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SN74CBT3257C: SN74CBT3257C instead of FST3257

Meir Battan
Expert 4050 points
Part Number: SN74CBT3257C


Hello,

One of my customers have used the Fairchild IC - FST3257 in their projects and recently they decided to replace it to a direct replacement from TI - the SN74CBT3257C. They have 3 ICs of the FST3257 in their board and the design used to work for them with the Fairchild ICs. With our IC they have a problem and the IC (U11) is burning when they use TI chip instead of the Fairchild chip. Can you please inspect the attached schematics and let us know what is the cause of this issue? are there any differences between the two chips?

P.S VBS is 5.4V  

Regards,

Meir.

 

  • 0
    TI__Mastermind 45625 points
    Meir,

    Thank you for sharing the schematic. I do not see any issue with the schematic as all pins are connected within the recommended operating conditions.

    1) Can you describe what the symptom of "burning"? Is the device catching fire or smoking? Is it getting too hot? Is it being permanently damaged? etc.

    2) How many boards is this happening? Have you replaced the SN74CBT3257C with another IC and it fixes the issue? Does the issue follow the device or does it follow the board?

    3) What is the difference between U11 and U12. Based on the schematic I would expect both ICs to be "burning" since they look identical

    4) what is the voltage on the S and/OE pins?

    Thank you,
    Adam
  • 0 in reply to Adam Torma
    TI__Expert 4050 points
    Hi Adam,

    Thanks for the fast reply.

    to answer your questions:

    1. “Burning” means that when the board is powered up, U11 becoming very hot and white smoke emits from the unit. It is permanently damage. This happen on most boards of this design but not all the time.
    2. This happens on all the boards they have tested (they tested ~50 boards). If they change this IC to new TI SN74CBT3257C the problem returns but if they use Ferchaild on the same board the problem disappeared.
    3. Theoretically there are no differences between U11 and U12 - the only change is that U11 begin to work first and then U12.
    4. S and /OE is 3.3V.

    Please let us know if you would be able to make a short conf call with the customer.

    Regards,
    Meir.
  • 0 in reply to Meir Battan
    TI__Mastermind 45625 points
    Yes this sounds like very strange behavior especially for a low power switch. I would be happy to support a conference call this week.

    Thank you,
    Adam