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SN74CB3Q3257: what will happen if pin7 short to GND

Intellectual 1650 points

Replies: 15

Views: 195

Part Number: SN74CB3Q3257

Hi Sir,

Could you please check what will happen if SN74CB3Q3257DGVR pin7 short to ground?

Will it result in system/board damage or burn out issue?

Thank you.

  • Pin 7 is 2A. This pin will be connected to either 2B1 or 2B2.

    The switch itself does not care about the voltage. The other devices connected to 2B1/2B2 must be able to cope with a connection to GND.

    If other devices connected to 2B1/2B2 actively drive high, they will source current that flows through the switch into GND. This will exceed the absolute maximum rating of 64 mA, which is likely to cause damage.

  • In reply to Clemens Ladisch:

    What if the pin7 has low resistance to GND (~10 ohm) due to IC itself damage, will that cause board burnt or system shut down?

  • In reply to Aaron Chiu1:

    The effects are the same.

  • In reply to Aaron Chiu1:

    Hi Aaron,

    As Clemens mentioned - this configuration by itself will not cause damage, however since this part is low resistance with a typical on resistance of 4 ohms. This means relatively small voltages across the switch can create relatively large currents. For example, this parts max current is 64mA; you would only need 256mV across the switch to hit the max current limit before you may start seeing damage. 

    In short as long as the absolute max ratings, such as voltage and current, are respected the switch will not sustain damage.

    If you have any other questions please let me know!

    Best,

    Parker Dodson

  • In reply to Parker Dodson:

    Hi Parker,

    We suffer from bus check error due to this IC damage recently. And our factory sent 2pcs IC for failure analysis. Attached is the FA report we got today.

    The FA report says it's caused by EOS. Do you have idea which one (ESD, over voltage/current, over heat) may cause this from the picture you see?

    We want to get this issue repro by ourselves first.

    Thanks,

    AaronQEM-CCR-2010-00484 Final Report.pdf

  • In reply to Aaron Chiu1:

    Hi Aaron,

    Based on what I am seeing it seems the cause of the issue is an overvoltage/overcurrent event. These events can cause ESD protection on the device to short the pin to ground or VDD. 

    Also due to the fact that only 1 pin is suffering damage I'd think its not temperature related. Based on the setup, I'd try to measure the voltage being put across the switch when it you connect the switch to ground, I'd imagine there might be some transient that occurs when switching to ground that could cause issues.

    However if the voltage across the switches induces too high of a current, or the voltage with reference to ground is higher than abs max that will also increase the chances of damage significantly as well.

    Best,

    Parker Dodson

  • In reply to Parker Dodson:

    Hi Parker,

    Thanks for the advise. 

    We did the voltage across the switch measurement when it's at static state, no matter in low or high level, it's lower than 256mV.

    However, we observed that the high voltage delta when it's in transient, such like '0' to '1'. But we think it's normal behavior as there has delay between the switch. 

    We'll try to follow your step to see if the issue could be repro.

    Thanks,

    Aaron

  • In reply to Aaron Chiu1:

    Hi Aaron,

    Another thing that you may want to check, if possible, is to see what the current being injected through the switch is. Overcurrent is typically the big killer for the mux, so if a transient current spike violated abs max, that could be possibly the source of the issue as well if the voltage tests don't prove to find anymore issues. Please let me know if you need anymore assistance on this problem!

    Best,

    Parker Dodson

  • In reply to Parker Dodson:

    Hi Aaron,

    One thing that may help reduce these transients is to add a bit more capacitance to the input of the switch can help smooth those out, at the cost of slightly slowing the response of the circuit. If possible adding a capacitor here may prove to be helpful - if you can get a reading of the peak of the transient I can probably help narrow down a capacitor value. 

    Best,

    Parker

  • In reply to Parker Dodson:

    Hi Parker,

    Thanks for the suggestion. We will consider to add the small capacitor for future product.

    We did some experiments but not yet to repro the issue. We'll perform the ESD and over-current test on these two days.

    Two questions that I would like to asks:

    1. If the over-current on the pin 7, then it should break this switch path, right? That means both switch side should have low resistance, pin 5 or pin 6.

    2. The fail symptom that we observed so far is pin 7 low resistance. Is this pin weaker than other pins?