,

Perhaps the turn on time of the PTC is too slow and that first peak is damaging the ESD diode before the PTC begins to clamp? The datasheet specifies a max time to trip as 200milliseconds which is quite a long time. 

Even if the switch time is a bit better than 200milliseconds, our device is only rated for 12 A of peak surge current protection for the IEC61000-4-5 waveform with an 8 us rise time and 20 us pulse width. A pulse of just a few A for a longer duration can definitely damage the device. 

Regards,

Sebastian 

Thank you Sabastian for that analysis.  Is there a better TI part to use vs the ESDS302 that I am using now?  It would especially be great if it were a drop-in replacement as I already have PCBs.  With much appreciation, Sean

One other thing Sabastian, if we raise the Vrwm for the new part, doesn't that remove the protection for the USB hub from a 12V pulse on DM/DP?

Hi Sean,

Yes the USB IC would need to be able to handle that overvoltage until the PTC turns on, which might be violating max ratings of the IC. 

The ESDS312 is a drop in replacement and has a higher surge current rating, it may be able to handle the overvoltage depending on the duration. What is the speed of the USB signals? 

Regards,

Sebastian 

That is awesome.   It is just a USB2.

Thanks,

Sean

Hi Sean,

The ESDS312 has a low enough capacitance to support the USB2.0 data rates, I'm not sure it will survive the overvoltage but since it's the same package you could drop it on the existing footprint and try to recreate the hotswap to test it out. 

Regards,

Sebastian